What kind of damp do I actually have, and how do I tell the difference?
Damp comes in three forms and each has a different remedy. Condensation forms when warm humid air meets cold surfaces. Penetrating damp enters through a failure in the external fabric. Rising damp travels up from ground level through capillary action in walls that lack a damp-proof course. The first step is correctly identifying which one is in front of you.
What it usually looks like
These are the symptoms readers describe most often. None of them alone is diagnostic, but together they build a picture.
- Black spotting on cold walls
- Window condensation in winter
- Musty smell
- Damp tide mark at low level
- Crumbling plaster behind skirting
Most common in: Victorian terrace · Edwardian semi-detached
Before you buy anything
Watch the house respond as you scroll.
These checks are listed in the order we would work through them. The illustration on the left changes with each one, so you can see what each check is actually addressing before deciding whether it is worth doing.
Warm wet air settles on the coldest surface and stays there. The window streams; mould follows.
Look at where the damp appears
Damp in upper corners is almost always condensation. Damp in patches on external walls suggests penetrating damp. Damp at low level around the base of walls points to rising damp.
Inspect external walls and roof for water ingress
Blocked gutters, cracked render and missing pointing cause many cases that are misdiagnosed as condensation. Fixing the leak makes everything else cheaper.
Measure humidity in the affected rooms for a week
Persistently above sixty per cent points to a ventilation problem rather than an envelope problem. The data prevents you spending on the wrong fix.
Get an independent damp survey if the cause is unclear
Many damp companies sell the treatments they recommend. An independent surveyor without skin in the game pays for themselves quickly.
Treat the cause before treating the symptom
Anti-mould paint over an unresolved damp problem fails within months. The cost saving comes from doing the underlying work once.
Products that may help
Only consider these once the checks above have been ruled out. A product fitted into the wrong cause is rarely satisfying.
The cheapest answer to damp, mould or condensation is usually the one that addresses the cause rather than the symptom. The list above is in the order we would work through it, because the checks at the top tend to rule out the most expensive mistakes further down.
Run the Home Comfort Score for this room
A two-minute reading gives you a number to compare against after each improvement, so you know what is actually working.