Damp is one of the most common worries for owners of old houses, and one of the most misunderstood. Many period homes are sold chemical damp-proof courses, waterproof renders and sealed plasters, only for the damp to get worse. Understanding how old walls handle moisture is the key to fixing it for good.
Old buildings were designed to breathe
Houses built before about 1920 usually have solid walls, no cavity and no damp-proof course. They cope with moisture by letting it move through the fabric and evaporate: out through the lime mortar and render on the outside, and through lime plaster and limewash on the inside. Floors were often stone, brick or earth, laid straight on the ground so moisture could escape.
As long as moisture can get out as easily as it gets in, the walls stay healthy. Problems start when that balance is upset.
Common causes of damp in old houses
- Cement render and pointing on the outside, which trap moisture in the wall and crack, letting rain in.
- Gypsum or cement plaster on the inside, which stops the wall drying into the room and often shows damp patches, salts and blown plaster.
- Modern masonry paints and vinyl emulsions, which seal the surface.
- Concrete floors with a membrane, which push ground moisture sideways into the base of the walls.
- Raised ground levels outside, from paths, patios or flowerbeds built up against the wall.
- Leaking gutters, downpipes and roofs, which are often the simplest cause of all.
- Poor ventilation and condensation, especially in kitchens and bathrooms.
What actually fixes it
The answer is usually to remove the barriers and let the building work as it was designed to:
- Fix the plumbing first: gutters, downpipes, roofs and drainage.
- Lower ground levels outside where they have been built up.
- Replace cement render and pointing with lime render and lime pointing.
- Replace gypsum plaster on solid walls with lime plaster, and use breathable paints.
- Where a concrete floor is causing problems, consider a breathable limecrete floor.
Walls that have been wet for a long time take a while to dry out, sometimes a full year or more, but once they can breathe again the improvement is usually lasting.
Talk to us about your house
Every building is different, and it is worth understanding the cause before spending money on a cure. If you have damp in an old house, get in touch and we will come and take a look.
