Condensation and black mould on walls: causes and how to stop it (UK guide)

If you've got black mould speckling the corners of a room, misting on the windows, or patches behind the wardrobe, the cause is almost always condensation — not water leaking in. It's the most common form of damp in UK homes, and it's a moisture-balance problem: too much humid air indoors, too many cold surfaces for it to settle on, and too little ventilation to clear it.

The good news is that condensation responds to four practical levers. Pull them together and the mould has nowhere to form. (If you're not certain condensation is the cause, check our damp diagnosis guide first.)

Lever 1: Reduce the moisture you put into the air

Everyday living adds litres of water to indoor air — cooking, showering, drying clothes indoors, even houseplants. Cover pans, dry washing outside or in a vented space, and keep bathroom and kitchen doors shut while moisture is being produced so it doesn't spread through the house.

Lever 2: Ventilate — get the moist air out

Moisture you can't avoid producing needs a way out. Use extractor fans in the kitchen and bathroom (and let them run on after you finish), open trickle vents, and air rooms daily. Sealing a house up tight without providing ventilation makes condensation worse — the two must go together.

Lever 3: Keep surfaces warm enough

Condensation forms on the coldest surfaces first. Low, intermittent heating lets walls get cold enough to hit dew point; steadier background warmth keeps surface temperatures up and moisture in vapour form. Leaving furniture hard against cold external walls creates exactly the kind of cold, still pocket where mould thrives — leave a gap for air to move.

Lever 4: Fix cold spots and air leaks

Two building issues make condensation far worse:

• Cold bridging — a localised cold spot (a concrete lintel, a junction, an uninsulated reveal) where condensation and mould concentrate.
• Air leakage — warm, moist indoor air leaking into the structure through gaps, then condensing inside it. Controlling that airflow is central to a dry, healthy build-up.

This is where an airtight, vapour-controlling internal layer helps. By sealing the surface against air leakage and controlling how moisture moves into the wall, products like Passive Purple Internal Airtight & Vapour Control Paint reduce the uncontrolled moist-air movement that drives interstitial condensation — particularly valuable alongside internal wall insulation, which must be detailed airtight to work. For more on why that layer matters, see which side the vapour control layer goes.

Remember the pairing, though: airtightness only improves condensation when it goes hand-in-hand with ventilation. Seal the leaks and give moisture a deliberate way out.

Dealing with the mould that's already there

Clean existing mould off hard surfaces with an appropriate mould treatment, and address the conditions above so it doesn't return — cleaning alone, without changing the moisture balance, is temporary. Persistent or widespread mould in the home is worth taking seriously; if anyone in the household is affected by it, speak to a qualified professional.

Related reading

• Rising, penetrating or condensation damp? How to tell the difference
• Which side of the insulation does a vapour control layer go?
• What causes damp walls and what is the solution

Frequently asked questions

Is black mould always condensation?

Most of the time, yes — it's the classic sign of moisture settling on cold surfaces. Occasionally a leak or penetrating damp creates the damp conditions instead, so rule those out if the pattern doesn't fit.

Will better insulation stop condensation?

It helps by keeping surfaces warmer, but only if it's detailed airtight and paired with ventilation. Insulation with cold bridges or air leaks can move the problem rather than solve it.

Does airtight paint cause condensation or prevent it?

Used correctly as a warm-side airtight vapour control layer — and combined with ventilation — it reduces the uncontrolled moist-air movement that causes condensation within the structure. The essential partner is adequate ventilation.

Tackling condensation as part of a retrofit or insulation project? Our technical team can advise on the airtight and vapour control detailing.