If you have cleaned mould off your walls and it has come back within weeks, you are not doing anything wrong. Mould comes back because the conditions that caused it have not changed. Cleaning removes the visible growth but leaves spores in the wall surface and in the air of the room, and as soon as the temperature drops and condensation returns, those spores germinate and the mould grows back. The only way to permanently stop mould returning is to remove the conditions it needs to grow.
Why mould keeps coming back — the cycle
Mould needs three things: moisture, warmth and something to grow on. In UK homes the something to grow on is never in short supply — plaster, paint, wallpaper, timber, grout and sealant all support mould growth. Warmth is present for most of the year. The variable is moisture. In a home with condensation problems, moisture appears on cold wall surfaces every night and on cold days throughout winter. Clean the mould on Monday, and by the following weekend the spores that remained in the surface have germinated again in the moist conditions.
Step one — clean it properly first
Before addressing the cause you need to remove the existing growth properly. Painting over mould does not kill it — it continues to grow under the paint and comes through within weeks. Wiping with a damp cloth spreads spores across a wider area.
Use a mould-specific fungicidal wash or a diluted bleach solution on non-porous surfaces — follow the product instructions and wear gloves. For porous surfaces like bare plaster and grout, a fungicidal solution needs to be left in contact long enough to kill the mould at the root, not just on the surface. After cleaning, allow the surface to dry completely before painting or decorating. Use a fungicidal paint or primer for any redecoration in areas that have had mould problems.
Step two — find out what type of damp it is
Condensation mould and structural damp need different solutions. Applying a damp proof injection to a condensation problem wastes money and does nothing. Improving ventilation in a house with penetrating damp from a leaking roof wastes time and does nothing.
- Condensation mould appears at high level on walls and ceilings, in corners, on windows and behind furniture. It is caused by warm moist air meeting cold surfaces.
- Rising damp appears as a tide mark at low level on ground floor walls, typically up to about a metre high. It may leave white salt deposits on the surface.
- Penetrating damp appears after heavy rain, often on a specific wall. It may follow the line of a crack, a failed joint or a roof defect.
See: Condensation mould vs penetrating damp — how to tell the difference →
Step three — fix the ventilation
For condensation mould — the most common type — improved ventilation is the primary long-term fix. The goal is to remove moist air from the home before it can condense on cold surfaces.
- Make sure extractor fans in the bathroom and kitchen work properly and are used every time the room produces steam. A fan that runs for 15 minutes after a shower removes far more moisture than one that is turned off as soon as you leave the bathroom.
- Open trickle vents on double-glazed windows — they exist specifically to provide background ventilation and are frequently closed by occupants who want to reduce draughts.
- In rooms with persistent mould, a passive air brick or a small continuous extract fan provides background ventilation without heating bills rising significantly.
- In severe cases, a whole-house mechanical ventilation with heat recovery (MVHR) system replaces moist stale air with fresh air without losing the heat from the outgoing air. This is a longer-term investment but eliminates condensation permanently in well-insulated homes.
Step four — warm the cold surfaces
Cold wall surfaces cause condensation regardless of ventilation. In a poorly insulated home, external walls and corners will always be cold enough to condense moisture on cold winter nights. Insulating external walls — either externally or internally — raises the surface temperature of the wall and reduces condensation dramatically. Even a thin layer of thermal insulation board on a problem wall can make a significant difference in a room with persistent mould.
At a minimum, make sure rooms are heated consistently rather than intermittently — a low background temperature maintained throughout the day and overnight is less likely to cause condensation than a room that is very warm for a few hours and then very cold for the rest of the time.
Step five — reduce moisture production
The less moisture you add to the air, the less there is to condense. Drying clothes indoors, unvented tumble dryers, cooking without lids and long hot showers all add substantial moisture to the air of a home. See: How to reduce condensation in your home →
If you are a tenant and mould keeps returning
If you have followed all the steps above and mould continues to return, the problem may be structural — inadequate insulation, failed cavity wall insulation, cold bridging through the structure, or defective ventilation systems that are your landlord's responsibility to maintain. In England, landlords have legal obligations under Awaab's Law and the Decent Homes Standard to investigate and remedy persistent damp and mould. See: My landlord isn't fixing the damp — what are my rights →
