Does My Old Cottage Need Damp Treatment?
Expert Douglas Kent answers one reader’s question
QWe are hoping to buy an old stone cottage in Yorkshire. The damp report we have had done suggests the house is damp and recommends a damp treatment. However, from reading around the subject of damp in old buildings I’m hesitant about whether this is the right thing. A lot of what I’ve read is that this is the worst thing to do, and that the house should be restored to traditional finishes. I’m not sure if we should just avoid it all together or if we should get a local heritage builder to go over and take a look instead. Can you help to advise on what’s best?
Angela Brook, Yorkshire
AYou are right to be wary about the advice you have been given, but I would not let this deter you from considering purchasing the cottage if it generally matches your requirements.
A common problem is that advice on damp is often given by remedial treatment contractors. They have vested commercial interests in their recommendations, which leads to self-serving reports and the over-specification of work. Additionally, many individuals advising on damp do not appreciate how the construction of older buildings pre-dating about 1919 differs fundamentally to that of most modern buildings.
As a result, unfortunately, recommendations are frequently made for work that is unnecessary, damaging and expensive — especially the retrofitting of damp-proof courses (DPCS) introduced to act as a barrier to rising damp in walls.
Most pre-1919 buildings are of ‘ traditional’ construction with solid walls that need to ‘ breathe’. Contrast that with post-1919 or ‘modern’ buildings that depend on barriers in order to stay dry. The former are analogous to an overcoat and the latter a raincoat. If barriers are added to a traditionally constructed building it is likely to exacerbate damp, whereas a modern building will become damp if barriers are removed.
Retrofit DPCS are often recommended unnecessarily just because they are absent. Sometimes rising damp is misdiagnosed solely on the basis of high electrical moisture meter readings; this could be due to penetration from rainsplash, for example. Elevated readings occur not infrequently in old buildings that are virtually dry, due to salt deposition on breathing walls. Damp-proof membranes (DPMS) are similarly recommended without good reason as a moisture barrier below old stone floors. By restricting evaporation, DPMS displace moisture into the previously dry adjacent walls, causing damp there.
If damp does actually exist, aim to tackle the cause. Measures that help the fabric ‘ breathe’, such as replacing a hard cement render or the pointing in masonry joints with a more suitable lime-based mortar, may be the best solution. If a floor has an inappropriate DPM, this might be substituted with a ‘ breathable’ construction or a strip for evaporation cut around the room perimeter and infilled with a material such as lime concrete, or grated over.
Condensation is an increasing problem due to laudable but misguided attempts to improve energy efficiency in old buildings. It can arise not only where incompatible nonbreathable forms of insulation are used (for example, PIR boards rather than vapour-open materials such as hemp) but over-zealous draughtproofing, including the installation of replacement windows. Condensation can promote rot and aggravate human health problems, including asthma, but can be controlled by improving ventilation, generating less moisture and increasing heating.
What’s more, the neglect of basic maintenance can cause rain penetration. Simple tasks such as clearing out blocked gutters and downpipes, and reinstating slipped or missing slates or tiles should be undertaken on a regular basis to prevent this. Rain penetration may also occur where hard surfaces are laid externally immediately alongside walls.
It is worth challenging any recommendation regarding damp that you believe is questionable. If necessary, seek a second opinion in writing from an independent chartered surveyor or consultant, not a builder. This will usually satisfy mortgage companies. The Society for the Protection of Ancient Buildings (SPAB) may be able to advise you on suitable names.*
“If dampness does actually exist, aim to tackle the cause”