What Causes Mould on Walls? | Damp & Mould Hub

Mould on walls always means one thing: that surface is staying damp. Understand why, and you can stop it for good rather than scrubbing it off again every few weeks. This guide explains the causes — and the fix.

General guidance. Recurring or widespread wall mould is worth getting the cause diagnosed — see damp and mould survey explained.

Mould needs moisture

Mould spores are always in the air; they only grow where they find moisture. So the real question isn’t “why is there mould?” but “why is this wall damp?”

The usual cause: condensation

On internal walls, the most common answer is condensation. Warm, moist indoor air meets a cold wall surface and deposits moisture there. That’s why wall mould typically appears:

in cold corners and where walls meet ceilings,
on external walls (colder than internal ones),
behind furniture pushed against outside walls (no air circulation), and
in rooms with lots of moisture and little ventilation.

See condensation explained for the full picture.

Other causes

Penetrating damp — water entering through an external defect, giving a localised damp patch that worsens after rain.
Rising damp — at the base of walls, where damp-proofing has failed.
A leak — plumbing or rainwater.

Tell them apart with the types of damp explained.

Why “it’s just dirty” is the wrong diagnosis

Black mould isn’t a cleanliness problem — it’s a moisture and ventilation problem. Treating it as dirt (wiping it off, repainting) without addressing the damp is why it keeps coming back.

How to stop mould on walls

Clean it off safely (mask, gloves, ventilation).
Fix the cause — usually by improving ventilation, heating and insulation to stop condensation, or repairing a damp defect.
Keep moisture down and watch the area.

Full method: how to get rid of mould on walls permanently.

When to get help

If wall mould keeps returning, the cause needs proper diagnosis — an independent damp and mould surveyor can find it. Renting? Don’t accept “it’s your lifestyle” without investigation — see your rights as a tenant.

Frequently asked questions

What causes mould on walls?

Mould grows where walls stay damp. On internal walls that moisture is usually condensation — warm, moist indoor air settling on cold wall surfaces — but it can also be penetrating or rising damp, or a leak. No moisture, no mould.

Why do I get black mould on my walls?

Black mould is simply the type that commonly grows on persistently damp internal surfaces, especially cold corners and behind furniture on external walls. It signals a moisture and ventilation problem rather than a dirt problem.

Why does mould keep coming back on my walls?

Because the moisture feeding it hasn't been removed. Cleaning mould off treats the symptom; unless you fix the condensation or damp causing it, it returns. See how to get rid of mould on walls permanently.

Is mould on walls a sign of damp?

Yes — mould is a visible sign that a surface is regularly damp. Identifying which type of damp (most often condensation) is the key to stopping it. See the types of damp explained.