Why your bedroom is still hot at midnight

The room was fine at nine in the morning. It is now half eleven at night, the outside air is pleasant, and the bedroom is still uncomfortable.

This is not your imagination and it is not a badly built house. It is a house built to do exactly this.

Thermal mass, working against you

British housing is heavy. Brick, block, plaster, concrete. All of it stores heat, and all of it releases that heat slowly.

In winter this is the point. The fabric absorbs warmth during the day and gives it back through the evening, which is why a solid brick house feels steady rather than draughty.

In summer the same physics runs in the same direction and you do not want it to. The structure absorbs heat all day, then spends the night radiating it back into the room. The air outside cools; your walls do not, or not nearly as fast.

A top floor bedroom is the worst case, because it has the roof above it. Roof tiles in direct sun get far hotter than the air, and a loft conversion puts a bedroom directly underneath that with, quite often, the insulation that was specified for winter heat loss and nothing else.

The nights are not yet the problem, and that is worth knowing

This site stores the daily record back to 1950 for twenty five British towns. On tropical nights, meaning nights whose minimum stays above 20 degrees, the count is still low everywhere. Brighton leads at 0.8 a year this decade, with Southampton and Portsmouth at 0.7. London manages 0.2, which is one every five years, and nine of the twenty five have effectively none at all.

The south coast beating London is not a mistake. The sea holds its heat through the evening and gives it back after dark, so a Brighton flat has a harder night than a London one.

That is genuinely good news, and it is the opposite of what most coverage implies. The overnight cooling that British houses rely on still works, most of the time, in most of the country.

What has changed is the days. London has gone from 3.3 days a year above 25 degrees in the 1960s to 17.7 this decade, and Cambridge from 3.2 to 20. More hot days means more heat pushed into the fabric, and a structure that starts each night warmer than it used to.

The mechanism has not broken. It is being asked to do more.

What actually works, in order

Stop the heat getting in. Close curtains and blinds on any window the sun is on, before it is on it, not after. External shading beats internal shading by a distance, because internal blinds absorb the energy on the inside of the glass and re-radiate it into the room.

Ventilate on the right schedule. Open up when the outside air is cooler than the inside air, and not before. In a British summer that is usually from mid evening. Windows open through a hot afternoon is heating, not cooling.

Get air moving across you. A fan does not cool a room, it cools a person. It also adds a little heat from the motor. Both things are true and the fan is still worth having.

Cool the person, not the building. A damp flannel on the wrists or the back of the neck, a cooler shower before bed, lighter bedding. Cheap, immediate, and it works on the actual problem, which is you rather than the plaster.

Then, if the load justifies it, consider a machine. The room cooling calculator will tell you the number. It will also tell you when the number is low enough that shading and ventilation are the better purchase.

When heat stops being a comfort question

Heat kills people in Britain every summer, and the deaths are concentrated in groups that do not always feel unwell first.

Confusion, a headache that will not go, cramps, nausea, hot dry skin or not passing urine are worth acting on rather than sleeping off. Older people, very young children, and anyone taking medication that affects how the body handles temperature are at higher risk than they feel.

If you are worried about somebody, contact NHS 111 or a doctor. That is not advice this page can give you.