Quick answer
Identify the cause first. If mould appears in cold corners in winter, it is condensation: ventilate, use a dehumidifier and clean with an anti-mould product. If the stain grows near a pipe, ceiling or outside wall, it is penetrating damp: stop the leak before covering anything. If it climbs from the floor with white salt deposits, it is rising damp and needs specialist treatment, not paint.
What you need
- Room hygrometer
- Electric dehumidifier
- Scraper
- Nylon brush
- Gloves, safety glasses and an FFP2 mask
- Roller and brush
- Anti-mould cleaner (Bruguer, Valentine or similar, from Leroy Merlin and Bricodepot)
- Anti-mould or anti-damp paint
- Stain-blocking sealer or primer
- 120 grit sandpaper
- Plastic sheeting and masking tape
Step by step
- 1
Measure the humidity
Leave a hygrometer in the room for 24 hours. If it reads above 65 %, there is too much vapour and it is probably condensation. If it is normal and the wall is wet, suspect penetrating damp.
- 2
Check for a leak
Turn off all the taps, look at the water meter and see whether it is moving. Check the ceilings of the neighbour above, under windows and next to downpipes. If you find the cause, repair it before going further.
- 3
Protect the area and ventilate
Cover furniture with plastic, open the windows and put on an FFP2 mask, safety glasses and gloves. Mould releases spores when you clean it.
- 4
Clean off the mould
Apply the anti-mould product with a brush or sprayer, wait as long as the label says (usually 15-30 minutes) and scrub with a nylon brush. Wipe with a damp cloth and let it dry.
- 5
Scrape off loose paint
Remove lifted or blistered paint with the scraper until you reach firm surface. If there is soft plaster or salts underneath, stop: it is not just condensation.
- 6
Let it dry for at least 48 hours
Ventilate daily and run the dehumidifier. A damp wall will not take paint. Check with the hygrometer that it drops below 60 %.
- 7
Sand and prime
Sand lightly with 120 grit, remove the dust and apply one coat of stain-blocking sealer. This stops yellow stains bleeding through the new paint.
- 8
Paint with anti-mould paint
Apply two coats of anti-mould paint with a roller, waiting the time on the tin between coats (normally 4-6 hours). Cover a hand's width beyond the affected area.
- 9
Check again in two weeks
See whether the stain reappears. If it comes back in the same spot, it is not condensation and you should have it looked at by a professional.
Condensation, penetrating damp or rising damp: how to tell them apart
Before you buy paint or a miracle product, look at what the stain looks like, where it is and when it appears. That is half the diagnosis and it costs nothing.
Condensation is water vapour from inside the flat that cools against a cold wall and turns into droplets. You get little black dots of mould, especially in corners, behind wardrobes, on bathroom ceilings and on north-facing outside walls. It shows up in winter or on very humid days, and improves when you ventilate.
Penetrating damp comes from outside the wall or from a leak. The stain has a brown or yellowish edge, grows after rain or when a tap is used, the plaster swells and the paint blisters. It is typical under a terrace, next to a neighbour's bathroom or on an outside wall with a crack in it.
Rising damp is ground water that climbs up the wall. You see it on ground floors and in old buildings, in the first 50-100 cm of the wall, with an uneven horizontal tide line, paint that flakes and white salts (salitre) that crumble off as powder.
| Type | Where it appears | Quick clue | Who fixes it |
|---|---|---|---|
| Condensation | Corners, bathroom ceilings, behind wardrobes | Black mould, improves with ventilation | You: ventilation and anti-mould |
| Penetrating damp | Under terraces, next to pipes or bathrooms | Stain with a brown edge that keeps growing | Plumber or the residents' association, then a builder |
| Rising damp | Ground floor, first 100 cm of the wall | White salt deposits, horizontal tide line | Damp-proofing specialist |
How to treat condensation at home, step by step
Condensation is the kind of damp you can best solve yourself, because the problem is about use and ventilation, not building work. Open the windows for 10 minutes a day with a through draught, even when it is cold, and do not dry washing in closed rooms. Use the extractor fan in the bathroom and kitchen and keep indoor humidity between 40 and 60 %, which you can monitor with a simple hygrometer from Leroy Merlin.
