Keep the heat out before it arrives

Almost all of this is won or lost before the heat is in the room.

Once a flat is hot, you are trying to cool air, and air is a poor thing to fight — it keeps arriving, and the walls and floor have already taken a charge of heat that they will hand back all evening. Keep the sun off the glass at seven in the morning and none of that starts. Keep the sunlight off the glass, shut the place up while the outside air is hotter than the inside, and open it again only when outside is genuinely cooler.

The sun, the glass, and the one test that governs everything

The single highest-value move is to stop sunlight before it gets through a window, and to do it on the outside if you possibly can — an awning, a board propped against the glass, a sheet on a line. Sunlight stopped outside never becomes heat in your room. Stopped by a curtain, it already has.

Inside, light colors beat dark ones, which is the opposite of what most people reach for: a black blackout curtain does block the light, and then it holds the heat it absorbed and radiates it back into the room all afternoon. White, or white-backed, is the version that works.

Then the rule that runs the whole day, and the test that overrides every version of it you have heard: windows and blinds shut while it is hotter outside, open when it is cooler outside — and “cooler” means actually cooler, not later in the evening. People run this on the clock and lose, because a warm evening is still warmer than the room they spent all day keeping cool, and opening up undoes it. If you open up at night, put a fan high in one window blowing out and open a low window elsewhere: hot air leaves at the top, cool air is drawn in at the bottom, and the building empties itself. That principle is older than electric fans — it is what tall windows and transoms were for. If insects are the reason you can’t do any of this, screens are not a nicety, they are the thing that unlocks the method.

Cool the person, not the room

When it is already hot, switch targets. You are much smaller than your flat and much easier to cool.

Cold water on the places where blood runs near the surface does the most work for the least effort: wrists, the back of the neck, the inside of the elbows, behind the knees, and feet in a basin of cool water. A cool bath before bed lowers you further and holds longer than a shower. Wet cloths from the freezer work as cooling packs, and so does the blunt version: ice, cold drinks, something frozen to hold. If the bed itself is the problem, people hang a sheet or blanket overhead and drape cloth around the sleeping space to trap a pocket of cooler air — thicker is better, and cardboard works where fabric runs out. A mist of water plus moving air is often enough to get back to sleep, which is sometimes the entire problem that needed solving.

Two honest limits on the tricks that get repeated most. Anything that works by evaporation — a damp sheet, a wet towel in front of a fan, a home-made cooler — depends on dry air. In humid weather the air is already close to full and those methods do little, and can leave you in a room that is both hot and clammy. And the bag of ice in front of the fan really does cool the corner you’re sitting in, but the freezer that made the ice put that heat straight back into the kitchen. It is a way of moving comfort to where you are sitting, not a way of cooling a home.

The building you actually have

Here is where the standard advice quietly stops applying, and it is worth knowing before you decide the method failed or you did.

Sealing up assumes your building can hold the cool. A well-insulated house behaves like a flask and holds the night air you trapped. A solid brick or adobe house with no insulation does the opposite on purpose: it absorbs heat all day and releases it after dark, and shutting that building up overnight bakes you. Some houses were built to shed heat in the evening so people could sleep; some newer ones were built to hold it, and hold it in the wrong season too.

So run it as an experiment rather than a doctrine. Try a day sealed and shaded, and notice what the room does. If your room — and upstairs rooms are the usual culprits — gets hotter and closer with everything shut, that is real information about your building, not a failure of nerve. Believe the room.

What this costs, which is mostly nothing

Nearly all of the above is free, and the pieces that aren’t tend to be available without paying. Fans turn up regularly on local giveaway and buy-nothing groups; community groups and church drives sometimes distribute them when a heat wave is forecast. Asking is the whole method.

And there is the option people skip because it feels like giving up, when it is among the most sensible things on the list: leave. A library, a public building, a shopping center — somewhere already being cooled at someone else’s expense. Many places open official cooling centers in a serious heat wave. Spending the worst hours of the day somewhere else is not a defeat; it is the cheapest cooling available to anyone.

Two gaps worth naming. If your room has no window to shade or to open, most of this page is not for you: the sun-and-airflow half needs a window, and what is left is the cool-the-person half — which still works, and is not nothing. And nobody here had an answer for a heat wave during a power cut, when the fans stop too. That question kept being asked and never answered, and it deserves better than silence.

On money: almost everything above is free. The real costs are the things this page never tells you to buy — a unit, a shading structure, insulation — and it can’t tell you what those run to.

The leverage, then, sits earlier than it feels. The room you are fighting in the afternoon was mostly decided that morning, by whether the sun got in. Most of the work happens before you are uncomfortable enough to think about it — which is inconvenient, and also the reason it works.