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Precision Treatment of Death Watch Beetle Attack

Precision Treatment of Death Watch Beetle Attack

By Robert Demaus

Over the past few years it has become increasingly apparent that existing methods, both for the assessment of Death Watch Beetle attack and its treatment, are of limited success, and in some cases actually counter-productive. New non-destructive techniques for locating and quantifying the extent of infestation provide a far more accurate assessment of the structural implications of Death Watch Beetle attack: the precision of these (damp and timber) techniques has in turn allowed a more conservative, effective and environmentally safer damp and timber treatment to be developed. This has particular relevance to ecclesiastical buildings, which suffer more than most from continuing attack by the beetle.


Death Watch Beetle (Xestobium rufovillosum) is a native British insect, which naturally inhabits the dead wood of several hardwood species found in the United Kingdom. For the larvae to flourish, the heartwood is usually required to have been modified by fungal decay, making the timber more palatable. The vast majority of structural oak used in historic buildings was converted and assembled green, when the moisture content was still very high, and it is likely that some timbers used had already suffered minor fungal attack before felling. In larger section timbers, the moisture content would have remained high enough to sustain fungal attack for many years, and so a suitable environment for long term Death Watch Beetle infestation was present in the building from the outset, and the beetle larvae themselves were probably introduced into the buildings in the timber used in their construction. Lack of maintenance over the ensuing years inevitably allowed periods of water ingress, setting up new fungal attacks, and consequent fresh food sources for the infestation.

In many cases of active infestation, the environmental conditions allowing the beetle larvae to survive are only just met, so that the life cycle is continuing, but at a very slow rate; and structural damage occurs at a proportionally slow rate. A relatively small change to the environment can cause the attack to die out, or conversely, to become more active. At present it is thought that a moisture content of 14 per cent is the lower limit for a flourishing colony of Death Watch Beetle larvae, and if the moisture content drops below 12 per cent, the larvae will die. It therefore ought to be a simple matter of ensuring that the moisture content is below this level, and the infestation would cease to be a problem. Unfortunately, even in a fairly well ventilated roof space, the normal moisture content of structural timber averages 14-15 per cent and in many buildings in which this beetle is a problem (such as irregularly heated churches), condensation coupled with poor ventilation, can significantly increase this moisture level. In the long term, therefore, every effort should be concentrated on ensuring that the environmental conditions are adjusted, first to slow down, and ultimately to kill off, the beetle attack: this improved environment must then be maintained year after year. Even if these improvements can be achieved, it may still be necessary, over the short term, to introduce chemical control where the beetle is particularly active. It is of course essential that moisture levels in surrounding masonry are measured and reduced as necessary: if this is not practicable, the timber should be isolated from the damp masonry as much as possible.

There are some situations where sufficient improvement to the environment cannot be achieved, and in these situations more intensive chemical intervention may well be necessary over a longer term.

For many years it had been thought that the life cycle of Death Watch Beetle was a maximum of five to seven years, and that the adult beetle laid its eggs on or close to the surface of the wood. The hatched larvae then burrow into the timber and continue to feed on the wood until they have grown sufficiently to pupate: it is the larval stage that does most of the structural damage to the wood. The adult emerges during the spring following pupation, mates and renews the cycle. However, it is now established that the life cycle depends on the suitability of conditions, and that the larval stage may vary from one year in ideal conditions to 12 years or more, if conditions are not favourable. It has also been shown that the adults do not necessarily need to emerge, and can mate in cavities within the timber, and further, that adult females, if they have emerged to mate, sometimes re-enter existing flight-holes and lay their eggs deep in the timber, rather than on or near the surface. What is still unknown is whether some adults have always mated and laid their eggs without emerging, or whether this behaviour has evolved to counter surface chemical treatments. The results of these, and other observations, have highlighted the ineffectiveness of existing treatments.


Surface spraying will only penetrate a few millimetres into heartwood and then only if the surfaces are very thoroughly cleaned down before application. It has been argued that this is sufficient as it will kill the adult as it emerges, but what tends to happen is that the beetles avoid the treated areas and instead emerge, if at all, through joints and other untreated areas. The darkness and relatively stable environment of joints is in any case a favourite habitat of the insect, and any treatment that tends further to concentrate attack in joint areas should be avoided. No new flight holes appear, and the problem is thought to have been solved, but is in fact continuing, unobserved and unchecked. Further, by discouraging their emergence, the beetle’s only natural predator within buildings, the spider, is prevented from exercising any control, if it has not already been killed by the spraying.

In an attempt to avoid the hazards associated with solvent-based chemicals, waterbased emulsion fluids have been introduced, but current evidence suggests that in oak, their depth of penetration is even less than the solvent-based fluids. Paste, commonly known as mayonnaise, which uses the same contact insecticide but in a thick emulsion carrier, does allow a slightly deeper penetration and greater effective concentration of chemical. This method of application still suffers the same limitations as surface spraying, and is even more difficult to apply where access is difficult. It also often leaves a waxy skin over the areas of timber treated.

Pressure injection or irrigation through one-way valves inserted into pre-drilled 10mm holes can be more effective in some cases, but there is no control over where the fluid is going, or how much is being used. It only needs one drill hole to enter a shake or mortice for gallons of fluid to run along the shake, emerging sometimes metres away from the injection hole, or into an unseen void, and not necessarily getting to the areas of larvae attack at all. Such uncontrolled use of large volumes of chemical (usually in a solvent carrier such as white spirit) introduces a number of potential hazards. First is the increased risk of fire; second, the risk of considerable damage and staining to plaster, decorative paint and other finishes; third is damage to electrical insulation; and fourth is potential damage to the health of those who inhabit the building. Water based emulsion cannot be used for injection or irrigation as the wood will swell, and there may also be a large amount of staining on decorative finishes.

Smoke treatments, set off around emergence time, are particularly ineffective, generally killing more spiders than beetles.

Gas fumigation can be effective, but it is extremely difficult adequately to seal a building or area of a building, of the type typically attacked by Death Watch Beetle. This, coupled with the hazards generally involved with using toxic gas (usually methyl bromide), render it impractical for use in buildings.

Heat sterilisation is currently receiving a lot of attention. It is claimed that a temperature of 52-55ºC maintained for 30-60 minutes will kill all wood-boring insects. Given that live Death Watch Beetle larvae have been found in the middle of large, recently fire damaged timbers, the duration of treatment would need to be very much longer than one hour if this temperature is to be achieved throughout a 300 x 250mm oak member, for example. The potential effects on delicate finishes, oak panelling and other fragile fabric of such a temperature for a prolonged period are likely to be considerable.


The identification of the beetle itself is amply covered in most building surveying books. The adults are 6-9mm long, dark brown with patches of yellow hair: the larva are up to 9mm long, cream and slightly curved, covered in fine yellow hairs. The flight holes and tunnels are circular and 3mm in diameter. The bore dust is cream coloured with bun-shaped pellets.

It is important to confirm whether a beetle attack is active or dead. It should always be borne in mind that the great majority of Death Watch Beetle attacks found in historic buildings died out many years – even centuries – ago. However, this has not stopped the unscrupulous from treating the attack by one system or another, and hailing the subsequent status quo as a success.

The extent of the attack within the timber is not always proportional to the number of flight holes visible, and the structural integrity of the timber should always be checked. Many visible attacks affect only the sapwood areas left on the outside of the timber after conversion, which has no structural significance: surface treatment will normally deal with this, but the attack has usually died out years ago.

The presence of fresh, brightly coloured bore dust and clean dust-free flight holes certainly indicates that the attack is active, but their absence may not necessarily mean that the attack is dead. It is quite possible that a previous unsuccessful remedial treatment has discouraged any flight holes in the visible area, but allowed the attack to continue within. Moisture content of the timber is a useful indicator: if it can be shown that the moisture content within the timber is below 14 per cent, then it is very unlikely to be active; between 14-17 per cent there is a good chance there will be some activity, and over 17 per cent the colony is likely to be thriving. It is essential that the moisture content is measured deep within the timber, not on, or near, the surface (as with most proprietary moisture meters), where daily or seasonal variations, condensation and other factors may well give misleading information.

One of the reasons why such a hit and (more often) miss, ‘carpet bombing’ approach has been so widely used is that, until relatively recently, there has been no way of accurately assessing the internal condition of large section timbers where Death Watch Beetle attack was suspected. New diagnostic techniques, using a combination of ultrasound and micro-drilling, allow very precise location of cavities and tunnels within the cross-section of the timber.

Ultrasound is a very quick and totally non-destructive method of locating areas where significant internal degradation of the timber has occurred. Microdrilling allows a very accurate measurement of the size of cavities and the depths at which they, and tunnels, occur. The microdrill leaves a hole of about 1mm diameter (it looks very like the exit hole left by Anobium punctatum, the common furniture beetle), and testing can be carried out through ornate plaster, panelling and other decorative finishes. This in itself is a great bonus, as it reduces or eliminates the damage and cost of stripping out. Because the depth at which the cavities and tunnels within the attacked timber occur can be so accurately measured (2mm), it is then possible to insert a 0.81mm x 200mm long hypodermic needle through the hole left by the micro-drill, and inject fluid precisely into the cavities and tunnels, and in controlled measured volumes. The coverage within the cavities depends on the size of the cavity, the design of the spray head on the needle, and the injection pressure used, but normally a spacing of around 150mm is required for adequate coverage. This may seem very close centres, but it should be remembered that the overall area being treated is vastly reduced by knowing exactly the extent of the attack before treatment is started. In situations where a small amount of damage to the fabric is acceptable, it may sometimes be more effective to drill 6mm holes and inject a bodied emulsion paste deep into the timber. It is essential that the timber is assessed first, to confirm exactly at what depth, and in what volume, the paste should be injected. The paste can be introduced using a caulking gun with extended delivery tube. Research and development of a new bulked paste is currently underway. This paste is specifically designed to fill internal cavities, but not to spread further.


Almost all problems of decay in timber structural components originate from faults in the design or maintenance of other components of the building: remedy of the timber problems must be considered as an integral part of a building’s repair and maintenance programme and not in isolation. Architects and others involved in the care of buildings must maintain their control over the methods used in Death Watch Beetle treatment, and should not merely pass the responsibility to a remedial treatment specialist, who has little or no control over the faults that set up the decay problem in the first place. The amount of treatment necessary, and certainly the volume of toxic chemicals used, can be vastly reduced, and the effectiveness of the treatment greatly improved, but only if a detailed assessment of the severity of the attack, and the mechanisms that allow that attack to continue, is carried out first.


(damp and timber article)

The facts about chemical damp-proof courses and replastering

 The facts about chemical damp-proof courses and replastering

By Graham Coleman

There are a number of factors to be taken into account when considering chemical damp-proofing systems and attendant replastering works. The following guide is intended to provide an objective insight into chemical damp-proofing, its performance and the importance of the replastering works.

