How to flush heat out of a flat at night
Night ventilation is the only free cooling most homes without AC get. It only works when outdoor air is cooler than indoor air, and city heat islands make that window shorter.
A heatwave day is defensive. The night is when you actually dump heat.
Once outdoor air is cooler than indoor air, open as wide as security allows and keep a path through the home. Walls, floors, and furniture give back what they stored. You are not trying to match the outdoor thermometer by midnight. You are trying to arrive at sunrise a few degrees down, then close up and protect that gain.
That is Phase 2 and Phase 3 of the daily heat cycle.
The window is shorter in a city
Urban heat islands hold the day’s sun in asphalt and walls. Rural air may drop at 9 pm. Street air often stays warm until midnight or later, and it climbs again earlier in the morning.
So the flush:
- Starts later than a countryside forecast suggests
- Ends earlier than you would like
- Matters more, because you recover less overnight
City-by-city, that lag is not the same. Paris nights stay warm in dense arrondissements. Berlin parks help some streets. See heatwave playbooks for European cities.
How to actually flush
- Confirm outdoor air is cooler than indoor air. Guessing from the clock is how people open at 7 pm onto 32 °C streets
- Open the cooler, shadier, greener side wide. Crack the hot street side for exhaust if you have two façades
- Use a fan to speed the exchange: layout and fans
- Keep interior doors open so bedrooms share the flush
- If dew point is high, prefer a burst rather than all night wide open: dew point
- Close before the morning crossover. East shutters down first
After the heatwave the same night routine still helps for a few days, because the building is still giving heat back. See why the home stays hot afterwards.