Seasonal · 4 min read · Updated 6 November 2026
Why mould gets worse in winter: an autumn prevention checklist
Why the cold months bring mould
Warm air holds more water vapour than cold air. When warm, moist indoor air touches a surface that is colder than its dew point, the vapour turns back into water. In summer the walls and windows are warm and the windows are open, so this rarely happens. From autumn onwards the outside temperature drops, external walls and glass get cold, and the same household moisture from cooking, showering, breathing and drying clothes has nowhere to go.
Condensation that sits on a surface for several hours a day, day after day, gives mould spores the damp they need. That is why black spots so often appear in the coldest places first: window reveals, the corners where an external wall meets the ceiling, behind wardrobes on outside walls and in rooms that are rarely heated.
The typical mould calendar
| Time of year | What usually happens | What to do |
|---|---|---|
| September to October | Heating goes on, windows start staying shut, first condensation on glass | Run the checklist below and fix fans and vents |
| November to February | Peak condensation, coldest walls, most indoor drying | Wipe windows daily, keep low background heat, ventilate after showers |
| March to April | Mould that started in winter becomes visible and spreads | Clean and treat early, before it roots into sealant and plaster |
| May to August | Walls warm up, windows open, mould looks dormant | Best time for redecorating with anti-mould paint and fixing causes |
Mould that seems to vanish in summer has usually just stopped growing. The staining and the spores are still there, ready to start again when the weather turns.
Autumn prevention checklist
- Test every extractor fan by holding a single sheet of toilet paper against the grille: it should be pulled in and held
- Clean fan grilles and check run-on timers still work after the light is switched off
- Open all trickle vents and leave them open through the winter
- Check external air bricks are not blocked by soil, ivy or garden furniture
- Bleed radiators and make sure every room, including spare bedrooms, can be heated
- Pull wardrobes, beds and sofas 5 to 10 cm away from external walls
- Clear gutters and check downpipes so rain is not soaking the walls
- Put a cheap digital hygrometer in the bedroom and bathroom
- Plan where washing will dry, ideally outside, in a tumble dryer or in one closed, ventilated room
- Treat any small patches left from last winter now, while the walls are dry
Daily habits that make the biggest difference
Ventilate at the source. Put lids on pans, use the cooker hood on its higher setting while cooking and for a few minutes after, and keep the bathroom door shut with the fan running during and after a shower. Opening a window a few centimetres for ten to fifteen minutes after cooking or bathing clears the moist air faster than leaving it to spread through the house.
Keep a steady low level of heat rather than letting rooms go very cold and then blasting the heating. Cold walls are the problem, and they take a long time to warm up. Many households find that keeping rooms at a moderate background temperature in the coldest weeks causes far fewer condensation problems than heating only in the evening.
Wipe the windows each morning when condensation has formed overnight, and wring the cloth out in a sink rather than drying it on a radiator. A squeegee or a small window vacuum makes this a two-minute job.
Rooms to watch most closely
- Bedrooms, where people breathe out moisture for eight hours with the door shut
- Bathrooms without a working fan, especially internal ones with no window
- Kitchens where the hood recirculates rather than extracting outside
- North-facing rooms and rooms over garages or under flat roofs
- Rooms used to dry washing
- Spare rooms that are left unheated with the door closed
When winter mould is not just condensation
If mould appears in a patch that gets wetter after rain, sits low on a wall, has a tide mark, or comes with crumbling plaster or salts, the cause may be penetrating or rising damp rather than condensation. Those need the building repaired, not just better ventilation. If mould returns within weeks of a thorough clean even though you are ventilating well, it is worth getting moisture readings and the cause checked properly before winter sets in again.
Costs of getting ready for winter
Most autumn prevention is cheap compared with treating mould across several rooms in spring. These are typical guide figures and vary by home and region.
| Item | Typical UK cost |
|---|---|
| Digital hygrometer | £10 to £25 |
| Window squeegee or window vacuum | £5 to £40 |
| Replacement bathroom extractor fan, fitted | £150 to £350 |
| Compressor or desiccant dehumidifier | £120 to £300 |
| Gutter clearing for a typical house | £60 to £150 |
If the same rooms get mould every winter, spending on a better fan or a dehumidifier once usually costs less than repeated cleaning and redecorating.
Frequently asked questions
Should I open windows in winter to stop mould?
Yes, but briefly and in the right places. Short bursts of ventilation after cooking, showering or first thing in the morning remove moist air without cooling the walls much. Leaving windows wide open all day mainly chills the room and can make condensation worse once they are closed. Trickle vents can stay open all the time.
Does a dehumidifier help in winter?
A dehumidifier can be very effective in a flat or a room where you dry washing, because it removes water rather than just moving air around. Desiccant models tend to work better than compressor models in cold, unheated rooms. It works best alongside working extractor fans, not instead of them, and it will not fix a leak or penetrating damp.
Why does mould only appear on one wall?
Mould usually picks the coldest surface. That is often an external wall, a corner where two external walls meet, a wall with a cold bridge such as a concrete lintel, or a wall behind furniture where warm air cannot circulate. If one wall stays wet in dry weather or worsens after rain, have it checked for penetrating damp.
winter condensation checklist prevention