It should be fully appreciated that free water in building materials is not desirable, it can lead to decorative spoiling and rot: in some cases it can lead to collapse of the material itself, eg, cob. Indeed, someone saw fit at the end of the 1800′s to stipulate the introduction of damp-proof courses as a whole and this practice is clearly an essential part of all properties constructed since that time; it certainly is now. If a damp-proof course was of no consequence it would not be necessary or be part of the ‘Regulations’ etc, and would not still be part of all new properties. Thus damp-proof courses are obviously beneficial to both the building and the occupants alike.

Another factor to consider is that, all things being equal, rising damp tends to rise higher in thick walls than thin walls; this is due to the lower surface to volume ratio of thicker walls, evaporation being mostly subject to surface area. This is an important feature to consider when dealing with properties with larger dimensioned walls – simply the so-called ‘allowing walls to breathe’ syndrome to stop the rising water may prove of little effect in such cases.

So there may be a case for chemical damp-proofing.

Chemical damp-proof courses are inserted to control the vertical passage of moisture from the ground and are almost all installed in properties where no damp-proof course exists or it has broken down with age.


Chemical damp-proof courses should be installed in a position in accordance with good practice as described in BS 6576:1985, “Code of Practice for the Installation of Chemical Damp-proof Courses”. They are installed in walls by various methods depending on the particular system being employed but the ultimate objective is to provide a water repellent or pore blocking material in a continuous horizontal band in the masonry thereby to provide a ‘barrier’ to water rising from the ground. Moisture paths

mpathsThe only continuous pathways through which water can rise through a wall are the mortar beds: for water to pass, say, from brick to brick it must still cross a mortar bed (bottom of figure, right). It is therefore essential that the damp-proofing material impregnates the mortar courses since these form the major pathway for the rise of water within walls. Damp-proofing the masonry units (eg, bricks) alone is of very little value! Porous mortar and impervious/water repellent brickwork will still allow rising dampness to occur. However, if the pores in the mortar line are made water repellent or blocked then the water cannot rise since it cannot traverse the mortar beds to do so (top figure, right).

In the majority of older properties the mortar is not alkaline so that the water repellent formulations based either silicone resins, aluminium stearate, or methyl siliconate (sodium or potassium methyl siliconate) can be used. Occasionally, however, the mortar may highly alkaline such as in a recently constructed wall (eg. where the physical damp-proof course has been omitted). This will exclude the use of the methyl siliconates since the highly alkaline conditions found, for example, in new mortars prevent the formation of the water repellent resin.

Care should be taken to ensure that the damp-proof course is not bridged by high external ground levels, blocked cavities or debris piled against the wall; ground levels should be lowered, cavities cleaned out or the area below the inserted damp-proof course might be ‘tanked’ internally if deemed necessary.

NOTE: Should minor bridging of an effective damp-proof course occur, for example by moderately porous plasterwork, then it is highly unlikely that the dampness would continue to rise to its original height. If the damp-proof course is effective the pathway for moisture should be limited within the wall itself. Any porous plasterwork is at the surface where evaporation would serve to restrict the flow of rising water through it. Therefore, moisture is unlikely to pass through this relatively narrow pathway at a rate sufficient for it to reach the height of rise prior to the insertion of the damp-proof course assuming, of course, that the injected damp-proof course is moderately effective. Problems of this type together with defects in construction of the floor/wall junction usually manifest themselves at the base of the wall.


Unlike a physical damp-proof course these injected damp-proof courses do not form a ‘discrete impermeable plane’, but more of a ‘diffuse band’.

ViscfingWhen fluids are injected into a heterogeneous substrate such as brick/mortar they do not totally Viscous fingeringfill up the porous structures and neither do they completely push out the water in front of the advancing injection fluid as is so often claimed. Instead, the fluid tends to ‘finger’ within the substrate, a process known as ‘viscous fingering’ (figure , left). The fingers of the injected material form when the fluid takes the lines of least resistance such as the larger pores and cracks. Unfortunately, such pathways are not the most important elements in the conductance of water up the wall. Furthermore, the damper the substrate the greater this fingering is likely to be, especially with solvent based systems since these are not miscible with the resident moisture. Fingering is also increased by injection at high pressure. Reduction of the phenomenon is obtained by low pressure injection or, better still, by gravity diffusion of the dpc fluid.

The result of the damp-proofing fluids forming fingers give rise to non-impregnated ‘pools’ within the wall through which water can continue to rise. In the case of pressure injection damp-proof courses, this suggests that it is unlikely that the diffuse band of the damp-proofing agent will be totally complete. The resultant chemical damp-proof course may therefore not stop rising dampness by causing an immediate cut-off of rising water above the damp-proof course like that effected by a physical damp-proof course. Instead, a relatively rapid decline in the moisture gradient should occur above the inserted chemical damp-proof course due to the ‘control’ exerted. Thus, in practice, the rising ground water should be reduced to such a level that, in association with specialist replastering, it should no longer cause decorative spoiling or damage.
The efficacy of the water repellent damp-proofing systems can be affected where there are detergents (surfactants) impregnated into the wall by, for example, past leakage from sink waste pipes. A similar problem may occur when walls are sterilized against dry rot infection by biocide formulations containing surfactants.

rdfigThe overall effectiveness of a remedial damp-proof course can be investigated by examining the relationship between the distribution of free moisture (water due to rising dampness or rising damp figuresother source of active water ingress) and contaminant salts (chloride and nitrate). Where rising dampness is still active capillary moisture will be found to the full height of salts (figure, left). The absence of capillary moisture in the presence of salts arising from rising dampness indicates that drying back has occurred (Figure centre) and that the damp-proof course is effective. Intermediary stages are also found which demonstrate different degrees of control of the rising dampness.

If chloride and nitrate are not detected in a sampled profile it is possible that the ingress of moisture is due to recently developed rising dampness or more likely through rainwater penetration, condensation, plumbing defect or other sources.

When evaluating the efficacy of remedial damp-proof courses care must be taken not to misinterpret electrical moisture meter readings; high readings might not indicate that the damp-proof course itself has failed. They may reflect a number of possibilities including contaminated or inadequate plasterwork. Thus, an accurate assessment of the efficacy of a damp-proof course can only be undertaken by determining full moisture profiles linked with analysis for contaminant salts. It is also important to give consideration to the expected performance and limitations of the installed system as described above.

NOTE: It is identified in BS 6576:1985 that where timber suspended floors are encountered the damp-proof course must be injected, where possible, below joist level; this is to protect the embedded timbers from dampness and the risk of fungal decay. However, given the likely efficacy of injection systems the embedded joist ends could still remain in contact with damp masonry even if above the injected damp-proof course and may therefore remain at risk to fungal decay. It would be considered prudent that in all cases where a damp-proof course is installed in relation to a timber suspended floor, action is taken to protect any embedded timbers just above and certainly below the injected damp-proof course to prevent potential decay!


A long term rising damp complex brings with it certain soluble ground water salts into the wall; these are left behind as the water evaporates, and become concentrated at such sites. A proportion of these salts are hygroscopic, that is they are capable of absorbing water from the surrounding environment. As a result affected plasters and masonry may remain damp even though the source of moisture which lead to the build up of the salts has been eliminated.

It therefore stands to reason that any property which has been subject to a long term rising damp complex must have some degree of salt contamination in the plaster and the underlying masonry. These salts can, on their own, cause spoiling to certain types of decoration, even in relatively low quantities.

Following the insertion of a remedial damp-proof course a damp wall can take many months to dry out (Building Research Establishment Digest 163). Furthermore, due to the limitations of chemical damp-proof courses the wall is always likely to remain damp at the base (this is an important consideration when evaluating the efficacy of a remedial damp-proof course).

Where hygroscopic salt contamination is very heavy the wall may never dry out adequately due to continued moisture absorption from the surrounding environment. Under conditions of very high humidity some of these salts can become deliquescent, ie, they can absorb so much moisture that they become liquid. This in itself can lead to wet masonry.

Removal of the old contaminated decorations and plasterwork are essential because:

  • It removes the contaminated surface which could cause spoiling of any new decorations.
  • In order to prevent the new surfaces from becoming contaminated and damp the new plasterwork has a most important and specific function. This is to prevent the passage of residual moisture and contaminant salts from diffusing from the underlying wall to the new surface thus preventing future spoiling, AND there is the limitation in the performance of the damp-proof course itself which is likely to leave the lower part of a wall permanently damp.

These functions and the importance of the new plasterwork are described in Building Research Establishment Defect Action Sheet 86.

The importance of the replastering works cannot be over-emphasised. It must be considered as important as the injected damp-proof course, indeed, if not more important. Many disputes are centered upon whether a dampness problem is due to the failure of a remedial damp-proof course or inadequate new plasterwork. In such cases plasterwork should be examined as well as the efficacy of the damp-proof course since it is usually the more expensive of the two to put right.

In order to obtain the above design function it is essential to use either

  • A dense sand/cement mix, preferably incorporating a ‘waterproofer’ or ‘salt inhibitor’. The ‘integral waterproofer’ and ‘salt inhibitors’ specified to be incorporated in cement based internal renders both perform the same function–they are used to help restrict the passage of moisture to the decorative surface. Salts can only move or diffuse in solution therefore restriction of water flow also restricts the passage of salts.
  • Alternatively, one of the special premixed ‘renovating plasters’ designed for use in such conditions and which preferably carry an Agrement Certificate could be used.

Limited bridging by plasterwork (but not the gypsum finish) should not cause the complete failure of a damp-proof course (see above). It is certainly advisable to keep the new plasterwork cut well short of any solid floor; this reduces some of risks of spoiling which are greatest at the base of the wall for the reasons described above. Under no circumstances must lightweight premix gypsum based backing and bonding plasters or other highly porous plasters be used.

Note on historic/listed buildings:
Whilst the insertion of a remedial damp-proof course will control the rising dampness it is unlikely to stop it; nevertheless it will do no harm in that it will at least reduce the flow of moisture into the material (NB. Certain considerations however are necessary for injection damp-proofing cob construction {Trotman , P.M. “Dampness in Cob Walls” BRE, 1995}).

The main problem is where old lime plasters still remain. This may be badly stained, heavily salt contaminated and deteriorated. In this state it clearly shows that there has certainly been a problem, and replacing it with a similar material isn’t likely to do much better in the longer term! So to remove these material and apply a new lime plaster is highly likely to lead to similar problems – lime plasters are very permeable (although apparently no more vapour permeable than sand/cement mixes of the order of 1 : 6) and as such are highly likely to let through the dampness/colouration/salts already in the substrate. As such, spoiling may occur again in a relatively short time and lime plaster is a very expensive sacrificial coating! Also note that hygroscopic salt contamination alone can pass into new permeable material from the substrate without any ‘free’ water being present; this can occur when such salts become ‘deliquescent’ (ie, they become liquid and therefore mobile) under conditions of high humidities.

