In winter most homes have the opposite problem to summer: not too much heat, but too much moisture. Breathing, cooking, showering and drying laundry release water vapour every day, and in a closed, heated home it has nowhere to go. It ends up as condensation on windows and cold walls — and, after a few weeks, as black mould in corners and behind furniture.
The fix is counterintuitive: open the windows, even when it is cold and wet outside. Done the right way it takes a few minutes a day and costs very little heat.
The humidity in a weather forecast is relative humidity: how full the air is compared with how much water it can hold at that temperature. Cold air can hold very little water. Air at 0 °C that is completely saturated (100%) carries so little vapour that, once it is warmed to 20 °C inside your home, its relative humidity drops to about 26%.
So even on a foggy winter day, every minute of fresh air replaces moist indoor air with air that becomes dry as soon as it warms up. This is why winter airing works in rainy cities like London or Manchester too.
Target: keep indoor relative humidity between about 40% and 60%. If a hygrometer shows more than 60% for long periods, or windows fog up every morning, you need more airing.
Short, wide airing barely cools walls and furniture, which store most of the heat in a room. The air warms up again within minutes after you close the window.
In winter the best moment is usually the mildest hour of the day, in the early afternoon, when the outdoor air is least cold. There is one exception: on smog days. In many cities fine particles from heating and traffic peak in the evening and at night, so a quick airing in the middle of the day is usually cleaner.
Here is what the data says for the coldest month in 12 cities:
| City | Coldest month | Avg high | Humidity | Mildest hour | Cleanest air (day) |
|---|---|---|---|---|---|
| London | January | 7 °C | 87% | 15:00 | 16:00 |
| Manchester | January | 7 °C | 87% | 15:00 | 15:00 |
| Glasgow | January | 6 °C | 88% | 15:00 | 15:00 |
| Dublin | January | 8 °C | 83% | 15:00 | 14:00 |
| New York | January | 4 °C | 68% | 15:00 | 15:00 |
| Chicago | January | -0 °C | 76% | 15:00 | 08:00 |
| Boston | January | 3 °C | 68% | 15:00 | 15:00 |
| Toronto | January | -1 °C | 75% | 15:00 | 08:00 |
| Montreal | January | -5 °C | 79% | 16:00 | 08:00 |
| Melbourne | July | 13 °C | 80% | 15:00 | 16:00 |
| Paris | January | 7 °C | 86% | 16:00 | 16:00 |
| Berlin | January | 5 °C | 85% | 15:00 | 15:00 |
Averages for the coldest month, Open-Meteo historical data 2021–2025 (air quality 2023–2025), local time. Click a city for its full month-by-month table.
Keep furniture a few centimetres away from outer walls so air can circulate, and do not block radiators with curtains or sofas. If mould keeps returning even with regular airing, the cause may be a structural problem (a cold bridge or water infiltration) that ventilation alone will not solve.
OpenWindow.live checks outdoor temperature, humidity and air quality for your location in real time and tells you whether now is a good moment to open the windows.