Germans have a word for proper ventilation — Stoßlüften, or "shock ventilation" — and in a heatwave it's your most powerful free cooling tool. Used correctly at night, it can transform a stuffy flat. Here's how to do it.
What Stoßlüften means
Stoßlüften is opening windows wide for a short, intense burst of ventilation, rather than leaving them tilted open for hours. The wide opening creates strong airflow that exchanges the room's air quickly and efficiently. In winter it's used to refresh air without losing too much heat; in summer, applied at the right time, it becomes a heat-flushing tool — a fast, complete air exchange when the outside air is finally cooler than inside.
Timing is everything
The whole method hinges on one rule: ventilate only when it's cooler outside than in. During the hot day, throwing windows open pulls heat in. But late evening through early morning, when the outside temperature drops below your indoor temperature, a burst of Stoßlüften flushes the day's stored heat and banks cool air. Get the timing wrong and you heat your flat; get it right and you cool it for free.
How to do it at night
- Wait until the outside air is cooler than indoors — usually well after sunset.
- Open windows on opposite sides of the home fully to create a strong cross-breeze.
- Add a fan facing out of a window to actively push hot air outside while cooler air is drawn in. See fans in stock.
- Let it run through the coolest hours — overnight is ideal in a heatwave.
- Close and shade everything before the day heats up, to trap the cool you just banked.
Pair it with daytime sealing
Night ventilation and daytime sealing are two halves of one strategy. You flush heat and bank cool overnight, then trap that cool by sealing and shading through the day — see our daytime cooling guide. Either half alone underperforms; together they keep a flat noticeably cooler with no electricity beyond a fan.
Boost it with a cross-breeze and fan
The more airflow, the faster the heat leaves. Opening windows on truly opposite sides maximises the cross-breeze, and a fan at one window facing out turns passive ventilation into active heat exhaust. For a top-floor flat that stores a lot of heat, aggressive overnight Stoßlüften is essential — see our attic guide.
The takeaway
Stoßlüften is the most effective free cooling you have: wide-open, well-timed night ventilation that flushes heat and banks cool air, sealed in by daytime shading. Add a fan to supercharge it. Check which fans are in stock near you to make your night ventilation work even harder.