Which rooms actually cost the most to heat

Most people assume the largest room is the most expensive to heat. Sometimes it is. More often the expensive room is a smaller one with a particular combination of problems.

Surface area, not volume

Heat leaves through surfaces that face outside. A room with three external walls loses far more than a room of identical size with one, and an upstairs room under an uninsulated roof loses more again.

This is why a small bedroom in a corner of the house can cost more to hold at temperature than a large central living room. The living room is surrounded by other heated rooms; the corner bedroom is surrounded by weather.

Glazing

Glass is the weakest part of almost every wall. A room with a large window, a patio door or a bay is losing heat through the worst-performing surface in the house, continuously.

Older double glazing is not equivalent to new double glazing — sealed units degrade, and a failed unit performs closer to single glazing while looking identical.

Air movement

The third factor is the one people underestimate. A room with a draught is not merely losing heat slowly; it is continuously replacing warmed air with cold air, which is far more expensive than conduction through a wall.

Hallways with external doors, rooms with old chimney openings and anywhere with a letterbox tend to be the worst offenders.

What follows from this

The useful conclusion is that heating the whole house to one temperature spends most of the money on the rooms that leak fastest, regardless of whether anyone is in them.

Closing doors and heating occupied rooms harder is not merely a folk remedy — it directly targets the mechanism that costs the most.