There is no problem with the above provided that the owner is prepared to accept that this may occur, but as stated above, this will make lime plastering a very expensive ‘sacrificial’ material, ie, as it spoils it will have to be periodically replaced if the owner requires a clean decorative surface. There is always an argument that lime plastering should be used but if it allowed the walls to ‘breath’(?) then there shouldn’t have been a problem in the first place!

There may also be a problem with the use of the stronger sand/cement mixes as described above; to meet their required design functions mixes of the type necessary may be far too strong for the background. It may be possible to use expanded metal lath to aid bonding but perhaps the best practice, should it be acceptable, would be to use a dry lining technique to provide a non-spoiling decorative surface. However, it may be possible to use one of the specialist ‘renovating’ plasters which are usually less ‘strong’ than the traditional dense sand/cement mixes (see below).

The answer to damp-roofing and replastering such properties is clearly with the owner – what are they expecting and what are they willing to accept. If they expect clean non-spoiling decorative surface then some kind of action with reference to replastering/finishing will need to be taken, but if they are quite happy with some degree of staining/spoiling then consideration can be given to leaving the old material but being aware what has actually caused it to deteriorate in the first place. BUT REMEMBER – make sure no wood or other biodegradable material is left in contact with any dampness – it will be at a high risk to rot developing!!


Replastering must be carried out to a height in excess of the maximum rise of the dampness and the salt contamination. Dampness can frequently rise in excess of 1 metre, the height being governed by numerous factors including pore structure and rates of evaporation. For example, restriction of evaporative processes causes dampness to rise higher than if the wall surface was well ventilated. This is well illustrated in thick walls where moisture tends to rise higher than in thinner walls due to the lower surface area to volume ratio.

Sometimes, where a remedial damp-proof course is ineffective the moisture can rise above the new plasterwork as the result of its low permeability retarding evaporation of water from the underlying masonry. This tends to ‘drive’ the active rising dampness higher. Similarly, new plasterwork may not have been removed to sufficient height so leaving the old salt contaminated plasterwork above. Both cases may give a similar pattern of readings on an electrical moisture meter, ie, very high readings just above the new plasterwork line, but analysis for moisture and salt distributions may be necessary to properly distinguish between the two causes of the problem. Where the problems only occur above the new plaster line it does demonstrate the efficacy of good plasterwork in performing its required design functions.


Perhaps the most common defect encountered in replastering is the use of weak sand/cement mixes. Building Research Establishment Defect Action Sheet No. 86 identifies that where cement/sand mixes are used these should be 1 : 3 cement to sand, or alternatively, use a specialist premixed render designed for the purpose; these latter materials are especially useful on ‘weak’ backgrounds.

The use of much weaker mixes, ie, often weaker than 1 : 6 cement to sand or when lime has been added (eg, 1 : 1 : 6 cement to lime to sand), are more likely to lead to more porous plasterwork which are unlikely to achieve their required design function. Lime/sand mixes will also suffer from the same problem (see above).

SandgradThe use of poorly graded sand containing a high proportion of fines, especially in cement weak mixes, also exacerbates problems. The figure below shows the result of grading 2 sands to BS882:1992: the lower sand contains far to much ‘fines’ and would be unsuitable for use following damp-proofing works. Insufficient thickness of plasterwork can also add tosand grades the apparent failure in the required design function. In the above cases the cement weak mixes will not prevent the diffusion of salts and residual moisture from the underlying masonry which can potentially damage the new decorative surface.

Porous cement/sand mixes may also become contaminated with soluble sulphate diffusing from the underlying masonry. While still alkaline sulphate attack can occur which causes serious disintegration of the cement render. Similar damage also occurs where cement renders have been applied over gypsum (calcium sulphate) plasters or where a proportion of gypsum plaster has been added to a cement mix to obtain a rapid set. In the latter case small flakes of exfoliated mica (vermiculite), part of some lightweight gypsum plasters, can sometimes be observed on close examination of a sample so identifying the probable addition of such lightweight gypsum materials. Pieces of grey or pink gypsum plaster might also be seen in the mix if a sample is closely examined. Where gypsum plaster has been used to fix metal angles, severe expansion and disintegration may occur to overlying/adjacent cement render; this, again, is caused by sulphate attack.

A common problem associated with building practice is taking new plasterwork, including the gypsum finishing coat, behind and below the damp-proof membrane and floor screed. This occurs when replastering has been completed before laying of a new solid floor. Frequently, the edge of the damp-proof membrane is cut very short or rolled under during the laying of the floor. This not only fails to comply with the recommendations described in BS CP102:1973 but also serves to cause other problems. Such cases usually result in the dampness being restricted close to floor level or just above the skirtings and also around the perimeter of the solid floor.

Finally, in situations where light coloured wallpapers have been used, especially relatively impervious papers, diffuse dark areas can appear. On examination, these are shown to be caused by black mould growth on the back of the wallpaper. The cause is due moisture in the wall leading to high humidities/dampness behind the paper so leading to mould growth. It is sometimes encountered following damp-proofing works but in can appear in almost any situation where moisture/high humidities are present in the underlying masonry. Care must therefore be taken in selecting new decorative finishes; initial decoration should be regarded to be of a temporary nature whilst the drying processes take place.


The importance of the replastering works associated with the insertion of chemical damp-proof courses cannot be over-emphasised. Chemical damp-proof coursing must be regarded as an integrated system, the damp-proof course and the replastering. The chemical damp-proof course will control the rising dampness and the new plasterwork will complete the system by preventing residual moisture, especially at the base of the wall, and contaminant salts in the underlying masonry from passing to the new decorative surface. Where one is dealing with the considerations of historic/listed buildings there are clearly other factors to be taken into account in relation to finishes (see above).

Because chemical damp-proofing is a system it is prudent to avoid problems of ‘split responsibility’ – that is where the installation of the damp-proof course is undertaken by one contractor and the replastering is carried out by another. The problem of split responsibility is the most common cause of system failures and the associated disputes which develop, ie, the plasterer blames the damp-proofing installer for any developing problem and the damp-proofer holds the plasterer responsible. The simple answer is to avoid ‘split responsibility’, so frequently the origin of expensive problems and disputes. This is easily achieved by allowing the specialist installer to undertake both the damp-proof course installation and the replastering. Experience has shown that this approach significantly reduces problems; it also has the advantage to the client of only one single chain of responsibility.

(damp and timber article)

Electrical moisture meters Vs. Carbide moisture meters

Electrical moisture meters -v- carbide moisture meters

By Graham Coleman

In the recent press there has been considerable ‘sniping’ against the use of electrical moisture meters for diagnosing dampness in masonry substrates. The snipers rightly point out that the meters do not measure dampness but show changes in electrical resistance, capacitance or radio wave reflection/absorption in the substrate, depending on the type of electrical moisture meter in use.

They also tell us that certain salts and electrically conducting materials including foil-backed paper will give erroneous and misleading results: all the above, of course, are true. On the other hand the Carbide meter, they tell us, will give a true quantitative measure of moisture in the substrate, and that it is a recommended method of moisture measurement described in the BRE Digest 245.


But let’s have a closer look at these two fundamental methods of diagnosing dampness/damp related problems more closely. Please note that I use the word ‘diagnosing’ carefully and specifically because diagnosis is the identification of a problem, and this should be definitive. Once diagnosed, however, there may always be arguments over what treatment is necessary, if any at all.


Electrical moisture meters:

Depending on the type of meter in use electrical moisture meters measure changes in the electrical resistance, capacitance, etc, in the substrate. In over 99% of cases this change is brought about by water and/or certain salts. Clean uncontaminated substrates that are electrically conducting are rare, and foil-backed paper is usually readily identified and of course if present it is usually because of dampness problems in the first place.

As far as salts are concerned, efflorescent salts alone do not cause electrical moisture meters to respond; hygroscopic salts do! Again in over a 99% of cases hygroscopic salts in building materials arise through long term rising damp and perhaps around chimney flues from burning of fossil fuels; in a few cases they may arise from other sources. Basically, clean and dry mortars, plasters, brick, etc, do not cause electrical moisture meters to respond. However, very low levels of free moisture in some materials e.g., 0.1-0.2% free water in permeable lime plaster for instance, can cause very high meter readings as can relatively low levels of contaminant hygroscopic salts. But neither free moisture or the salts should be present – they must have come from somewhere (remember, we are only talking diagnosis – not treatment). The question is from where have the free water/salts originated?

Careful and proper use of the electrical moisture meter can guide us, and experience has shown that over 95% accuracy can be obtain in diagnosis using an electrical moisture meter properly and with common sense.


Electrical moisture meters
protism Unfortunately, the majority of surveyors do not appear to understand how an electrical moisture meter should be used, and the manufacturers of the meters do little to educate the user either. With most surveyors the meter is simply stuck into the surface of the wall just above the skirting, the indicator goes into the red and, hey presto, rising dampness!That is certainly not the way to use electrical moisture meter and using it like this will almost certainly lead to a high number of wrong diagnoses! Similarly, it is not a valid exercise to directly compare readings from one material with that of another in respect of moisture content.


In most cases only surface readings are taken, and lets face it, the surface is of most interest since it is here that the problem becomes manifest. Furthermore, with one major exception, condensation, what is happening at the surface usually reflects what’s going on deeper in the substrate. So surface readings alone should generally not be a problem in the warmer months but condensation may be a factor during the winter: this will have to be eliminated as an interference factor during this time. Fortunately, in most cases, surface condensation is very much limited to the surface so by simply scraping away the outer 2mm of perhaps a plaster finish, the probes of a resistance type meter can be put beneath the surface for a quick check. Clearly, if condensation is a problem then a different approach will be required to examine the wall – but the condensation may be the actual problem in the first place.

The correct method for using an electrical moisture meter is to take a vertical series of readings in order to obtain a vertical pattern of readings – this is how an electrical moisture meter should be used. It is the pattern of readings which gives us guidance on the diagnosis. And this is where skill and expertise of the investigator are required in the interpretation of these patterns. Moisture meter ‘profiles’ with their possible interpretations have been published, and should be a part of every damp investigators background knowledge. The isolated individual readings obtained during most surveys are of little value and will certainly increase the chances of misdiagnosis! Such patterns of meter readings can be obtained very quickly and therefore a number of areas can be rapidly evaluated.


The Carbide meter:

The Carbide meter is held up by many who own them as the definitive instrument in diagnosing dampness problems. After all, they give a true quantitative result of moisture content. We are also told they are recommended in the BRE Digest 245 – more of this later.

The Carbide meter has one enormous disadvantage as a preliminary survey tool – it is a destructive method of moisture determination, i.e., it requires samples to be removed. This is hardly likely to be acceptable to a vendor as part of a pre-purchase survey!

