How to treat damp walls: condensation, penetrating or rising damp

The three main types

Condensation is the most common. Moisture from everyday life condenses on cold surfaces such as outside walls, corners and behind furniture. It is usually worse in winter and in bedrooms, bathrooms and kitchens.

Penetrating damp is water getting in from outside through a defect: a leaking gutter or downpipe, cracked render or pointing, a damaged roof, or poor seals around windows. It tends to be in one area and gets worse after rain.

Rising damp is moisture drawn up from the ground into the lower part of a wall, usually where a damp-proof course has failed or been bridged. It is less common than often claimed, and a proper diagnosis matters before paying for treatment.

Simple checks to tell them apart

These checks narrow it down but are not a substitute for a survey. Some homes have more than one type at once.

1. 1

   Check humidity with a hygrometer; regular readings above about 60 % RH point to condensation.

2. 2

   Look outside the wall for leaking gutters, downpipes, cracked render or missing pointing.

3. 3

   Note whether the damp worsens after rain or on cold, still mornings.

4. 4

   Look at how high the damp reaches on a ground-floor wall.

5. 5

   Check whether furniture is pressed against the damp area.

Fixing condensation damp

Cut moisture at the source, ventilate, and keep walls warmer. Use extractor fans, keep trickle vents open, dry washing with a dehumidifier, and keep a steady low heat. Move furniture a few centimetres away from cold outside walls. A dehumidifier set to around 50 to 55 % RH holds the air below the point where walls get wet.

Our guides on how to stop condensation on windows and damp walls in a bedroom go into more detail. Clean any mould with the steps in how to remove black mould.

Best dehumidifiers for condensation and damp rooms →

Fixing penetrating damp

Find and repair the defect: clear or fix gutters and downpipes, repair cracked render or pointing, and fix roof or window seal problems. Once the source is fixed, the wall needs time to dry out; a dehumidifier can speed up drying of the room, but it will not fix the cause. Roof and high-level work should be done by a qualified tradesperson.

Note:Do not work at height or on roofs without the right equipment and training. Use a qualified tradesperson for roof, gutter and render repairs.

Rising damp and when to call a professional

If you suspect rising damp, get an independent survey from a qualified surveyor before agreeing to any treatment. Many cases that look like rising damp turn out to be condensation or penetrating damp, or a bridged damp-proof course, for example soil or a path built up against the wall, which can be fixed more simply.

Call a professional if damp keeps coming back after you have dealt with condensation, if plaster is crumbling, if there is a musty smell from floors or timbers, or if you are unsure of the cause. Tenants should report damp to their landlord in writing; GOV.UK sets out landlords' repair responsibilities.

• Damp that returns after condensation fixes
• Patches that grow after rain
• Crumbling plaster or rotten skirting
• Damp in timbers or under floors
• Any doubt about the cause

Drying the room after a fix

Once the cause is dealt with, walls can take weeks to dry. Keep the room heated and ventilated, and run a dehumidifier to speed things along. A desiccant model suits a cold room; a compressor model suits a heated one. Avoid painting or replastering until the wall is dry, and check the maker's guidance for any products you use. See where to put a dehumidifier.

Frequently asked questions

Will a dehumidifier fix damp walls? It fixes condensation damp and helps dry walls after a repair, but it will not stop penetrating or rising damp. Those need the cause fixed.

How can I tell if it is rising damp or condensation? Condensation is usually on cold walls and corners and worse in cold weather; rising damp sits low on ground-floor walls. A qualified surveyor can confirm.

Who is responsible for damp in a rented home? Landlords are responsible for many repairs, including those causing damp. Report it in writing and see GOV.UK's guidance on private renting repairs.