Airing a Room Costs Less Heat Than You Expect

Ventilating in winter feels wasteful, but a short wide opening loses far less heat than a window left slightly open. The difference is what actually stores the heat.

Air holds very little heat

The air in a room carries a small fraction of the heat energy present. Most of it is stored in the walls, floor, furniture and everything solid.

Replacing the air therefore removes a small amount of heat, and the solid mass reheats the incoming air quickly once the window is shut.

This is why a room feels warm again within minutes of closing a window that was fully open, provided the opening was brief.

A window left ajar cools the fabric

A small permanent opening exchanges air slowly but continuously, and over hours it draws heat out of the walls and furniture rather than only out of the air.

Once the fabric has cooled, reheating it takes far longer and costs far more than reheating a room's worth of air.

The heating system also runs continuously against the loss, which is the mechanism by which a slightly open window costs more than a briefly open one.

Humidity is the reason to ventilate at all

A person living in a flat produces a substantial amount of moisture through breathing, cooking, washing and drying clothes.

Without ventilation that moisture accumulates, and it condenses wherever a surface is cold enough, which produces the mould found in corners and behind furniture.

Removing humid air and replacing it with cold outside air, which holds less moisture, is the only mechanism that reduces indoor humidity in winter.

Cross ventilation shortens the time needed

Opening windows on two sides of the flat creates a through-draught that replaces the air in minutes rather than tens of minutes.

A short, vigorous exchange is what the approach depends on, since the aim is to change the air before the solid surfaces have cooled.

Turning radiator valves down during the exchange prevents the heating from running against an open window, which is the only real waste in the process.

Timing it around the sources

The most useful moments are immediately after showering, during and after cooking, and while clothes are drying indoors, because that is when moisture is being released.

Closing the door of the room where moisture is produced contains it, so the ventilation deals with one room rather than with the whole flat.

A morning exchange also clears the moisture that accumulates overnight in a bedroom, which is why bedroom windows show condensation before other rooms do.