The carbide meter


When used as an on-site instrument (as they mostly are), the investigator is usually looking for moisture contents of less than 5%. Why? Because BRE Digest 245 allegedly says that moisture contents up to 5% are acceptable. Actually, the Digest doesn’t state this at all! It clearly states, “If the found moisture content (MC) is less than 5 per cent at the base, it is unlikely that the wall is severely affected by rising damp” and that “Although only a rough indicator, the five per cent threshold does represent a reasonable general guide to whether or not some kind of remedial treatment is needed. This emphasises the importance of the difference between the HMC and MC measured on samples.” This clearly raises four points: (1) the guidance figure of 5% refers to the base of the wall, not 150mm or above where most samples appear to be removed! (2) these statements have nothing to do with diagnosis – they refer effectively to treatment regimes, (3) you cannot distinguish between MC and HMC on site! and (4) nowhere in the Digest does it describe or suggest the on-site use of the instrument! Unfortunately, most seem to ignore the above items, or misquote or misinterpret them.

The Carbide meter also responds to hygroscopic salt contaminated material, i.e., hygroscopic moisture; this adds to the total moisture content of the sample. When used on site the meter cannot distinguish between a free and hygroscopic moisture, and heavily salt contaminated materials can have it moisture contents well in excess of 5% without free moisture. Indeed, there is one a classic case where rising damp had been effectively eliminated but on-site use of a Carbide meter persistently recorded moisture contents of 10-12%. After 4 damp-proof courses had been installed it was subsequently shown that all this moisture was due to hygroscopic salt contamination, not a continued failure of the damp-proof course! This is precisely why the Digest emphasises ” – the importance of the difference between the HMC and MC measured on samples.

Perhaps one the most fundamental flaws in the use of the Carbide meter is the attempt to compare moisture contents of two or more totally different materials, and from this try to diagnose the origin of the dampness. Different materials have differing saturation moisture contents and so direct comparisons cannot be made between different materials. This again has led to misdiagnosis in interpreting on-site results.

Thus, as an on-site diagnostic tool the Carbide meter has a number of disadvantages, and as an on-site diagnostic tool its results are frequently misinterpreted and can lead to more errors (not treatment practices) than a surveyor competently using an electrical moisture meter. In effect the meter is subject to the same type of interpretation problems levied at the electrical moisture meter. There is also a distinct shortcoming in the education of the operator as to the limitations of the meter as well as a failure to appreciate the contents of BRE Digest 245, a document which has been so widely misinterpreted and misquoted. Furthermore, as on on-site tool it is very slow to use, a single sample taking in excess of 5 minutes to get a result. Hence the reason why most of those using such meters never take more than one or two samples and certainly not enough for use in diagnostics.

However, where the Carbide meter comes into its own is using it as described in the BRE Digest, that is to determine total moisture content (MC), hygroscopic moisture content (HMC) and hence a free moisture content in a vertical series of samples. Effectively, it is an off-site instrument taking the place of the laboratory method described in the Digest: it now becomes the true diagnostic tool as it was intended to be. Like the laboratory method also described in the Digest it is a destructive method of full moisture analysis and is thus best suited to investigative work. It requires a vertical series of samples to be removed from site and analysed for the total, hygroscopic and hence free moisture contents; the method is a fully described in the Digest.


The reality:

As general on site diagnostic tools electrical moisture meters are far more suitable provided they are used correctly with the necessary skill and expertise, together with a full understanding of their limitations. They are clean and non-destructive and will not effectively damage the property, a factor most important in a pre-purchase survey.

Carbide meters as an on-site diagnostic tool, even when used with skill and expertise, do not match the advantage of skilled use of an electrical moisture meter. Carbide meters are destructive, and it is still very difficult to interpret the results and certainly their site use has resulted in some catastrophic and expensive misdiagnosis. Whilst their ability in on-site diagnostics is certainly questionable, they can be very useful as a quick guide as to whether treatment may be required once diagnosis has been correctly made.

But the real advantage of the Carbide meter is off-site, where it can be used in place of laboratory facilities; it now becomes a very powerful diagnostic tool provided it is used fully along the guidelines as described in the BRE Digest for full moisture analyses. Unfortunately, most using the instrument do not follow these guidelines.



The two basic types of meter play totally different roles: electrical moisture meters are by far the best general survey tool for diagnosing damp problems provided the investigator is skilled in interpreting the results of the meter patterns and understands the instruments’ limitations. The Carbide meter on the other hand is far more of an in-depth investigative diagnostic tool for use off-site to determine full moisture profiles which is how it was designed to be used; this is clearly identified in the BRE Digest. As an on-site diagnostic instrument the Carbide meter is surpassed by the advantages of the electrical moisture meter, although it may be helpful in giving guidance on the level of treatment required, if that is treatment is required at all.
(damp and timber article)



Tim Hutton

Mould growth can occur in carpet and underlay in both bathrooms and showers with inadequate ventilation. This results in moisture-laden air pulsing into adjacent areas and provides the conditions for condensation, mould growth and damage to finishes.

Despite the best efforts of the ‘damp-proofing industry’ and the proliferation of ‘waterproof’ products in the second-half of the 20th century, it is not possible to keep water out of buildings. Our grandfathers knew this and relied instead on drainage details and breathable materials, so as to allow any water entering the structure to dissipate. Failure to maintain these systems or the inappropriate introduction of ‘waterproof’ materials or ‘damp-proofing systems’ will result in the build-up of moisture and damp problems in both historic and new buildings. This is well illustrated by considering the phenomena of condensation in buildings.

Strictly speaking, ‘condensation’ describes the physical process by which substances change from a gas or a vapour to a liquid phase, usually as a result of a drop in temperature. However, the term is commonly used to describe the process when moisture in the air condenses out to form liquid water as fine droplets in the air, or on a relatively colder material. Common examples of the former in the natural environment are the formation of clouds when warmer moisture-laden air mixes with colder air above, and fog, where this occurs at ground level. Similarly mist forms when warm moisture laden air is cooled by heat loss over night. Examples of the latter include the misting up of car windows when the warm moisture-laden air within cools on the surface of the window screen, and the misting on the surface of a mirror when held in the moist air exhaled from the mouth. This occurs because reducing the temperature of the gases that make up air reduces the energy available to keep the molecules whizzing around randomly within the available space, and lets a proportion of the molecules settle down into a less mobile liquid phase, in which the motion is more limited. Conversely, molecules in the liquid phase may pick-up enough energy to leave the liquid and ‘evaporate’ off to join the other gas molecules randomly moving around the available space once more. In fact, at any time molecules will be ‘condensing’ and ‘evaporating’ from any liquid water. The more active and energetic the molecules are, the greater ‘pressure’ they exert. This is described as ‘partial vapour pressure’. If the energy and hence the partial vapour pressure of the molecule in the liquid is higher than those in the air, then there will be a net movement of water into the air resulting from net evaporation or drying. Conversely, if the temperature and hence partial vapour pressure of the water molecules in the air is higher than that in the liquid or other adjacent material, there will be net condensation.


An exhaust duct from an extractor fan in a shower unit installed in an 18th century country house, is shown discharging into the roof void and providing the conditions for condensation, mould growth and insect decay in roof timbers. It also shows an uninsulated cold water tank which resulted in cold bridge condensation of moisture laden air.

At any given temperature and pressure there is a limit to the amount of water molecules that a given volume of air can hold. From the above it will be appreciated that this will rise and fall with the temperature. When a given volume of air contains the maximum amount of water possible at any given temperature it is described as ‘saturated’, and the moisture content of air at any given temperature is often described as a percentage of the maximum amount of water it could hold if saturated at that temperature. This is the ‘relative humidity’ (RH) percentage. Conversely, it will be appreciated for any given amount of moisture in a given volume of air, there will be a temperature at which the air would be ‘saturated’ and that any further drop in temperature could result in net condensation. This is called the ‘dew point’ temperature. Although the relative humidity of air is often measured and discussed, it will be appreciated from the above that it is not really a useful figure unless the temperature is also considered. Because the factors affecting condensation are so complex, specialists concerned with moisture movement will refer to tables or psychrometric charts’ to determine the relationship between the moisture content of air, the temperature, the partial vapour pressure, the dew point and the specific ‘enthalpy’ – the latter may be considered as representing the energy available within the system.


This can all seem very confusing even for specialists with a scientific background. However, when considering moisture movement and condensation in buildings, there are a few simple rules of thumb that are adequate for most practical purposes. Firstly, moisture can generally be thought of as moving from relatively wet to relatively dry areas or structures, and from relatively warm to relatively cold areas in buildings. Secondly, it is generally only necessary to determine and consider the dew point of the air, and the probable temperature of the fabric, in order to identify where condensation may occur. It should also be remembered that this is a dynamic process with continuous fluctuations in the temperatures and moisture content of air as a result of annual and diurnal changes, as well as the result of local heating and ventilation. Because of these factors it would be necessary to measure the temperature and moisture content of air over time and in a large number of representative locations to determine if net condensation was occurring. This is why the common practice of referring to individual RH percentage readings is often confusing and counter-productive; and why in most cases it is better to focus on possible moisture sources, and to look for evidence of moisture accumulating in vulnerable materials or on vulnerable surfaces, when investigating possible condensation-related problems.

This mould growth on contaminated wallpaper represents a potential environmental health hazard to those occupying the building.
At the Monument in the City of London we see corrosion of railings and spalling of the staircase and spiral stair, due to water draining down the inside of the tower and staircase as a result of warm front condensation.
Decay to window frame is evident from condensation on the glazed surface, resulting in water penetration to timber elements which are prevented from drying due to the application of relatively impermeable gloss paint finishes. This had resulted in wet rot decay to the timbers. Original breathable paint finishes had previously allowed drying, preventing decay in the past.


The air in occupied buildings will always contain moisture. This is because we are all mostly made up of water, and add water to the environment at every breath. Occupancy will also introduce water into the built environment with activities such as bathing, washing and cooking. In modern and refurbished buildings the installation of shower units, Jacuzzis, swimming pools and saunas in particular can add significant quantities of water to the internal air. Moisture will also enter the air within structures due to the evaporation of water penetrating from the exterior. This occurs mostly from ground and surface drainage via the foundations, and through walls and roofs due to defective roof drainage. Moisture-laden air may also enter the structure from the exterior when it is warm and wet outside relative to the interior environment. Any sources of moisture into the internal environment may result in moisture-laden air being cooled to below its dew point at relatively cool surfaces or within relatively cool materials within the building structures; resulting in net condensation and the accumulation of liquid water causing localised damp conditions. This localised accumulation of moisture as a result of condensation can result in a number of damp-related problems in buildings, including the decay or damage of building materials or contents, and affecting the health and comfort of occupants.

