Is this condensation or rising damp?

Condensation mould appears on cold surfaces — reveals, wall corners, behind furniture — and is worst in winter. Rising damp shows as a tide mark low on a wall with salts in the plaster. Penetrating damp appears as a localised patch traceable to a defect outside. They need different trades.

Why does it come back after I clean it?

Because the moisture has not changed. Cleaning removes the growth; ventilation, heating and fixing the source are what stop it regrowing. Mould that returns within weeks of a proper clean is telling you the cause is still there.

Should I paint over it with anti-mould paint?

Not while the surface is damp and the cause is unresolved. Paint applied over mould or onto a damp substrate fails, and covering it delays finding a defect that will keep growing behind the finish.

I rent — who is responsible?

Ventilating, heating reasonably and wiping small patches falls to the tenant. Broken extractors, blocked vents, failed sealed units, leaks and building defects causing persistent damp and mould are the landlord's repairing obligations. Report in writing and keep the record.

Caution

Updated 30 August 2026

Condensation Mould On Walls And Window Reveals

Black mould spreading on the window reveal, in the corner of a cold external wall, or behind a wardrobe.

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What's happening?

Condensation mould grows where warm, moist indoor air meets the coldest surface in a room. That is why it appears in the same places in every UK home: window reveals, the corners of external walls, behind furniture pushed against a cold wall, and around a cold lintel. A cleaner will remove it, and that is worth doing — leaving it is not neutral. But cleaning treats the symptom. If it returns within weeks the moisture balance has not changed, and the causes are almost always ventilation, heating or a defect: a blocked trickle vent, an extractor that vents into a loft, washing dried indoors, or an actual leak. Being clear about which you have matters, because the difference between condensation mould, penetrating damp and rising damp decides who fixes it.

What usually causes it?

Warm, moist air condensing on cold surfaces — reveals, wall corners, lintels, and behind furniture
Blocked, closed or painted-over trickle vents removing the background ventilation the house relies on
An extractor fan that is broken, too weak, or ducted into a loft rather than outside
Drying washing indoors or on radiators, adding litres of water to the air every load
Heating used in short bursts, so surfaces never warm above the dew point
Furniture tight against a cold external wall with no air movement behind it
A genuine defect — failed sealed unit, leaking gutter, penetrating damp or a bridged damp proof course

Is it dangerous?

Mould in the home is a recognised health risk. Damp and mould are associated with respiratory infections and with allergy and asthma symptoms, and the risk is higher for babies and young children, older people, and anyone with an existing lung condition or a weakened immune system. Extensive or recurring mould in a rented home is a housing condition, not a housekeeping failure, and should be reported in writing. The cleaning products carry their own risk: never mix a bleach-based mould spray with an acidic or ammonia-based cleaner.

Can I fix it myself?

The cleaning, yes, if the patch is small. Ventilate the room, wipe the growth off with a damp cloth and a suitable mould cleaner rather than dry-brushing it, and dry the surface. Then change the moisture: open trickle vents, run extractors during and after showering and cooking, stop drying washing indoors, pull furniture a few centimetres off cold walls, and keep low background heat rather than short bursts. Do not paint over mould — including with an anti-mould paint — while the surface is still damp, and do not treat a recurring patch as a decorating job.

When should you call a cleaner?

Call a cleaner to remove the growth properly, particularly before a tenancy inspection or a sale, and for the deep clean of affected soft furnishings, curtains and wardrobes people forget. Call them again if it comes back within a few weeks — that is the signal it is not a cleaning problem. At that point it is a damp and ventilation investigation, and if you rent, a repairing obligation for your landlord. Mould between the panes of a window is never cleanable; that sealed unit has failed.

What will the cleaner actually do?

Remove surface growth with an appropriate treatment, wet rather than dry, and ventilate through the work
Clean affected curtains, blinds, soft furnishings and the insides of wardrobes and cupboards
Clean window reveals, sills, frames and gaskets where the growth starts and reseeds
Clean the extractor grille and accessible fan cover so airflow is not the limiting factor
Leave the surfaces dry rather than damp, so growth is not immediately re-established
Say plainly when growth is recurring, extensive or behind a finish, so it is investigated as damp rather than cleaned again

Frequently asked questions

We’re happy to answer all your questions.

Sources & References

Black Mould On Bathroom Sealant And GroutCaution Stained Sofa And MattressLow risk Black Mould On Window Frames And SealsCaution

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