Where the water comes from
Warm air holds moisture. When it meets a cold surface, such as a window, an outside wall or the corner behind a wardrobe, it cools and the water condenses. Black mould grows where that happens often enough for the surface to stay damp.
An ordinary household puts a lot of water into the air every day:
- Cooking, especially boiling pans without lids
- Showers and baths
- Drying clothes indoors, and tumble dryers that are not vented outside
- Breathing, which adds up overnight in a closed bedroom
Ventilation: give it a way out
Moisture needs a way out of the house. Opening windows helps, but few people do it in January, which is what the mechanical options are for.
- Extractor fans in the kitchen and bathroom, used every time and left running for a while afterwards. Many can be fitted with an overrun timer or humidity sensor that does this for you
- Check a fan actually works: hold a sheet of toilet paper to the grille and it should be pulled in
- Trickle vents at the top of the windows, left open
- Positive input ventilation (PIV): a unit, usually in the loft, that gently pushes filtered air into the house and damp air out
- Mechanical ventilation with heat recovery (MVHR): whole-house ventilation that recovers heat from the outgoing air. Best suited to well-sealed homes and major renovations
Heating: steady beats bursts
Short blasts of heat warm the air quickly but leave the walls cold, and warm, moist air against a cold wall is exactly the problem. A steady, lower background temperature through the day keeps surfaces warmer, above the point where water condenses on them.
Try not to let unused rooms get very cold, and leave a gap between furniture and outside walls so air can move behind it. Good heating controls make a steady pattern easier to set and forget.
When it is not condensation
Condensation is usually worse in winter, appears on the coldest surfaces and shows up as black, spotty mould. Damp that comes from outside or from a leak follows a different pattern and needs a different fix, often from a different trade:
- Penetrating damp: patches that appear or grow after rain, at any height, often from gutters, roofs, pointing or render. A job for a roofer or builder
- Rising damp: a tide mark low on ground-floor walls, often with white salts, where moisture is coming up from the ground. Get an independent surveyor’s opinion before anyone sells you a treatment
- Plumbing leaks: a patch that stays wet whatever the weather, often below a bathroom or near pipes or a radiator. Heating leaks we can find and fix
For landlords and tenants
Landlords are responsible for repairs and for making sure a rented home is fit to live in, and serious damp and mould can make it unfit. The rules differ between England, Scotland and Wales and have been tightened in recent years, so check the current guidance for where the property is.
Tenants: report damp and mould to your landlord or agent in writing, with photos and dates. Landlords: treat it as a building problem, not a lifestyle one. Find the cause, fix the ventilation, heating or leak, and keep a record of what was done. Painting over mould without fixing the cause only buys a few weeks.