Cold-bridge condensation occurs when relatively warm moisture laden air comes into contact with surfaces, at or below its dew point, which are relatively cold as a result of locally reduced insulation values between the warm air and a relatively cold area. Typical examples of this process are condensation at the base of external walls, where it may be confused with rising damp, condensation on window panes where it often results in accelerated decay to the lower parts of window frames, and condensation to the undersides of roof surfaces. The latter may result in accelerated corrosion of lead roof surfaces. Liquid water penetration into structures will usually degrade their insulating properties and may therefore form a ‘cold bridge‘, resulting in further condensation. Because of this it is not unusual to find water penetration at the base of walls or through roofs also causing local condensation. Cold bridge condensation can also occur on relatively cold internal structures, such as inadequately insulated cold water tanks or refrigeration units.

Warm front condensation occurs when relatively warm moisture-laden air from the exterior enters into a relatively cold building, following a change in weather from cold to warm. This usually occurs in the UK with a ‘warm front’ arriving from the Atlantic from November through to February, and can result in water running down the interior walls of massive masonry structures under reduced occupancy, especially in the towers of churches or castles, and in subterranean structures.

Interstitial condensation occurs when relatively warm moisture-laden air diffuses into a vapour-permeable material or structure such as fibrous insulation or a porous brick wall. If it is relatively warm on one side and below the dew point temperature on the other; this can result in the moisture-laden air reaching ‘dew point’ within the material and depositing liquid water at this point. This becomes a particular problem if the diffusion of the moisture vapour through the material is restricted towards the cold side of the structure and if the insulation or thermal conductivity of the structure is such that the temperature profile is skewed towards the relatively warm side. The risk of condensation in these circumstances can be calculated using graphs and formulae, or using specialist computer programmes, and it can become a particular problem in heavily insulated or air-conditioned buildings. This is especially important when dealing with the conservation of buildings in extreme environments such as the conservation of buildings in the tropics, which tend to be airconditioned on refurbishment. In this situation, interstitial condensation can be a significant problem; and it is necessary to turn the usual calculations back to front, as conditions will be warm and wet on the outside and cold and dry on the inside of the structure. Similarly, extreme conditions can occur in very cold environments, and when refrigeration units are introduced, without adequate ‘vapour checks’ or insulation.


Historically these problems have been controlled by ensuring that moisture laden air can exit to the exterior, and by controlling the effect of fluctuating air temperature by a continuous low level of structural heating. For example, historically buildings had relatively gappy structures and through ventilation was ensured, particularly in cellars and roof voids. Buildings were also ventilated by the passive stack effect via chimneys and staircases. Historically materials used in buildings such as thatch, lime plaster and traditional paints, were microporous or permeable allowing the movement of moisture vapour and drying. Low level structural and radiant heating was also provided in the past by the use of fires or stoves situated in massive chimney breasts and walls, or lately by the installation of massive hot water low level central heating systems, or in classical times by the hypocaust. More recently in new buildings reliance has been placed on insulation and ‘vapour barriers’. Unfortunately, these are generally imperfect, especially when retrofitted to existing structures, resulting in localised cold bridging or interstitial condensation. From the above it will be noted that the key factors in controlling condensation are ventilation, heating and insulation. These are the factors that usually require modification when dealing with an apparent problem with condensation in an existing building, often because they have been compromised by a previous refurbishment or change in occupancy.


Defects introduced in the refurbishment of older buildings or during the construction of new extensions may cause damp problems as a result of condensation. Some common examples are listed below:

  • the sealing of gaps around windows without provision of appropriate supplementary ‘trickle’ ventilation
  • the introduction of showers, jacuzzis, saunas or swimming pools with insufficient provision of extractor fans or passive stack ventilation
  • the installation of laundry units without proper installation of exhaust vents to the exterior
  • the installation of broken or crushed ducts from extractor fans in showers or bathrooms
  • the installation of extractor fan ducts exhausting into building voids such as roof spaces, rather than to the exterior
  • the failure to provide adequate ‘makeup’ air or trickle ventilation into areas fitted with extractor fans, to allow proper through ventilation
  • the blocking of existing flues and chimneys preventing passive stack ventilation
  • the blocking of existing vents or plenums designed to vent air to the exterior, in particular through the ceilings and roofs over function rooms, or at the skylights over staircases
  • the installation of intermittent heating, especially hot air heating systems, allowing warm moist air to ‘pulse’ into unheated areas under reduced occupancy – for example, in churches in reduced or intermittent use
  • the inadequate provision of low level structural heating to massive structures under reduced occupancy, such as churches or castles, allowing cold front condensation.
  • the provision of inadequate through ventilation to rooms under reduced occupancy
  • the introduction of security locked windows with no provision for locking in a partially opened position
  • the sealing of roof voids by the installation of insulation or sarking felts, preventing adequate through-ventilation
  • the sealing of floor voids by the blocking of airbricks, and the installation of fitted carpets or other impermeable floor coverings
  • the blocking of windows, hatches or other vents to cellar or basement areas, preventing adequate through-ventilation
  • the introduction of defective insulation and ‘vapour barriers’ or ‘vapour checks’; especially in extremely hot or cold environments, or around cold structures within buildings, such as cold water tanks or refrigeration systems.


This sensor probe is designed to monitor temperature and moisture profiles through a wall to identify and resolve problems of interstitial condensation as part of an H+R Curator building monitoring system.

From the above it can be seen that in most cases the appropriate remedial measures to control condensation are often directly related to correcting defects previously introduced. This should be done as far as possible by putting the building back to the way it was originally designed, with the use of original materials and detailing. This is not always possible given the changes in use and modern styles of occupancy. However, in nearly all cases problems can be reduced by reference to the recommendations for new buildings found in Building Regulations and the associated British Standards. These are especially useful in specifying remedial works and in providing ‘comfort’ to organisations such as building guarantors, mortgage lenders and official bodies such as Building Control. In these circumstances it is often cost effective to seek advise from an independent specialist.


As the temperature of moisture-laden air approaches dew point and the relative humidity rises, a number of moisture associated problems can become apparent, even before condensation occurs. In particular, superficial and interstitial mould growth can occur, especially on surfaces or in materials contaminated by dust or other organic materials. This typically occurs in poorly ventilated areas such as behind furniture and pictures, behind the glazing of pictures, in soft furnishings or carpets, and in poorly ventilated cupboards or corners of rooms, both at ceiling and floor level. This can cause serious damage to decorative and historically important finishes, as well as representing a significant health hazard, especially to sensitive individuals. Interstitial mould growth in contaminated carpets, soft furnishings or insulation materials due to this raised moisture content and poor ventilation is a particular health hazard, and appropriate respiratory protection should be used in affected areas. Hygroscopic salts can also cause significant damage as moisture from the air is absorbed and evaporated from affected plaster, masonry or brickwork, with fluctuating temperature and air moisture contents above dew point. This can cause damage to decorative stonework and plaster finishes, even at relative humidities fluctuating around 75 per cent or less. Although these problems are not caused by condensation, in the way the term is usually used; they can be understood and managed using methodologies similar to those discussed above.


In recent years it has become more common to try and control condensation in buildings using dehumidifiers. These may be useful in unoccupied buildings where environmental control can be achieved, such as in storerooms or museums, where it is the contents rather than the structure that is thought to be at risk. However, the use of dehumidifiers in occupied historic buildings is rarely cost-effective. This is because of the difficulty of effectively controlling air movement, and the high management and maintenance input required to ensure the efficient operation of the dehumidifiers themselves. It is common to find dehumidifiers installed in such circumstances vainly trying to dehumidify the entire external environment of the United Kingdom, or merrily extracting water from the air which is then allowed to recycle into the environment that is being attempted to be controlled. Efficient and cost effective use of dehumidifiers requires a high level of technical input, long-term monitoring and control. This usually requires independent specialist specification and supervision. Where possible it is therefore generally better to rely on the ‘fail safe’ and buffered systems inherent in the natural ventilation and structural heating of historic buildings.

In conclusion, historic buildings with their original materials and detailing should not generally suffer from problems resulting from condensation. Where there have been changes in occupancy, and where new materials or new extensions have been added, problems resulting from condensation can usually be easily and cost-effectively controlled by an understanding of the control mechanisms inherent in the original structure. Remedial measures are then generally associated with passive ventilation and structural heating systems. Where new materials or structures are introduced or problems persist, the information contained in Building Regulations and British Standards can be used to solve most problems. In these cases an holistic investigation of the building structure, materials and occupancy should be carried out, and short- or long-term monitoring may be required to ensure that the most cost-effective remedial measures are undertaken. This will ensure the conservation of the maximum amount of original materials and detailing.

(damp and timber article)

Rising Damp

Rising Damp

Tim Hutton

Rising damp is widely misdiagnosed in existing buildings, based on the incorrect interpretation of visual evidence and the readings of moisture meters. Because of a highly successful sales campaign for over 30 years by specialist remedial contractors installing injected ‘chemical dampproof courses’, this misdiagnosis of rising damp has also become synonymous with a diagnosis of a lack of an ‘injected chemical damp-proof course’. Although this has been very good for business, it has often resulted in a waste of the clients’ money and resources; original plasters and finishes have been destroyed in the process of installation, and unnecessary damage has been caused to original structures by the drilling of irrigation holes. In addition, money that might have been spent on more cost-effective maintenance or repair works has been wasted.

Whilst injected chemical damp-proof courses may provide some protection for certain types of structure if properly specified, their general application is rarely the most cost-effective way of controlling damp problems in buildings, and may be wrongly specified and ineffective. In particular the more generally available water based products may only form an effective ‘hydrophobic band’ if applied to a dry wall after it has dried out. This can prevent their effective installation in damp walls.


Rising damp actually describes the movement of moisture upward through permeable building materials by capillary action. It becomes a problem if the moisture penetrates vulnerable materials or finishes, particularly in the occupied parts of a building. This moisture will dissolve soluble salts from the building materials such as calcium sulphate, and may also carry soluble salts from its source. If the moisture evaporates through a permeable surface, these salts will be left behind and form deposits on or within the evaporative surface. Where there is a large evaporative surface, salt crystals are deposited as a harmless flour-like dusting on the surface. If evaporation is restricted to localised areas such as defects in an impermeable paint finish, then salt deposition is concentrated, forming thick crystalline deposits with the appearance of small flowers; hence the term ‘efflorescence’. When evaporation occurs within the material, salts can be deposited within the pores. The expanding salt crystals in these locations may result in fractures forming in the material and spalling of the surface. This type of decay may be seen in porous brickwork or masonry.

When there has been a long-term problem with moisture penetration, evaporation at the edge of the damp area leads to a distinctive ‘tide mark’ as a result of salt deposition. Where this occurs at the base of a wall, the tide mark is often taken as a typical diagnostic feature of ‘rising damp’. However, these salt accumulations may remain even when the water penetration that originally caused them has long gone. Similarly, water penetration may have occurred from causes other than ‘rising damp’.

The most common source of moisture in the base of the walls of buildings is from defective ground and surface drainage. This is present to some degree in almost every building in the country, due to a combination of such factors as rising ground levels, the failure of ground drainage systems, and the increased use of concrete or finishes around buildings without consideration of drainage slopes.

