Fixing Condensation and Black Mould in UK Flats

Condensation Is Not Rising Damp

This is the single most common confusion in UK housing. Rising damp affects ground-floor walls near floor level; condensation affects any room, at any height, and is caused by warm moist air meeting a cold surface — most often in flats with poor ventilation, single-glazed windows, or homes where drying laundry indoors is the only option.

Where and Why It Happens

• Bathrooms — steam from showers/baths with inadequate extraction
• Bedroom corners and behind wardrobes — cold external walls with poor air circulation trap moist air against the surface overnight
• Kitchens — cooking steam without extraction
• Window reveals — single glazing or poorly insulated frames are the coldest surface in a room, so moisture condenses there first

Immediate Fixes

• Run bathroom and kitchen extractor fans during and for 15-20 minutes after showering or cooking — not just during
• Open trickle vents on windows if fitted; do not tape or block them, a common but harmful habit in cold weather
• Dry laundry outdoors or in a room with an extractor fan and closed door, never on radiators in an unventilated room
• Wipe down window reveals and sills each morning if condensation has formed overnight — this alone prevents most mould growth

Treating Existing Mould

• Small patches: clean with a dedicated mould and mildew spray, not just bleach diluted in water, which often only bleaches the colour without killing the spore root
• Always wear gloves and ventilate the room while treating mould
• After treatment, apply an anti-mould paint additive or specialist bathroom paint to the affected area to slow recurrence
• Large or recurring areas of black mould, especially with a musty smell that persists after cleaning, may indicate an underlying ventilation or insulation problem worth a professional assessment

Longer-Term Prevention

• Consider a Positive Input Ventilation (PIV) unit for flats with persistent whole-property condensation — these draw in filtered fresh air and gently pressurise the home to push moist air out
• A standalone dehumidifier is a practical short-term fix in a single problem room while you address ventilation more permanently
• If secondary glazing is out of reach, thermal curtain liners meaningfully reduce condensation on single-glazed windows by insulating the glass surface

When It's Not Condensation

If mould appears in a straight horizontal band roughly 1 metre up an external wall on the ground floor, or if it's accompanied by crumbling plaster or salt staining, treat this as possible rising or penetrating damp instead — see our separate rising damp guide.