The first cold snap does two things at once: it chills the outside of your windows and sends you reaching for the thermostat. By morning there's a film of water on the glass, and within a fortnight the first grey speckles are showing in the corner of the window reveal. This isn't a housekeeping failure. It's a moisture balance that has tipped the wrong way — and it's fixable.
Why condensation arrives with the cold
Warm air holds more water vapour than cold air. When that air meets a cold surface — a single-glazed pane, a north-facing wall, the top of a bathroom ceiling — it cools and can no longer hold the moisture, so it releases it as droplets. Mould spores are already in the air; give them a damp surface and they settle in.
Modern homes are especially prone. Double glazing, draught-proofing and loft insulation keep heat in, which is what we want, but they also keep moisture in. A household of four adds litres of water to the air every day through breathing, cooking, showering and drying washing. If it can't escape, it lands on the coldest surface it can find.
Ventilate on purpose, not by accident
Short bursts of vigorous ventilation beat leaving a window ajar all day, which just wastes heat. Try these:
- Open windows wide for ten to fifteen minutes, morning and evening, then close them. Open one at each end of the house if you can — a cross-draught clears the air fastest.
- Keep trickle vents open all winter. They provide background ventilation without noticeably cooling the room.
- Run the kitchen extractor every time you cook, and leave it going for ten minutes afterwards.
- In the bathroom, run the fan throughout your shower and for at least fifteen minutes after it, with the window on the latch.
- Test the bathroom fan. Hold a sheet of tissue against the grille; if it isn't held firmly, the fan is weak or blocked. Clean the grille in warm soapy water — it makes a surprising difference.
Heat steadily rather than in bursts
Surface temperature matters as much as air temperature, because warm damp air gives up its moisture to the coldest thing in the room. A house that's frozen all day and then blasted with heat at six o'clock is a recipe for condensation: the walls are still cold when the wet air arrives.
- Aim for a steady 18–20°C in living rooms, and resist switching the heating off completely when you go out. A low background setting keeps walls and floors warmer.
- Keep radiators clear. Sofas, long curtains and airers in front of them steal the warmth that would otherwise reach the wall.
- Close the kitchen and bathroom doors while you cook and shower, so steam doesn't drift into cooler rooms. Open them again afterwards.
Cut the moisture at the source
Before buying a dehumidifier, look at what's putting water into the air.
- Put lids on pans. A simmering pot with the lid off sends a steady plume of steam into the kitchen.
- Dry washing outside when you can. If you can't, use a vented tumble dryer, or one with a condenser that you empty and clean properly. Washing draped over radiators releases every drop into the room.
- Hang wet towels and bath mats out to dry rather than leaving them in a heap.
- Squeegee the shower screen and wipe the tiles after the last shower of the day.
- Empty and refill a dehumidifier regularly — a full tank does nothing — or use crystal moisture traps in wardrobes, cupboards and the airing cupboard.
Give the cold spots a hand
Some surfaces will always run cooler, and they need a bit of help.
- Pull wardrobes and beds about 5cm away from external walls so air can move behind them.
- In the bedroom, wipe the window and sill each morning. A minute with a cloth now saves a weekend of scrubbing later.
- Secondary glazing film on single-glazed windows, and thermal linings on curtains, both keep glass and the wall around it warmer.
- Leave the bathroom fan running after a shower and the window open for a quarter of an hour. Wipe down the ceiling and the top of the walls weekly so any film of moisture doesn't sit there.
If mould has already appeared
Check for other causes first. A leaking gutter, a slipped roof tile, a dripping overflow or a plumbing leak all look like damp, and no amount of ventilation will fix them. Damp patches low on a solid wall, or plaster that feels cold and wet rather than the air around it, are worth a professional opinion.
For surface mould on paint, grout or sealant: ventilate the room, wear gloves, and don't brush it dry — that flicks spores into the air. Wipe with a damp cloth and a cleaner suitable for the surface, then rinse and dry the area, and wash the cloth afterwards. Check labels before using bleach-based products on sealant, as some can be damaged by them. Sealant that has gone black inside usually needs replacing rather than cleaning.
If mould returns within a few weeks of cleaning, or covers more than a few small patches, ask a qualified damp and timber surveyor to investigate. If you rent, report it to your landlord in writing and keep a copy. Where someone in
Photo: Dimhou / Pixabay
