The instinct in winter is to close everything up and keep the heat in. It makes sense on the heating bill but it is the single biggest contributor to condensation and mould in UK homes. Warm moist air with nowhere to go finds the coldest surfaces — windows, external wall corners, ceiling edges — and deposits moisture on them. The solution is not to choose between warmth and fresh air but to ventilate smartly — removing moist air efficiently without losing unnecessary heat.
Why ventilation matters more in winter
In a typical UK home in winter, a family of four adds around 10 to 12 litres of water vapour to the air every day — from breathing alone, before cooking, showering, washing up or drying clothes are factored in. In summer that moisture escapes through open windows and doors. In winter, with everything closed, it has to go somewhere and that somewhere is the cold surfaces of your home.
Good winter ventilation is not about having cold draughts — it is about controlled removal of moist air and replacement with drier outside air. Outside air in winter, even when it feels damp, is typically less moisture-laden than the air inside an occupied home once it is warmed to room temperature.
Trickle vents — the easiest starting point
If your home has double-glazed windows installed in the last 20 years, it almost certainly has trickle vents — small slots at the top of the window frame with a sliding cover. These provide continuous low-level background ventilation even when the window is closed. Many people close them in winter to reduce draughts, which removes the background ventilation the building was designed to have.
Open your trickle vents and leave them open through winter. The heat loss from a trickle vent is minimal compared to the benefit of continuous low-level air exchange. In rooms with persistent mould, open trickle vents are one of the simplest and cheapest interventions available.
Extractor fans — use them properly
The bathroom extractor fan is the most important ventilation component in most homes. A shower produces around 200 grams of water vapour in 8 minutes — all of which ends up in your bathroom air and then migrates through the house if the bathroom door is open.
- Run the extractor fan throughout your shower and for at least 15 minutes afterwards
- Keep the bathroom door closed while showering — this contains the steam and gives the extractor fan a chance to remove it before it spreads
- If your fan is old and slow, consider replacing it with a higher-flow model or one with a humidity sensor that runs automatically when moisture levels rise
- Check the fan actually vents outside — some fans installed in flats recirculate air through a carbon filter rather than exhausting outside, which removes odour but not moisture
The same principle applies in the kitchen. Cook with lids on pans. Use the cooker hood when frying or boiling. If your hood recirculates rather than exhausting outside, open a window while cooking even in winter.
Brief purge ventilation — opening windows strategically
You do not need to leave windows open all winter to ventilate effectively. Brief purge ventilation — opening windows fully for 5 to 10 minutes at a time — removes moist air from a room far more efficiently than leaving a window slightly ajar for hours.
The best times to purge ventilate in winter:
- First thing in the morning after everyone has been in the house overnight — bedrooms and living areas will have accumulated moisture from breathing
- Immediately after a shower or bath
- After cooking
- After drying clothes indoors if unavoidable
Open two windows on opposite sides of the house for cross-ventilation if possible — this moves air through the building faster than opening a single window.
Dehumidifiers — targeted moisture removal
A dehumidifier removes moisture from the air directly and is particularly useful in rooms where condensation is a persistent problem despite good ventilation habits. Modern dehumidifiers are quiet and efficient. Running one in a bedroom overnight — where 8 hours of breathing adds significant moisture to a closed room — can make a substantial difference to morning condensation on windows and walls.
Set the dehumidifier to maintain 50 to 60 percent relative humidity. Below 40 percent the air becomes uncomfortably dry. Above 65 percent mould growth conditions are present. A good dehumidifier will cycle on and off to maintain the target level rather than running continuously.
Whole-house mechanical ventilation — the permanent solution
For homes with serious, persistent condensation problems that cannot be resolved by the measures above, mechanical ventilation with heat recovery (MVHR) is the permanent solution. An MVHR system extracts stale moist air from wet rooms (bathroom, kitchen, utility) and supplies fresh air to living rooms and bedrooms. The heat exchanger in the unit recovers 70 to 90 percent of the heat from the outgoing air and transfers it to the incoming fresh air, so the ventilation system does not significantly increase heating costs.
MVHR is most effective in well-insulated, reasonably airtight homes — it is standard in new builds to Passivhaus and Future Homes Standard specifications. In older draughty properties it is less effective because uncontrolled air infiltration through gaps in the building fabric bypasses the heat exchanger.
Simple winter ventilation checklist
- Trickle vents open on all windows throughout winter ✓
- Bathroom extractor fan used every shower, runs for 15 minutes after ✓
- Bathroom door closed while showering ✓
- Cooker hood used when cooking, or window open if hood recirculates ✓
- Brief purge ventilation first thing in the morning ✓
- Clothes dried outside or in a vented tumble dryer where possible ✓
- Furniture moved away from external walls ✓
- Consistent background temperature maintained overnight ✓
Why mould grows in autumn and winter →
How to reduce condensation in your home →