If your home has a lot of humidity in the air, an electric dehumidifier, such as the De'Longhi DEX series, takes litres of water a day out of the air in a flat. Move furniture 5 cm away from the cold wall so air can circulate. After cleaning off the mould, paint with an anti-mould or anti-damp paint, such as Bruguer or Valentine, not ordinary emulsion: it hides the stain but does nothing to stop the mould.
For the mould itself, see the guide on how to remove mould from a wall, and the one on how to paint a wall once the wall is dry.
Common mistakes with damp stains
- Painting over it without finding the cause. The stain comes back within weeks, and the moisture trapped behind the paint lifts even more paint.
- Using anti-damp paint to fix penetrating damp. It helps with condensation, but it does not replace repairing a leak.
- Hacking off the render straight away. Before you chip anything, confirm with a wall hygrometer or a technician that the problem really is rising damp.
- Waterproofing the outside of the wall without knowing where the water gets in. It can push the damp to another area.
- Drying with the heating on and no ventilation. It turns the wall into a sauna and the moisture moves to the cold areas.
- Not telling the comunidad de vecinos (residents' association) or the neighbour. If the stain comes from above, from the patio or from the facade, the cause is often outside your own flat.
When to call a professional and what we can do
Call us when the stain is growing, when the source is a bathroom, a pipe or a terrace, or when you have a flaking patch with salts and no idea where it comes from. At Manitas Costa we look for the source with a hygrometer and a visual check, and if it is a leak a plumber stops it. We do the repairs for dry damp, meaning once the source has been dealt with: chipping off the damaged area, repairing the render and leaving it ready to paint.
A builder costs €30-40 an hour and repairing a crack or a flaking patch comes to €50-120. A diagnosis and small repair usually fits into one or two hours. See our small building works at home service and the rates in how much a builder charges.
What is not handyman work. Rising damp across a whole wall, damp from missing waterproofing on roofs or facades, and drainage systems or resin injections need a specialist damp-proofing company, and sometimes an architect or surveyor to oversee it. These are licensed works and we do not do them, but we can put you in touch with a company we trust. If the leak comes from a pipe, start with a plumber at home.
Damp on the coast: salt, closed flats and residents' associations
In Valencia, Alicante, Torrevieja or Benidorm the outdoor relative humidity is high almost all year, and outside walls near the sea absorb salts. Condensation appears even in summer if you shut the flat up with the air conditioning on and the blinds down. If you have a second home and leave it closed, mould appears in wardrobes and bedroom corners: ventilate, leave wardrobe doors slightly open and put moisture-absorbing crystals in the wardrobes.
In old buildings in Valencia or Alicante and in ground-floor flats in developments with gardens, rising damp is common. In a building with a comunidad, check whether it affects common elements: downpipes, the roof terrace, the light well or the facade. Tell the administrator, take dated photos and call your home insurance before repairing if the cause comes from a neighbour. That avoids arguments about who pays.
Frequently asked questions
How do I know whether the damp is condensation or penetrating damp?
Condensation shows as black mould in cold corners and improves when you ventilate. Penetrating damp leaves a stain with a brown edge that grows after rain or when water is used, and the paint blisters. If in doubt, send us photos on WhatsApp and we will point you in the right direction before we come.
Does anti-damp paint hide a damp stain?
Only if the cause is condensation and you have already cleaned off the mould. With penetrating or rising damp the stain returns within weeks because water keeps getting in. First stop the source, then dry the wall, and paint last.
How much does it cost to repair a damp wall?
It depends on the cause. Repairing a flaking patch or crack costs €50-120 and a builder's hour is €30-40. Stopping a leak is a plumber's job, charged separately. Rising damp treatment is done by damp-proofing companies who quote per metre. We give a fixed price after seeing photos.
How long does a damp wall take to dry?
A wall with condensation dries in 2-3 days with ventilation and a dehumidifier. After a leak is repaired, a brick wall with render can take 2 to 6 weeks. Do not paint until the hygrometer drops below 60 % and the plaster feels hard to the touch.
Who pays if the damp comes from a neighbour or the residents' association?
If the source is a neighbour's pipe or terrace, you claim on their insurance. If it is a common element, such as the facade, roof terrace or a downpipe, the community's insurance covers it. Take dated photos, tell the administrator in writing and do not repair until the insurer has seen it.