Search for how to treat damp walls and you’ll find anti-mould paint, damp-seal primers, dehumidifiers, tanking slurries and injection kits. Every one of them works in the right situation. Every one of them fails in the wrong one. The reason damp comes back after “treatment” is almost always the same: the treatment matched a symptom, not a cause.
So before anything else, work out what kind of damp you have.
Step 1: Identify which type of damp it is
There are three common types, and they need completely different fixes.
Condensation — by far the most common in UK homes.
- Black mould spotting, especially in corners, behind furniture, on window reveals and on cold external walls.
- Worse in winter, in bedrooms overnight, and in kitchens and bathrooms.
- Streaming windows in the morning.
Penetrating damp — water getting in from outside.
- Damp patches at any height, often after wind-driven rain.
- Usually traceable to a defect: failed pointing, a leaking gutter or downpipe, cracked render, a defective window seal, a chimney or flashing.
- Patches that grow after wet weather and shrink in dry spells.
Rising damp — ground moisture wicking up the wall.
- Confined to the lower part of the wall, typically below about a metre.
- Tide marks and salt deposits, crumbling plaster at skirting level.
- Fairly steady whatever the weather. We explain rising damp in detail here.
If you’re unsure, our damp checker ranks the likely causes from what you’re seeing, and our guide to telling the three apart goes further. And it’s common to have more than one at once.
Step 2: Fix the cause
If it’s condensation
The aim is to produce less moisture, remove what you do produce, and keep surfaces warmer.
- Ventilate at source. Use extractor fans in kitchens and bathrooms (and check they actually extract), keep trickle vents open, and open windows after bathing and cooking.
- Dry clothes outside or in a vented dryer where you can. Drying washing indoors releases litres of water into the air.
- Heat more evenly. Cold rooms and cold walls attract condensation. Low, steady background heat is better than short bursts.
- Leave a gap behind furniture on external walls so air can circulate.
- Treat mould correctly. Clean it with a suitable fungicidal wash, not by dry brushing (which spreads spores). Anti-mould paint helps afterwards, but only once the humidity is under control. Our guide to getting rid of black mould goes step by step.
- Consider a dehumidifier as a support, not a cure.
Persistent condensation on one wall can also point to a cold bridge or missing insulation, which is worth investigating.
If it’s penetrating damp
Find the entry point and repair it. Painting or plastering the inside won’t stop water coming through from the outside.
- Clear and repair gutters and downpipes, and check where they discharge.
- Repoint failed mortar joints — in older stone and brick, with a lime mortar rather than hard cement.
- Repair cracked render or, in older buildings, consider whether cement render is itself trapping water.
- Check window and door seals, sills, flashings and chimney details.
- In exposed locations, consider whether cavity wall insulation is bridging the cavity.
Once the source is fixed, the wall needs time to dry before redecoration.
If it’s rising damp
Look for bridging first: external ground or paths above the damp proof course, soil banked against the wall, render or plaster running past the DPC line. Remove the bridge, fix any drainage, and let the wall dry. Only if the DPC itself has failed or is absent is a new one the answer — and even then it’s worth understanding the options.
Treating damp walls internally: what works and what doesn’t
The “how to treat damp walls internally” question usually means someone wants a fix they can do inside without dealing with the outside. Be careful:
- Damp-seal paints and primers can stop staining showing through, but they trap moisture in the wall, which usually moves the problem somewhere else.
- Tanking (waterproof slurries or membranes) is designed for below-ground rooms such as cellars. It has a place, but used above ground on a wall with an untreated cause, it hides damp rather than solving it.
- Replastering helps where the old plaster is salt-contaminated, but only after the cause is dealt with. In older solid-walled homes, a breathable lime plaster usually performs better than gypsum.
The honest rule: internal treatments manage the look of a damp wall. Fixing the cause is what makes it dry.
Old houses need a different approach
In traditional solid-walled buildings, much of the “damp” homeowners fight is caused by modern materials — cement render, cement pointing, gypsum plaster and plastic paints — trapping moisture that the wall was built to release. Treating these buildings like modern cavity walls, with sealants and injection, often makes things worse. Restoring breathability is frequently the real fix.
When to get an independent survey
If the damp keeps coming back, if you’re being quoted for injection or tanking, or if you’re buying a house with a damp flag on the report, an independent diagnosis is worth it before you spend money. We carry out independent damp and timber surveys with no treatment to sell: we find the cause and tell you the simplest fix that will work. Get in touch.
Sources and further reading
- Damp and mould: understanding and addressing the health risks — GOV.UK
- Damp in historic buildings — Historic England
- Homeowner advice hub — Property Care Association