The accumulation of ‘moisture reservoirs’ in the foundations may also arise as the result of chronic plumbing leaks or floods from catastrophic plumbing or drainage defects.

Damp conditions at the foot of walls may be greatly increased by condensation. This occurs when warm moisture-laden air cools to dew point (the temperature at which moisture condenses) against a cold surface. Such cold surfaces commonly occur when the insulation value of the external wall is reduced by water penetration, as described above. Intermittent occupancy with intermittent heating provides the conditions for condensation of further water on these cold damp surfaces, particularly in ground floor bedrooms. These phenomena are the main causes of damp in the base of walls rather than ‘rising damp’ alone.

Concentrations of hygroscopic salts, which are often found in masonry, can also absorb moisture from the air, especially at relative humilities above 75 per cent. In a room that is sometimes unoccupied, with fluctuating relative humidity levels, this can result in the regular appearance of salt blooms on the surface (‘cyclical efflorescence and deliquescence’), resulting in damage to vulnerable materials, and giving the appearance
of rising damp.

Damp masonry at the base of walls may lead to a number of problems:

  • The moisture content of the structure may rise to a level at which decay organisms may grow, or the materials themselves may be adversely affected. For example, timber skirting boards or built-in bonding timbers along the base of walls may become infected and decayed by dry rot, wet rot, weevils or woodworm.
  • In very damp conditions, the inorganic materials themselves may lose their structural strength. This occurs most spectacularly with walls made of cob (earth) soaked with water.
  • Damp conditions on the surface of walls, particularly in conjunction with condensation, allow the growth of moulds both on the surface and within porous or fibrous materials, such as wallpapers or carpets fitted against the base of the wall. Not only is this aesthetically unacceptable and damaging to finishes, but it can be a significant health hazard to occupants.
  • Where evaporation takes place, the deposition of soluble salts on the surface or within the pores of materials can cause aesthetic and structural damage.
Diagram of building showing moisture sources Diagram of building showing moisture reservoirs



As described above, ‘rising damp’ is only one of many mechanisms resulting in high moisture levels in the base of walls, and even when it is a significant factor, it is rarely the primary source of moisture. The management of problems due to high moisture levels requires the proper identification of the moisture source and the defect responsible, before the most cost-effective solution to the problem can be determined.

Damp and its effects may then be controlled by adopting one or more of the following measures:

  • The provision of suitable moisture sinks to dissipate the moisture at its source without causing problems to the structure or occupants, and the repair of any contributing defects acting as moisture sources, such as broken pipes.
  • The introduction of either physical barriers using damp-proof membranes or materials to form a ‘damp-proof course’ or hydrophobic (water-repellent) materials as in ‘chemical damp proof courses’.
  • The isolation of vulnerable materials such as timber and interior finishes from damp fabric.

Moisture Barriers

The control of moisture movement using either damp-proof or hydrophobic materials to create a relatively less permeable ‘moisture barrier’ is not necessarily a cost-effective option in controlling damp problems and may even be counter-productive. This is because use of relatively impermeable materials will restrict moisture movement and hence drying. As a result, moisture may be ‘locked’ into damp materials for many years causing chronic problems. Moisture may also be prevented from dissipating from permeable materials, resulting in the build-up of moisture or even damper conditions in localised areas. This may result in moisture moving into previously dry structures or evaporating from previously unaffected surfaces, causing further salt efflorescence. One reason why those injecting ‘chemical damp-proof courses’ generally insist on re-plastering treated masonry with a salt-proof and waterproof mixture, is to cover up these potential problems.

A relatively common example of the effect of inserting a damp-proof material into a structure is the appearance of fresh ‘rising damp’ in walls following the laying of a new concrete floor with a damp-proof membrane. This is most often done when a suspended floor structure is replaced by a solid floor, or when a breathable stone slab floor is lifted and re-laid. Before the alteration of the original floor, moisture would have been able to evaporate off a large surface, without affecting internal finishes. However, a new impermeable membrane allows the water to accumulate beneath, forcing it to the sides of the room and into the base of the walls. This causes damp and decay problems unless appropriate ventilation has been provided at the floor/ wall junction. These damp problems are then often used as justification for the injection of a moisture-barrier and the removal and replacement of plaster with remedial mixes. In fact, the more cost effective solution would have been to allow the floor structure to continue to breathe. This can be done with a suspended floor or by re-detailing the floor/wall junction in such a way as to allow moisture to dissipate, for example, with a vented skirting detail.

If it is decided that a moisture-barrier at the base of the wall is essential, the most reliable method is to introduce a physical barrier rather than a chemical one. This involves cutting in a layer of damp-proof material to form a barrier which is continuous with the damp-proof membrane under the floor. As the wall above this barrier will remain damp for some time, it is then necessary to isolate all vulnerable materials above as well as below the barrier, such as skirting boards, from the base of the wall with a damp-proof membrane or ventilated air gap.

However, a damp-proof barrier is always vulnerable to local failure and will tend to concentrate moisture and damp problems at these points. This is a general characteristic of all impermeable materials, including those used in tanking systems, which are generally found to fail at some point or at some time. This results in more ‘concentrated’ moisture at the points of failure, and hence more severe damp problems locally when they fail. Because of this, the more robust, fail-safe, and traditional building techniques rely on the use of permeable materials and ventilation details in order to dissipate moisture and prevent it coming into contact with vulnerable materials or interiors.

‘Chemical damp-proofing’ may provide a useful barrier to damp in the short to medium term, or at least a ‘nominal damp proof course’, where the walls are of uniform construction such as sound brickwork laid with strong cement mortar; especially if they are combined with a ventilated dry lining system or other building detail which allows moisture to dissipate. However, any gaps which are left, or which appear over time as the material deteriorates, may lead to an accelerated rate of decay.

This method is most unreliable where walls are of natural stone, because the injected hydrophobic material will follow the lines of least resistance and may not accumulate in sufficient quantities where it is needed. This is particularly true when the wall is made up of materials of different permeability, as is common in the thicker walls of older buildings where the bricks and mortar may be of variable consistency and the structure may include cavities, particularly when the wall consists of brickwork or masonry skins containing a rubble infill.

Surface drainage channel

A traditional ventilated and drained ‘dry area’ around the foot of the wall of a church, partially covered with stone slabs.



Surface Water Drainage

The most cost-effective way of preventing damp problems in buildings, including those resulting in damp masonry at the foot of walls, is to minimise moisture sources and provide adequate passive moisture sinks to dissipate any penetrative moisture so as to make the system fail-safe. This should start with the provision of adequate ground drainage around the building to minimise water penetration to the foundations, and the re-detailing of surface drainage so as to ensure surface water is drained clear of the foot of the walls.

It has become fashionable to specify ‘French drains’ to help with this process. However, these are often poorly specified and soon become ‘French ponds’ in UK conditions. This may be because the base of the drain has been inadequately levelled or drained to keep water out of the foundations and the gravel infill has become contaminated with soil and debris, preventing proper moisture drainage and evaporation from the foot of the wall.

In the UK, the more traditional and more effective detail is to use a ventilated and drained ‘dry area’ around the foot of the wall. These are commonly covered with York stone slabs in order to prevent debris accumulating in the drained dry area and to minimise maintenance.

Alternatively, a perforated plastic land drain can be laid to falls in a trench lined with geo-textile and back filled with ‘beach cobles’ or large diameter hard core. Proprietary external ‘drained cavity systems are also available.

Wall Construction

The use of impermeable finishes, such as sand/ cement renders, around the base of external walls is a common cause of damp problems. These prevent moisture evaporating from the foot of the wall, forcing it into the interiors. As with all impermeable materials, they eventually fail, generally due to cracking. This allows water to penetrate into the foot of the wall, but prevents drying. The use of more traditional breathable lime mortar renders, and the correct detailing of renders to shed water clear of the base of the wall and to prevent ‘bridging’ of any existing damp-proof course, would be the preferred solutions.

Cavity wall construction may provide a way of dissipating moisture and preventing it penetrating into the building, provided the cavity is through ventilated. This may be compromised by debris or the ill-advised injection of proprietary insulation foams. These defects may also bridge existing damp-proof courses, allowing water to penetrate to interior finishes. In some cases, the most cost effective solution is to reinstate a through-ventilated cavity.

Generally, failures in existing damp-proof courses are the result of bridging by inappropriate repairs and alterations, by raised ground levels or by localised damage due to structural movement or poor building work. If a damp-proof course is an original design detail to control moisture movement in the structure, it may be necessary to carry out local repairs. This is best done by ‘cutting in’ a new layer of damp-proof material locally rather than by the general injection of hydrophobic solutions into the masonry to create a ‘moisture movement restricting barrier’.


Traditional buildings built in damp or potentially damp sites commonly included through-ventilated sub-floor cavities, cellars or basements. These act as sumps to allow the evaporation and dissipation of moisture from the structure before it reaches occupied areas or vulnerable finishes. Indeed, in some parts of the country it is not uncommon to find streams running through the cellars or basements in old farmhouses. These were presumably retained as a source of water for domestic use. However, if the ventilation of a basement, cellar or sub-floor cavity has been restricted, moisture can build up and penetrate vulnerable structures. This can occur, for example, by earth and plants clogging air bricks or by the ill-advised application of relatively impermeable materials. The solution to these problems if they develop, is to re-establish ventilation, not to start applying further damp-proof materials.

Diagram showing typical ventilation and drying measures

As described earlier, the reinstatement of a through-ventilated suspended floor is generally preferable to its replacement with a concrete slab. The requirement for the continued dissipation of moisture does not preclude the use of basements and cellars as occupied areas, but means that walls should be kept ventilated and not sealed. This can be achieved by using through-ventilated dry lining systems rather than impermeable finishes or tanking materials, which would only force moisture into adjacent structures above or to the side. Traditionally, dry lining has been produced by the use of timber panelling spaced from the masonry with battens or the use of lath and plaster. In all cases, the cavity behind should be ventilated at the top and at the bottom to allow through-ventilation to dissipate moisture, as otherwise moisture will accumulate to cause damp and decay problems. This commonly happens when insulation material or debris is allowed to block the cavity behind lath and plaster or when impermeable paint layers accumulate over timber panelling. These defects are easily solved and the traditional ‘farmhouse’ technique of timber panelling to dado level can be an attractive and cost-effective solution to problems of damp penetration or condensation affecting the foot of masonry walls. Modern materials and techniques may be used to achieve the same end, and many products are available on the market to allow the cost-effective provision of through-ventilated dry lining systems, including specialist plasterboard systems and studded plastic membranes which can be used to form vertical damp proof course details behind the dry lining.


