Opening the windows in winter feels like throwing money out of them. In reality the air in a home holds surprisingly little heat: most of it is stored in walls, floors and furniture. Air it the right way and the cost is small — usually much smaller than the cost of damp walls and mould.
Warming one cubic metre of air by one degree takes about 0.34 Wh. A 70 m² flat with 2.5 m ceilings holds 175 m³ of air. Replacing all of it with 5 °C outdoor air and warming it back to 20 °C takes about 0.9 kWh — roughly what an electric oven uses in half an hour. Try it with your own numbers:
Calculation: volume × 0.34 Wh per m³ per °C × temperature difference, assuming each short, wide-open airing replaces the air once. Heat stored in walls and furniture is not counted. Enter the price of heat in your own currency (for a gas boiler, roughly the gas price per kWh divided by the boiler efficiency).
The colder the winter, the more heat each full air change takes. Here is the coldest month in 12 cities, for the same 70 m² home:
| City | Coldest month | Mean temperature | Heat for one full airing* |
|---|---|---|---|
| London | January | 5 °C | 0.92 kWh |
| Manchester | January | 4 °C | 0.95 kWh |
| Edinburgh | January | 4 °C | 0.94 kWh |
| Dublin | January | 6 °C | 0.84 kWh |
| New York | January | 1 °C | 1.15 kWh |
| Chicago | January | -4 °C | 1.40 kWh |
| Boston | January | -1 °C | 1.26 kWh |
| Toronto | January | -4 °C | 1.41 kWh |
| Montreal | January | -9 °C | 1.70 kWh |
| Melbourne | July | 10 °C | 0.60 kWh |
| Berlin | January | 2 °C | 1.06 kWh |
| Paris | January | 5 °C | 0.92 kWh |
*Energy to reheat the air of a 70 m² home (2.5 m ceilings) from the mean outdoor temperature to 20 °C, after one complete air change. Temperatures: Open-Meteo historical data 2021–2025.
A closed home in winter fills with moisture from breathing, cooking and showers. Damp walls conduct heat better than dry ones, and condensation and mould appear. Treating mould and repainting costs far more than a few cents of heat a day. That is why energy agencies across Europe recommend short, wide airing several times a day rather than keeping windows shut.
OpenWindow.live tells you in real time whether it is a good moment to open the windows where you are, based on outdoor temperature, humidity and air quality.