When and how long to air your home in winter to freshen the air without chilling the flat

01The principle

Why short and wide works better than long and narrow

In a flat in winter, the heat is stored mainly in the walls, floor and furniture, not in the air. Air is light and holds very little heat, so swapping it for cold air loses you only a small share of the warmth. The walls and furniture usually do not cool much in a few minutes, and once the window is closed they quickly pass their heat on to the fresh air.

This approach is known as shock ventilation. The aim is to replace as much air as possible in a short time, before the walls and other surfaces have a chance to cool down much. That is why several windows are opened at once and opened fully.

Opening windows on opposite sides of the flat at the same time greatly increases the exchange of air. A through draught then carries air across the whole flat, and less time is needed. If your windows are all on one side, air the rooms a little longer.

A window left permanently tilted open does the opposite. The air is replaced only slowly, and because the window stays open for hours, the stream of cold air gradually chills the wall around it. Water vapour from the air then condenses more readily on the cold wall, and the damp patch can start to grow mould. All the while the heating is running and warmth escapes through the gap.

02Duration

How long to air when no single figure suits every flat

There is no exact time that suits every flat. When there is no wind, the exchange of air is driven mainly by the difference in temperature. Cold air is heavier, so it flows in through the lower part of the window while warm air leaves through the top. The harder the frost, the stronger this flow and the less time you need. The wind also plays a part, as do whether a through draught forms and the size of the windows and the room. It usually takes a few minutes, a little longer in mild weather than in frost.

A simple check is more practical than a stopwatch. Keep airing until the air feels fresh, but stop before the walls and furniture near the window become noticeably colder. If the room quickly returns to its earlier temperature after the window is closed, the timing was right. If it takes a long time to warm up, air it for a shorter time next time.

03Timing

When to air during the day

We breathe and sweat overnight, so by morning the air in the bedroom is stale and damp. Airing in the morning is therefore the most important. Other good moments are after showering, cooking or drying laundry, when humidity rises most. When airing the bathroom or kitchen, close the doors to these rooms, otherwise the moisture will spread through the rest of the flat.

A brief airing before bed freshens the air for the night. Then close the window so the bedroom does not get needlessly cold.

04Checking

How to tell whether you are airing too little or getting it right

A lack of ventilation usually shows first on the windows. Typical signs of damp are excessive condensation around the windows, black mould and a musty smell. Condensation, the turning of water vapour into droplets, forms when moist air meets a cold surface such as a window.

Occasional morning condensation on a window need not be a problem in itself, provided it clears quickly. Warning signs are water that lingers or runs down into the frames, or dark patches appearing in corners and behind furniture against outside walls.

A cheap hygrometer is more reliable than guesswork. It shows relative humidity, meaning how much water vapour the air holds compared with the most it could hold at its temperature. If the reading stays high long after cooking or showering, air more often. Stale air is better detected by a carbon dioxide monitor than by feel, as the gas builds up in a room as people breathe. Without one, judge by whether the air feels heavy or musty when you walk into the room.

05Exceptions

When the general rule does not apply

Keep the door to a cooler bedroom closed. Warm, moist air from the other rooms would quickly condense on its cold walls, and mould could develop. Air the bedroom separately, briefly and with the windows wide open, and keep it at least lightly heated in winter.

An insulated flat with new windows lets little air in through gaps, so moisture builds up faster unless you air deliberately. Here regular short airing matters even more. The exception is a flat with mechanical ventilation, a unit that supplies fresh air and extracts stale air by itself. In that case follow its instructions. If the unit serves only some rooms, keep airing the others through the windows.

An old, draughty window lets air in on its own, but unevenly and with draughts, so regular shock ventilation is still worth doing. If mould keeps coming back despite diligent airing, the cause may lie elsewhere, for instance in leaks, damp masonry or cold spots in the wall where more heat escapes than elsewhere. In that case an expert assessment will help, not longer airing.

On days of heavy smog, keep airing short and choose times outside the rush hour. People who are sensitive to cold or have health problems can air the flat in stages, one room at a time.

06How to

A four-step daily summary

Airing in winter step by step

1. 01

   Turn down the radiator valve

   In the room you are airing, turn down the thermostatic radiator valve so the heating does not warm the outdoors through the window.

2. 02

   Open the windows wide

   Open several windows at once, ideally on opposite sides of the flat, to create a through draught and replace the air quickly.

3. 03

   Wait a few minutes

   In frost a shorter time is enough, in mild weather a longer one. Stop when the air is fresh and the walls are not yet cold.

4. 04

   Close and check the windows

   Close the windows, turn the valve back up and look at the frames. Condensation that lingers for a long time means too little airing or another source of damp.