🗓 Updated September 17, 2026 · EcoBack editorial team
British houses have three separate faults that all get called "damp", and they have almost nothing in common except the stain on the wall. They arrive by different routes, they cost wildly different amounts to fix, and the dehumidifier that solves one of them does nothing whatsoever for the other two.
Before you spend anything, work out which one you are looking at. Two questions get you most of the way: where is the patch, and when is it worst.
The two-question test
| Condensation | Penetrating damp | Rising damp | |
|---|---|---|---|
| Where | Coldest surfaces: outside walls, corners, window reveals, behind furniture, unheated rooms | One localised patch, any height, usually on a wall with something wrong outside it | Bottom of a ground-floor wall, fading out at a fairly level line |
| When | Winter, and within hours of showering, cooking or drying washing indoors | During and after heavy or driving rain; can persist in summer if the weather does | All year round, roughly constant, indifferent to the weather |
| Looks like | Black spotty mould, misted windows, damp-smelling fabric | An irregular blotch, often darker in the middle, may be tide-free | A tide mark, blown or crumbling plaster, white salt deposits |
| Fixed by | Ventilation, heating the surface, a dehumidifier | Repair to the outside: gutters, pointing, render, flashing | Building work to the wall base — a surveyor's job |
Condensation is the common one, and the only one we can help with
Condensation damp is a humidity problem, not a building failure. Warm indoor air holds a lot of water; a cold surface chills the air touching it; and at the point where that air reaches saturation the water comes out on the surface. That is all it is.
The reason it looks like a wall problem is that the humidity at a cold surface is much higher than the humidity in the middle of the room, so your hygrometer can read a reassuring 55% while the wall behind the wardrobe is sitting near the threshold where mould germinates. The mechanism, the arithmetic and the surface-humidity calculator are on our winter condensation page.
Three things move it, in order of cost:
- Move the furniture. A few centimetres of gap behind a wardrobe on an outside wall lets air reach the surface. Free.
- Ventilate briefly and fully rather than leaving a window ajar all day. Cold outside air is dry air, and in winter it is almost always drier than the air indoors even when it is raining.
- Dehumidify, if the moisture load is genuinely high — drying washing indoors will do it on its own. Which type depends on how cold the room is: desiccant or compressor.
Penetrating damp is a repair, not a purchase
Water is getting in through the fabric. The usual culprits are outside and visible on a wet day: a blocked or leaking gutter running down a wall, cracked render, failed pointing, a slipped tile, flashing that has come away at a chimney or an extension join. Look at the wall from outside while it is actually raining. That is worth more than any meter.
A dehumidifier cannot help. It will lower the humidity in the room slightly and the wall will keep getting wet, because the water is not coming from the air.
Rising damp is the rarest, and the most over-diagnosed
Rising damp is groundwater drawn up through masonry by capillary action, where a house has no damp-proof course or the one it has is bridged — by raised ground outside, a rendered-over DPC, or a path laid above it. Its signature is the level line and the salts: as the water evaporates from the wall face it leaves behind the minerals it carried, which is why the plaster blows and why a white bloom appears.
If you rent
Which of the three you have also changes whose problem it is. Damp arising from the condition of the building is generally the landlord's to repair, and that is a legal question rather than a technical one — so rather than paraphrase it, here is where the official answer lives: gov.uk on repairs in private renting, and Shelter's free advice on damp and mould in rented homes.
We sell nothing on this page and we are not lawyers. If your damp is a repair the landlord owes you, buying a dehumidifier is not the answer to it — at best it manages a symptom you should not be paying to manage.
If it turns out to be condensation
Then it is a humidity problem and the rest of this site is about it. Start with what the air in your rooms is actually doing: the type question first, then size. The rules behind every figure we publish are open data at /daten.html, so you can check them or reuse them.
Frequently asked questions
Will a dehumidifier fix rising damp?
No. A dehumidifier takes water out of the air in the room. Rising damp is water arriving through the masonry from the ground, and it will keep arriving at the same rate however dry you make the air. The machine will run all winter, cost you every hour of it, and the wall will still be wet.
How do I tell condensation from rising damp?
Look at where it is and when. Condensation sits on the coldest surfaces — outside walls, corners, window reveals, behind wardrobes — usually as black spotty mould, and it is worst in winter and after showering, cooking or drying washing. Rising damp starts at the bottom of a ground-floor wall, stops at a fairly level line, and often leaves a tide mark with salt deposits in the plaster.
Is damp the landlord's responsibility?
It depends on the cause and on your tenancy, and it is a legal question rather than a technical one. Damp caused by the condition of the building is generally the landlord's to repair. Official guidance for England is on gov.uk, and Shelter publishes free advice for renters. We are not lawyers and this page is not legal advice.
Why is there mould behind my wardrobe?
Because the wall behind it is colder than the rest of the room and the air there barely moves. Mould germinates on the surface when the humidity at that surface stays high, and a cold surface raises the humidity right against it even though the room reading looks fine. Pulling furniture a few centimetres off an outside wall is the cheapest fix there is.