Why airing is so effective in winter
It sounds paradoxical: it is foggy outside, the air feels wet — and yet airing is supposed to dry your home? The explanation lies in the difference between relative and absolute humidity. Cold air can hold only little water. Even when it is almost saturated, it contains little water vapour in absolute terms.
An example: outdoor air at 0 °C and 85% relative humidity contains about 4.1 g of water per cubic metre. When this air is warmed to 20 °C in your home, the amount of water stays the same, but the capacity rises to about 17.2 g/m³. The relative humidity of the fresh air therefore drops to about 22%. At −5 °C and 80% outside it is only about 14% after warming. The stale indoor air at 20 °C and 50%, on the other hand, contains about 8.6 g/m³ — so every cubic metre exchanged carries several grams of water outside.
How often and how long to air?
For the cold season, Germany's Environment Agency (UBA) recommends airing two to three times a day for about five minutes per room with windows wide open. Bedrooms should be aired for five to ten minutes in the morning after getting up. After cooking and showering: open the windows wide during or immediately afterwards for five to ten minutes.
In the warm season, air exchange takes longer because the temperature difference between inside and outside is small. Here the UBA recommends ten to twenty minutes per airing.
Burst ventilation instead of tilted windows
A permanently tilted window exchanges little air but strongly cools the wall around the window. The UBA explicitly points out that this significantly increases energy consumption and can cool the window reveal so much that mould forms there. Short, intensive burst ventilation with windows wide open is better; cross-ventilation through opposite windows exchanges the air even faster. Walls and furniture hardly cool down and quickly release their heat to the fresh air afterwards.
Close the radiator valves while airing so the thermostat does not heat against the cold air. Then turn them back to the usual setting.
Don't heat rooms via other rooms
A common mistake: the cool bedroom is not heated but warmed “on the side” through the open door to the living room. The UBA advises against this, because moisture travels into the cool room along with the warm air. There, relative humidity rises and the mould-critical limit of 80% surface humidity is quickly reached at the cold exterior walls. Keep doors to cooler rooms closed and heat each room on its own.
When airing does not dry
In summer the calculation can reverse. Warm outdoor air at 28 °C and 60% contains about 16.3 g of water per cubic metre. If it flows into a cellar at 14 °C, the cooler air there can only hold about 12 g/m³ — the rest condenses on walls and objects. It is therefore better to air cellars in summer during the cool night and early morning hours.
Whether airing dries right now is shown by the ventilation check in the dew point calculator: it compares the absolute humidity indoors with the outdoor air warmed to room temperature.
The hygrometer as a check
According to the UBA, a simple hygrometer helps keep an eye on indoor humidity. As a guideline, the agency recommends 40 to 60% relative humidity. If the value stays above that despite regular airing, it is worth looking for additional sources of moisture — drying laundry indoors, many houseplants or an aquarium. With poorly insulated walls, even a value in the upper range can be critical; more on this in the article Mould on exterior walls.
In a nutshell
1. In winter, airing almost always dries — even when it rains or is foggy.
2. Burst-ventilate two to three times a day for about five minutes; bedrooms five to ten minutes in the morning.
3. Air immediately after showering and cooking; avoid tilted windows.
4. Don't heat cool rooms via warm neighbouring rooms.
5. Air cellars at night or in the morning in summer.