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Damp bathroom walls: causes and fixes

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In short

Most damp bathroom walls are caused by condensation: steam from baths and showers settles on cold walls and ceilings faster than it can escape. The fixes are a working extractor fan run during and after washing, wiping down, keeping the door shut and the room warm, and checking sealant and pipes for leaks. A dehumidifier can help next door, but must stay out of the wet zones.

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Bathrooms produce more moisture in a short time than any other room. A hot shower fills the air with water vapour that has to go somewhere, and in a small, cold room it ends up on the walls, ceiling and window. Over weeks that leads to peeling paint, mould on sealant and a musty smell. This guide works through the causes in order of how common they are and the fixes for each, and covers where a portable dehumidifier can and cannot be used.

Why bathroom walls get damp

Warm, steamy air holds a lot of water. When it touches a surface colder than its dew point, the vapour turns back into liquid. Outside walls, ceilings under a cold loft and window reveals are usually the coldest surfaces in a bathroom, so that is where the damp appears.

If damp shows up in only one patch, for example below a shower tray, around a bath panel or on a wall shared with a kitchen, a leak is more likely than condensation. Our guide on signs of damp walls explains how to read the pattern.

Fix 1: get the steam out

An extractor fan is the most effective fix. Run it throughout every bath or shower and for at least 15 to 20 minutes afterwards, or fit a fan with an overrun timer or humidity sensor that does this for you. Building regulations in England set minimum extract rates for new bathroom fans; if your fan barely moves a sheet of tissue held to it, it may be too weak, blocked or venting poorly.

Clean the fan grille regularly. If you have a window but no fan, open it during and after washing. Keep the bathroom door shut while the room clears, so steam does not spread to colder bedrooms and landings.

  • Run the fan during and after every shower
  • Keep the door closed until the mirror clears
  • Open the window if there is no fan
  • Clean the fan grille every few months
  • Ask an electrician about a humidity-sensing fan if yours is weak

Fix 2: wipe down and keep it warm

A squeegee on the tiles and screen and a quick wipe of the window after showering removes a surprising amount of water that would otherwise evaporate back into the room. Hang wet towels to dry somewhere with airflow, not in a heap.

A colder bathroom suffers more condensation. Keeping some background heat on in cold weather raises the wall temperature above the dew point more of the time. A heated towel rail helps both towels and the room.

Fix 3: check for leaks

Look for gaps or black, lifting silicone around the bath and shower tray, cracked grout, and drips at taps, waste pipes and the toilet cistern. A damp patch that stays wet even when the bathroom has not been used for a day points to a leak. Reseal joints and fix drips, or ask a plumber if the source is hidden.

If the wall is an outside wall and damp appears after rain, check gutters, downpipes and pointing outside. Our guide on how to treat damp walls covers penetrating damp.

Where a dehumidifier can go

Most portable dehumidifiers are not designed for use inside bathroom zones near the bath or shower. Check the manual: many say not to use them in a bathroom at all, or only well away from water. Never place one where it could be splashed or knocked into water.

A safer approach is to run a dehumidifier on the landing or in the room next door with the bathroom door open after the steam has cleared, or in an adjoining bedroom where the bathroom's moisture spreads. Our guide on where to put a dehumidifier has more on placement.

Note:Do not use a portable dehumidifier inside bathroom zones or where it could get wet. Follow the manual, and use an electrician for any fan or wiring work.

See dehumidifiers for condensation →

Cleaning existing mould

Once the moisture is under control, clean any mould from tiles and grout. Mould inside silicone usually needs the sealant cutting out and replacing. Our step-by-step guide on how to remove black mould covers safe cleaning. Anti-mould bathroom paint can help on ceilings, as explained in does anti-condensation paint work, but only after the steam is dealt with.

Frequently asked questions

How long should a bathroom fan run after a shower? Run it for at least 15 to 20 minutes after you finish, or until the mirror clears. A fan with an overrun timer does this automatically.

Can I put a dehumidifier in my bathroom? Check the manual. Many portable units are not designed for bathrooms, and none should be used in the wet zones. Running one in an adjoining room is usually safer.

Why is only one bathroom wall damp? It is often the coldest wall, usually an outside wall. A single localised patch near pipes or a shower tray can mean a leak.

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