Even with the loss of traditional skills and the complexities introduced into building by new materials and new styles of occupancy, the conditions resulting in damp to the base of walls can easily be avoided with a little thought and scientific understanding. Indeed, new materials and techniques can often be used to advantage if their properties are analysed as potential environmental controls. In contrast, the misdiagnosis of rising damp and the general application of particular products and techniques without considering the consequences lead to the unnecessary waste of the increasingly limited budgets available for maintenance and refurbishment. A more rational approach to the diagnosis and treatment of damp problems in buildings is only good building practice, which independent surveyors and their scientific consultants should promote in the interest of sound building and public health.

(damp and timber article)

Timber Decay

Timber Decay

Dr Jagjit Singh

Sporophores of the dry rot fungus Serpula lacrymans affecting floor joists

Building materials are decayed by the effects of adverse environmental conditions and the extent of damage depends on both the materials and the conditions. Among the most vulnerable materials are timber, paint, textiles and paper. Timber remains one of the most useful in a world of diminishing resources and is a major component in most historic buildings. It has many positive structural and aesthetic properties as well as being an energy-efficient and renewable resource. However, timber provides specialised ecological niches and many organisms have evolved to use it as a food. The most common and destructive to timber are dry rot, wet rot, common furniture beetle, and death watch beetle.

Orthodox remedial treatments often entail the loss of irreplaceable decorative finishes, floors and ceilings. Furthermore, treatment of the infestations with insecticidal fungicidal chemicals is not only expensive, inconvenient, hazardous to the operatives and occupants but also environmentally unacceptable and usually unnecessary. Environmental control and preventative maintenance provide an alternative, less destructive solution, and remain the most widely used methods for preventing biological decay.


Biodeterioration of materials was defined by Hueck in 1968 as ‘any undesirable change in the properties of material of economic importance brought about by the activities of living organisms’. A wide range of materials are subject to microbiological deterioration, which are caused by a broad spectrum of micro-organisms.

Beetles responsible for the decay of timber principally include woodworm (Anobium punctatum), death watch beetle (Xestobium rufovillosum), powder post beetle (Lyctus spp), and house longhorn beetle (Hylotrapes bajulus). It is their larvae which cause most damage as they bore through the wood, feeding off it and causing damage to the structure and strength of the timber.

Decay fungi are capable of enzymatically degrading complex cellulosic materials, such as wood, into simple digestible products. The decay of wood cells by these fungi results in the loss of weight and strength of the wood. There are two main types of wood-rotting fungi found in buildings; wet rot and dry rot (see Table 2).

The principal environmental factors favouring the biodeterioration of building materials are temperature, humidity and a lack of ventilation. Moisture may be contributed by penetrating or rising damp; condensation; building defects and disasters such as leaks; and from construction moisture introduced in mortar, concrete and plaster for example.


Environmental control relies on controlling the cause of the problem by controlling the environment. It is designed to ensure the future health of the building and its occupants by avoiding the unnecessary use of potentially hazardous and environmentally damaging chemical pesticides where possible and their consequential legal and management complications. Eradication of dry rot spores or insect pests in an historic building and its contents is in practice, impossible. The volumes of chemicals necessary and the toxicity required would be damaging both for the buildings and all its occupants. Where chemical treatment cannot be avoided materials and techniques should be used which have minimum adverse environmental effect.

By reducing the need to expose and cut out infected material, environmental control also reduces damage to the fabric and the finishes of a building. Where an historic building is concerned, this is particularly important, and the specification should ensure maximum conservation of existing materials to maintain the historic integrity of the fabric, as well as to avoid unnecessary expenditure. Its success depends on a thorough investigation of cause and effect. Through a methodical approach such as this, it is possible to decrease the cost of remedial timber works significantly or in some cases eliminate it altogether.

First the building should be thoroughly inspected using non-destructive techniques to locate and identify all the significant decay organisms within it. In cases of actual or suspected problems of woodrot or wood boring insects in buildings, investigation should be by an independent specialist consultant, architect or surveyor to establish the cause and extent of the damp and timber decay, including the potential risk to the health of occupants before specification or remedial work. Correct identification of the fungi and insect material is important as not all fungi are equally destructive. Some rots are present in timber when it is cut or are acquired in storage. Fungal material may also be dead or dormant, the product of conditions now past.

Having identified the nature of decay, the environmental conditions which are required to support it should be considered (see Table 1). Only then will it be possible to devise a scheme to deal with the problem.

The aim of remedial building works is to control the timber decay, to prevent further decay and to correct any significant building defects resulting in conditions of high moisture content or poor ventilation of timber. In particular, it is important to reduce sub-surface moisture content of all timber to below 16-18 per cent. Timber should be isolated from damp masonry by air space or damp proof membrane, and free air movement should be allowed around timber in walls, roofs and suspended floors. All other sources of water should also be eliminated, such as overflowing gutters, leaking plumbing, condensation and rising or penetrating damp. Humidity in voids should not exceed an average relative humidity of 65 per cent. In addition, all active fungal material should be removed together with all rotten wood, and the structural strength of the remaining timber and fabric construction should be assessed to determine whether reinforcement or renewal is required. In the case of insect infestation, measures should also be introduced to avoid recontamination. Dirt, dust and builders’ rubbish provide a haven for insects and fungi. Voids and cavities should be cleared and the areas cleaned with a vacuum cleaner to remove dust. A programme of building maintenance and monitoring may then be instigated to prevent any future problems.





Environmental Factors


fungi (dry rot, wet rot, moulds and others) bacteria; actinomycetes; lichens, mosses and algae wood-boring insect larvae (woodworm, death watch beetle and others) carpet beetle, moths, book lice and silverfish termites

moisture and humidity
air movement
food source


acids, alkalis and solvents

remedial treatment


mechanical abrasion, general handling and others, decomposition by physical agents such as prolonged heating, fire and moisture

normal use, visitor wear
accidental damage
sunlight, heating, fire, damp

Radiation ultraviolet light exposure to sunlight

Moisture Conditions

Temperature Requirements


Minimum moisture content in timbers of about 20 per cent
Optimum growth occurs at 30-40 per cent
Spore germination requires wood moisture content of 30 per cent

The optimum temperature for dry rot growth in buildings is about 23°C, the maximum temperature for continued growth is about 25°C and the fungus is rapidly killed above 40°C

WET ROTS Wet rot fungi usually occur in persistently damp conditions needing an optimum moisture content of 50-60 per cent Wood-rotting fungi differ in their optimum temperature but for most the range is between 20-30°C
Dry rot:
 Serpula lacrymans
Wet rots: Cellar rot fungus (Coniophora puteana); Poria fungi, (eg Amyloporia Xantha; Fibroporia vaillantii and Poria placenta); Phellinus continguus; Donkioporia expansa; Oyster fungus (Pleurotus ostreatus);Asterostroma spp; Paxillus panuoides; Lentinus lepideus; Dacrymyces stillatus; Ptychogaster rubescens
Soft rot: Chaetomium globosum
Moulds: Cladosporium spp; Penicillium spp; Aspergillus spp; Trichoderma spp; Alternaria spp; Aureobasidium spp
Slime moulds: Myxomycetes
Plaster fungi: Coprinus spp; Peziza spp; Pyronema spp
Stain fungi: Cladosporium spp; Aureobasidium spp


Remote monitoring systems can be very useful in increasing the efficiency and decreasing the cost of maintenance programmes. They can be especially useful for checking the moisture content of inaccessible timbers in roof spaces, behind decorative finishes and in walls.

Sensors can be placed at all critical points after the investigation or after remedial building works. Areas can then be closed up and finishes reapplied; for example sensors may be placed in lintels, joist ends, valley gutter soles or in damp walls to monitor drying. It is important to use enough sensors and to place them with an understanding of the moisture distribution processes, because conditions can vary even in a small area. It is these local variations in conditions that produce the environmental niches which decay organisms exploit.

If more than 30 sensors are deployed, taking the readings can become onerous and this may result in human error or negligence. In these situations automatic monitoring systems become desirable and a number of specialised systems have been developed. With larger systems the wiring of sensors can also become a problem. For systems requiring 100 or more sensors, a computerised unit is used, working via a single four-core mains cable connecting up any number of nodes, each supporting four sensors. This system can be programmed to record and log data at regular intervals with alarm limits for each sensor. The data is then transmitted to a remote computer via a modem connected to a telephone line. Data from the system can then be analysed using CAD and programs for statistical interpretation.


The water content of building materials can be determined through a range of direct and indirect methods. Direct methods involve removal of a sample of the material to be tested which is weighed and then dried to determine its water content. This has the disadvantage of being destructive and it cannot be used for remote monitoring.

Indirect methods are based on measurement of characteristics related to the moisture level in the testing material. These involve thermal conductance, electrical capacitance and resistivity. Measurement of a surrogate material in equilibrium with the first material is another method. The use of electrical resistance moisture meters provides a quick and relatively accurate method of determining the moisture content of wood if a knowledge of their limitations is taken into account. Moisture meters measure the changes in resistance, due to changes in moisture content, between two electrodes placed in the timber. Increasing moisture content results in a reduction in electrical resistance.

Miniature sensors are fabricated from hygroscopic material which has been calibrated to match the moisture content changes in timber. They are encased in a protective shell. The sensor is then inserted into a previously drilled hole to the required depth and the hole sealed. In most instances the sensor cable seals the hole to the outside. The sensor will fairly rapidly come into equilibrium with the atmosphere within the hole. Due to their small size the sensors can be inserted into the centre or ends of large dimension timbers allowing the best chance of detecting defects early.


Systems for use with masonry can be based on the direct measurement of the material’s moisture content or by the use of a surrogate material which changes in moisture content in a similar way to the host material. This material may be of any hygroscopic type which, providing it has been calibrated correctly, can be used as the basis of a remote sensing system.

The sensors are placed in the material to be tested at the required depth, or in an array and the hole sealed to the external atmosphere. Sensors will come to equilibrium with the relative humidity within the cavity or drilled hole and hence with the surrounding material. Single sensors can be placed at varying depths but must be sealed within the area to be measured. A series of sensors individually sealed within the drilled hole can provide a profile of readings across the material. These can either be wired up and resistance measured remotely or can be removed, weighed and oven dried to calculate their water contents. Changes in the water content of masonry can be rapid when wetted so that it could provide an early warning of building defects leading to water penetration. However, drying down can take many weeks or years.

(damp and timber article)

How extensive is rising damp?

