Condensation, Damp and Mould: Find the Cause First
How to tell condensation, penetrating damp, leaks and rising damp apart, what to do first and why the cause must be fixed before redecorating.
Painting over mould or a damp patch looks tidy for a few weeks, then the mark comes back. The paint was never the problem: the moisture is. This guide helps you work out what is causing it, what you can do yourself and when to call in a specialist.
The quick answer
- Mould in corners, behind furniture and on window frames is usually condensation
- A patch that grows after rain is usually water getting in from outside
- A patch that grows without rain may be a plumbing leak
- A tide mark rising from the floor may be rising damp, but get it properly diagnosed
- Fix the cause first, then treat, prime and redecorate
Which type of damp is it?
| Type | Typical signs | Common causes |
| Condensation | Mould in corners, on windows and behind furniture, worse in winter | Poor ventilation, drying washing indoors, cold walls |
| Penetrating damp | Patch in one place, worse after heavy rain | Blocked gutters, damaged pointing, slipped tiles, failed seals |
| Plumbing leak | Patch grows with no rain, often under baths, sinks or radiators | Leaking pipes, joints, seals or wastes |
| Rising damp | Tide mark up to about a metre from the floor | Failed or bridged damp-proof course |
Condensation: the most common cause
Warm, moist air hits a cold surface and turns to water. Cooking, showering, drying clothes and even breathing add moisture. It is especially common in winter and in well-sealed homes with poor ventilation.
- Use extractor fans when cooking and showering, and leave them running for 15 to 20 minutes afterwards
- Open trickle vents on windows, or open a window briefly each day
- Dry washing outside, or in a ventilated room with the door shut
- Keep furniture a few centimetres away from external walls
- Keep a steady, low background heat rather than short bursts
- Put lids on pans
Penetrating damp
Water gets into the wall from outside. Look for a stain that appears or grows after heavy rain, often near the ceiling, a window or a chimney. Check gutters, downpipes, pointing, roof tiles and window seals.
Leaks
A stain that spreads without rain points to a plumbing leak. Check under baths, showers, basins and around radiators. Turn off the water if you find an active leak, and get it repaired before any decorating.
Rising damp
True rising damp is less common than many people think, and other problems are often mistaken for it. A high external ground level, blocked air bricks or a leak can all cause similar marks. Get an independent diagnosis before paying for expensive damp-proofing.
What to do first
- Photograph the area and note when it gets worse
- Wipe small areas of surface mould with a suitable mould cleaner, wearing gloves and ventilating the room
- Improve ventilation and heating straight away
- If you rent, tell your landlord or agent in writing
- Do not paint over it until the cause is dealt with
Mould can affect health, particularly for people with asthma, allergies or weakened immune systems, so do not ignore it.
Redecorating after damp
Once the cause is fixed and the wall has dried out, clean the area, apply a stain-blocking primer and use a suitable paint. Anti-mould paint can help in bathrooms and kitchens, but it does not fix the cause. Our painting and decorating guide explains good preparation.
Frequently asked questions
Is black mould dangerous?
Mould can trigger breathing problems and allergies, especially for vulnerable people. Deal with it promptly and fix the cause.
Will a dehumidifier help?
Yes, for condensation. It reduces moisture in the air, but ventilation and steady heating are still important.
Does anti-mould paint work?
It slows mould growth on the surface, but it will not fix a leak, penetrating damp or poor ventilation.
Who is responsible for damp in a rented home?
Landlords must investigate and address damp, mould and disrepair where their legal duties require it, including relevant leaks and building defects. Tenants should report problems promptly and keep records. Ventilation or heating advice is no substitute for fixing a structural cause; legal response duties vary by tenancy type.
Need help?
We reseal baths and windows, fit extractor fans through qualified electricians, repair and redecorate once the cause is fixed, and will tell you if you need a roofer, plumber or damp specialist first. Send photos and your postcode.
Official guidance and further reading
GOV.UK: landlord safety responsibilities
GOV.UK: Awaab's Law proposed extension to private tenancies
Related guides
- Painting and decorating guide
- How to choose the perfect wall colour
- How to reseal a bath or shower
- Draughty or failing windows
- Winter-proofing: insulation and draught-proofing
Prefer to talk? Call 07444 262522.