Black mould on walls: why it grows and how to stop it coming back
Mould keeps coming back because cleaning removes the growth but not the conditions. Those conditions are almost always the same: a surface that stays cold enough, for long enough, to sit at high humidity.
Mould doesn't need visible water
Common household moulds can grow when the humidity right at a surface stays above about 80% for long periods. That's why BS EN ISO 13788, the standard used to assess building surfaces, uses 80% surface humidity as its mould criterion rather than 100% (condensation).
Because cold surfaces raise the local humidity, a room reading of 60–65% can mean 80–90% in a cold corner. For example, air at 18 °C and 65% humidity reaches 80% at a surface of about 14.7 °C. An uninsulated solid-wall corner can easily be colder than that on a winter night.
Why it appears where it does
- Corners and wall–ceiling junctions: more outside surface per inside surface, so they lose heat faster (a "thermal bridge"), and air circulation is poor.
- Behind wardrobes, beds and sofas: furniture insulates the wall from the room's warmth. The hidden wall can be several degrees colder than the open wall next to it.
- Window reveals and above lintels: often uninsulated even when the wall is insulated.
- Bathroom ceilings and grout: repeated soaking from showers with too little extraction.
- North-facing rooms and bedrooms: little sun, often heated less, and occupied all night.
Cleaning mould safely
Small areas (roughly up to 1 m²) of surface mould caused by condensation can usually be cleaned by the occupant. NHS and UK Health Security Agency guidance suggests:
- Wear rubber gloves, goggles and a mask, and open a window. People with asthma, allergies or other breathing problems, and children, shouldn't do it themselves.
- Wipe with a cloth dampened with water and a little washing-up liquid, or a fungicidal mould remover used according to its label. Don't brush dry mould — it releases spores.
- Dry the area thoroughly afterwards and bag the cloth.
- Don't mix cleaning products, especially bleach with anything else.
- Mouldy soft furnishings and clothing should be washed or, if badly affected, discarded.
Larger areas, mould that returns quickly after cleaning, or mould caused by a leak or flooding need the underlying cause fixed and may need professional cleaning. Painting over mould, even with anti-mould paint, only delays its return if the conditions stay the same.
Stopping it coming back
You need to change at least one of the three factors: less moisture, more ventilation, or warmer surfaces.
| Action | Effect | Cost |
|---|---|---|
| Move furniture 5–10 cm away from external walls | Warms the hidden wall by letting room air reach it | Free |
| Dry laundry outdoors or in a vented/heat-pump dryer | Removes one of the biggest indoor moisture sources | Free to low |
| Keep trickle vents open; run extract fans longer | Removes moisture continuously | Low |
| Keep problem rooms heated steadily rather than cold all day | Raises wall temperature | Medium |
| Dehumidifier in the worst room | Lowers humidity | See running costs |
| Insulate the cold wall, reveal or loft edge | Lasting fix for cold surfaces | High — get advice first |
The full assessment estimates the temperature behind your furniture, in corners and on open wall, and shows which of these lowers the humidity at those surfaces most.
A note on insulation
Insulation is often the lasting fix for a cold wall, but it needs care. The Energy Saving Trust and Historic England both advise resolving existing damp before insulating, and checking that the system suits the wall — solid brick and stone walls in older houses handle moisture differently from modern cavity walls. Badly detailed internal insulation can move condensation to the junctions around it.
Health and responsibility
The UK Health Security Agency's guidance on damp and mould is clear that it can harm anyone's health, particularly respiratory health, and that tackling it shouldn't be framed as a lifestyle issue for occupants. If you rent, report it to your landlord in writing — see your rights. If you have symptoms, speak to your GP.