It is a frequent worry for home owners and purchasers, but is “rising damp” really as common
as we are led to believe?
“Householders and even some surveyors are too quick to assume that problems with
dampness are caused by rising damp. In fact, true rising damp is not very common. Because
the remedies for rising damp are so expensive it is doubly important to ensure the diagnosis is
correct before starting work.” Building Research Establishment (BRE) Good Repair
Guide 6 – Treating Rising Damp in Houses – January 1997
The BRE, which until recently was UK Government funded, have been highlighting that
dampness was being misdiagnosed since the early 1980’s!
“Because of the high cost of remedial work, it is essential that the diagnosis is as positive as
possible to distinguish between rising damp and other sources of damp.”
BRE Digest 245 – January 1981 “Rising Damp in Walls: Diagnosis and Treatment
“Investigations have revealed many instances in which systems intended to combat rising
damp have been installed in buildings where rising damp is not occurring. A frequent reason
for this has been a wrong interpretation of high readings obtained when using an electrical
moisture meter. Another reason was the failure to recognize other causes of the damp
conditions.” Building Research Advisory Service, Technical Information Leaflet TIL 47
August 1982
“The diagnosis of rising damp needs careful and systematic thought because it can easily be
confused with penetrating dampness and condensation. The Building Research Establishment
(BRE) have suggested that only 10% of the dampness problems it investigates are attributable
to rising damp. Unfortunately, there are a number of companies specializing in d.p.c.
replacement who obviously have a commercial interest in finding problems with rising damp.
The diagnosis needs to be treated with caution. Although there are several reputable
companies working in this field, it may be wise to seek independent advice. Further
“encouragement” to find problems of rising damp is provided by banks and building societies
who often request a damp report as a condition of a mortgage advance. Understanding
Housing Defects (Estates Gazette) 1998
There are many contractors advertising specialist services to remedy dampness by installing
damp proof courses. Yet most apparent rising dampness cannot be attributed to the absence
or failure of a damp proof course.” The Remedial Treatment of Buildings by Barry
Richardson 1995

“Dampness of one sort or another is the most common problem in housing. It results in
visible wetting of walls, ceilings and floors, blistering paint, bulging plaster, mould on the
surfaces and fabrics and sulphate attack on brickwork It can also lead to less obvious
problems – thermal insulation is reduced in effectiveness or brickwork because metal
components imbedded in it have corroded. As with all repair work, the first step to solving
any damp related problem is to diagnose the cause correctly.” B.R.E. Good Repair Guide 5
– Diagnosing the Causes of Dampness, January 1997

“Before any measures are undertaken, the problem should be analyzed in order to identify the
cause properly. In the first instance professional advice should be obtained rather than that of
a specialist contractor.” The Repair of Historic Buildings (English Heritage) by Christopher Brereton

Often specialist remedial treatment companies report “they have diagnosed rising damp” and
specify remedial treatment which is possibly inappropriate, to be carried out by themselves. If
a Specialist Contractor is to be used they should be a member of the Property Care
Association who are more likely to provide an accurate assessment. However, an Independent
Surveyor with specialist knowledge of Historic Buildings will look at the property holistically
using his/her residential survey experience to provide the broader picture without the
potential influence of profit on the opinion given.
Building Research Digest 245 recommends that samples of brickwork are taken from within
the wall and laboratory analysis undertaken to determine the actual amount of capillary
moisture which is present. This test is invasive as holes are drilled (10mm diameter) in walls
to obtain plaster and brick samples but it is a lot less destructive than having plaster chopped
off to a height of approximately 1 metre all round the house in order to install a chemical
d.p.c. when the treatment is not necessary. After collecting samples we can make good walls
and the cost of accurate diagnosis usually results in avoiding unnecessary expenditure on
disruptive, messy work, which often results after incorrect diagnosis.
Moisture content of samples can be determined by 2 methods.

1. Carbide or Speedy test
A measured sample of brick dust or plaster and a measure of calcium carbide are placed in a
special pressure cylinder. The moisture in the test sample reacts with the calcium carbide to
form acetylene gas. This gas creates a pressure, which registers percentage moisture content
on an appropriately calibrated pressure gauge. The carbide meter reading is not affected by
salts and moisture content readings from within the thickness of the wall and can be obtained
in approximately five minutes. Actual moisture content is determined rather than the Wood
Moisture Equivalent (WME) readings displayed on a moisture meter.

2. Gravimetric or oven-drying method
The Carbide test provides actual moisture content but does not determine Hygroscopic
Moisture Content (HMC) and Capillary Moisture Content (HMC).
All building materials are hygroscopic and absorb a certain amount of moisture and no
amount of ‘damp-proofing’ will remove this and it is not doing any harm. The moisture that
concerns people is actually capillary moisture i.e. within the capillaries and pores of the wall.
Hygroscopic Moisture Content is determined by allowing the sample to come to its
equilibrium weight under controlled conditions and the sample is then oven dried to
determine the Capillary Moisture Content. If this sample is taken from the base of a wall and
has a moisture content of less than 5% it is unlikely to be affected by rising damp.
Other sources of damp ingress, such as leaking gutters and downpipes, bridging of physical
damp-proof course by external renders and paving etc., should also be checked and repaired
before going to the expense of expensive & damaging chemical treatments.

(damp and timber article)

North Wales Property Prices on the Up



HOUSE prices on Anglesey have rocketed by more than 5%. The Isle of Anglesey has seen the biggest increase in North Wales in the last 12 months with the new Land Registry figures showing the 5.1% hike took average prices to £134,328.

The annual change in Flintshire was also up by 4.5% (£126,656) followed by Gwynedd at 2.7% (£141,284) and Denbighshire 1.8% (£113,421), however Conwy and Wrexham saw average prices plummet by -2.2% (£130,680) and -2.7% (£114,865) respectively.

Last night, local estate agents predicted that the market will continue to slowly improve this year amid evidence that Government efforts to boost lending are having an effect.

Estate Agent Dafydd Hardy said: “The market is running as we predicted and showing a slow steady recovery. It is down to a combination of things including the mortgage situation with banks easing up on mortgage availability, lower deposits and affordability.  There is also a lot of pent-up demand from people in rented accommodation wanting to get on the property ladder. While recovery is slow we are optimistic that the market will benefit from banks offering that lending boost.”

A representative from Llangefni – based estate agents, Williams and Goodwin said: “The average annual increases are good news and what we’ve seen over the last 12 months is a much stronger second half of the year and the best last quarter which has flown through to January as well.  For example, in our Bangor office, we had the best January for ‘sales agreed’ since 2003, it’s not so much that average sale prices have risen, but that a few more expensive properties are being sold now which naturally puts the average price up.  In December, we sold a couple of dozen homes for £400,000 plus.  This year we expect that volume of sales will continue to contribute to the increase.  I think that people will be regretting it if they don’t buy in the next 12 months as prices will harden – but to counteract that, parts of North Wales are still patchy with some villages and towns selling better than others.”

Lenders and surveyors have previously said that they expect sales to pick up this year amid improvements to the mortgage market following the introduction of the Government’s Funding for Lending scheme.

Mortgage lenders have been slashing their rates since the scheme was introduced and the number of mortgages on the market has also increased.

The Council of Mortgage Lenders (CML) also recently reported an uplift in lending to first-time buyers.

(Article published in the North Wales Daily Post – 1st Feb 2013)

Rising Damp and Period Houses


The term rising damp is often used when referring to damp problems, or potential damp problems with houses, but if you own a period property there are certain issues you need to take into account

You have probably heard of the expression “Rising Damp”.  (Some of you may even remember Leonard Rossiter in the TV Sitcom!)  Here however, I am referring to that phenomenon of damp staining and failed plaster at low level, seen mainly on the external walls of properties built before 1900.
But what is rising damp?  Does it actually exist?  Now even to ask that latter question is controversial as there is a whole industry out there making its living providing reports and carrying out “remedial works” to cure rising damp and even offering 20 to 30 year guarantees.

Rising damp is the name given to dampness manifesting itself in the lower parts of external walls.  Tide marks may be present internally and sometimes “salts” are visible on the internal plasterwork which causes damage to the extent that the plasterwork fails.

In the later part of the 20th Century, the generally accepted treatment was to inject silicone into the walls and hack off all the plaster up to 1 – 1½ metres and re-plaster with sand and cement render often incorporating a water proofer.  This can cost anything from £5,000 – £10,000 for a three bedroom terrace or semi and causes lots of other consequential disturbance.

Is this necessary?  Probably not and it was mainly the water proofer that held back the damp.  Injecting silicone into bricks or their mortar is problematical. Injecting it into porous stonework, such as limestone or sandstone simply doesn’t work. Solid walls are not actually “solid” being usually two skins of stone with rubble infill and you can’t inject rubble infill.

The Society for the Protection of Ancient Buildings (SPAB) talk of “breathing buildings” and this is one of the mainstays of their philosophy of a period or listed building with no damp proof course.

Rising damp will only occur in a wall where there is no damp proof course (dpc).  This is a layer of something that is impermeable to water, sometimes under pressure.  Traditionally, from about the 1880’s onwards, slate was laid in two layers just above ground level.  Sometimes it was two courses of very hard dense engineering brickwork.  Later mastic asphalt was used and later still lead cored bitumen and latterly in the 20th century to the current day a variety of polymers.  The other innovation was to incorporate open cavities but pre-1880’s these are not generally present.

So why are we concerned about Rising Damp? It usually comes to mind after you have put in an offer on a house and the building society or bank surveyor has said to get your mortgage you will need to “cure the rising damp” and then insists on you getting a “specialist” to report and undertake works.

So you have a solid walled house with damp walls.  Why?  Rising damp? – it’s a possibility.
However look first for other causes such as:-

  • High ground levels, i.e. above the inner ground floor level.  It is generally accepted that the outside ground level should be about 15cm below the inner floor level.  However this is not always possible, especially when you don’t own the adjoining land.
  • Another very common cause of dampness at low level is leaking gutters or overflows and the inherent splash back resulting from this.  If you have thatch and no gutters then ensure that the ground is not hard paving around the building to reduce this splash back.
  • Defective render, poor or inappropriate pointing, poor brickwork or stonework can also be a cause of internal damp.
  • Depending on the internal pore size of the bricks or stones, liquid water may rise up through the individual bricks or stones mainly by capillary action. Once the water reaches the point where it can evaporate faster than it can rise up, it rises no further.  In a traditionally built property, this transpiration is called “breathing” and is the natural result of no damp proof course in solid walls. All such walls will be slightly damp at low level.  It is not a “fault” it is how they were built and how they work.

Action to take for rising damp

  • If the property has not already been “modernised” with cement render internally then replace any defective plaster with a breathable lime based plaster and apply a vapour permeable paint.
  • Note: It is very important not to use modern acrylic vinyl emulsion or other vinyl paints and preferably no wallpaper.
  • Deal with all the external housekeeping issues. If the house has cement based pointing consider removing and replacing it with lime based mortar. Be very careful to not damage the bricks or stones in the process and never ever use disk cutters or mechanical means.
  • If it is really bad or you have high ground levels outside, then use a hydraulic lime plaster that breathes but doesn’t allow liquid water to come through.

In conclusion..

Traditionally built properties are fundamentally different from modern buildings and require a holistic approach to the diagnosis of damp problems and their solutions. Initially, make sure you consult with a suitably qualified building surveyor to identify the cause of your damp issues and then employ a company experienced with the use of traditional materials to carry out remedial works.

(extract from Country Life Magazine)