The three phases of heat in your home
A heatwave is not one long problem but three, and each has its own answer. Keep heat out, cool down, then hold what you gained.
During a heatwave your home moves through a repeating daily cycle. Treating it as one continuous emergency leads to the classic mistakes: windows open at noon, shutters open in the afternoon, everything shut at 2 am when the outdoor air had finally turned useful.
Split the day into three phases instead, and each one has a clear answer.
Phase 1: keep heat away
This is the daytime phase, roughly from mid-morning until the outdoor temperature drops back below your indoor temperature. Usually that is late afternoon or early evening, not sunset.
The goal is defensive. Nothing you do now makes the flat cooler. It only makes it warmer more slowly.
- Close windows on any façade where outdoor air is warmer than indoor air
- Close shutters, blinds, or curtains on sun-facing windows, ideally on the outside of the glass
- Turn off anything that produces heat and is not needed: ovens, desktop computers, incandescent lights, tumble dryers
- Accept that the flat will warm up a little regardless. The aim is to arrive at evening at 27 °C instead of 32 °C
Phase 2: cool down
The moment the outdoor air is genuinely cooler than the indoor air, the strategy inverts completely. This is your only free cooling, and it is time-limited.
- Open as many windows as you can, as wide as security and safety allow
- Prioritise a through-draught if your home has windows on opposite sides
- Use a fan to push the exchange along, placed according to your layout
- Keep going as long as the outdoor air stays cooler, which often means overnight
Bear in mind that indoor temperature lags. Your walls and floors are still radiating stored heat, so the room will not drop as fast as the outdoor reading does.
Phase 3: maintain reasonable warmth
By early morning the outdoor air starts climbing again. There is a crossover point where continuing to ventilate stops helping and starts hurting.
Close up before that point, not after it. Every degree you saved overnight is capital you are now protecting for the day ahead. Shutters go back down on the east-facing windows first, because that is where the morning sun lands.
And look after your body
The building is only half of it. Fans, hydration, and light clothing do work that ventilation cannot, especially during Phase 1 when the flat is warming and there is nothing more to do about it. See how to cool your body down.
Where an app helps
The hard part of this is not understanding the phases. It is knowing exactly when the crossover happens today, for your flat, given your indoor temperature and the local forecast. That moment shifts by hours from one day to the next, and it is different for a top-floor studio and a ground-floor flat on the same street.
Ventilate works that out and tells you which phase you are in, window by window. What matters is your safety: always follow your local health agency for further recommendations.