Guides · Damp guides
Is it damp or condensation?
In short: Condensation is moisture from inside the house settling on cold surfaces, so it shows up as streaming windows and black mould in corners, behind furniture and on outside walls, and it is worst in winter. Rising and penetrating damp come from outside the room, so they show as stains and tide marks, follow the ground or the weather, and often leave salts or damaged plaster.
Where it appears
- Condensation: windows and window reveals, the top corners of outside walls, behind wardrobes and beds pushed against outside walls, bathrooms and kitchens. Often in rooms with little air movement.
- Rising damp: the bottom of ground-floor walls, at a fairly even height.
- Penetrating damp: any height, usually in patches that line up with something outside, such as a downpipe, a cracked section of render or eroded pointing.
When it is worst
Condensation is worst in cold weather, when walls and windows are coldest and windows are kept shut. It often gets better in summer. Penetrating damp gets worse after heavy or wind-driven rain. Rising damp changes little through the year.
What it looks like
Condensation produces water droplets and black mould spots on otherwise sound surfaces, without tide marks or salts. Rising and penetrating damp tend to leave brown staining, tide marks, white salt deposits, blown plaster and peeling paint.
Some homes have more than one at once: a wall made cold and damp by penetrating damp will also attract condensation and mould. That is why we look at the whole picture on a damp survey rather than treating the first thing we find.
A simple check you can do yourself
Tape a square of kitchen foil, about 30cm across, tightly to the damp area and leave it for a day or two. If moisture forms on the room side of the foil, the water is coming from the air in the room: condensation. If the wall side is damp and the room side dry, the moisture is coming through the wall. It is a rough guide rather than a diagnosis, but it helps.
What fixes each one
Condensation is fixed by getting moist air out and keeping surfaces warmer: better extractor fans, trickle vents, sometimes a positive input ventilation unit. See condensation control. Penetrating damp is fixed by repairing whatever lets the rain in (penetrating damp repairs), and rising damp by a new damp-proof course and salt-resistant replastering (rising damp treatment).
Frequently asked questions
Is it damp or condensation: FAQs.
Will a dehumidifier fix condensation?
It helps in the short term by taking moisture out of the air, but it treats the symptom. Improving extraction and ventilation deals with the cause.
Can condensation cause a damp patch low on a wall?
Yes. Cold areas behind furniture or low on outside walls can stay damp from condensation, which is one reason it is often mistaken for rising damp.
Is condensation the tenant's fault?
Not necessarily. Everyday living produces moisture, and weak extraction, missing trickle vents and cold walls make condensation much worse. Those are building issues, not lifestyle.
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