Roof Windows and the Stack Effect
On a still summer evening, opening two windows on the same floor barely moves any air. Yet crack a roof window and a low window at the same time, and you can feel a draft appear out of nowhere. No wind required — your house just became a chimney.
Warm air wants out — give it a way up
Warm air is less dense than cool air, so it floats. All day, the warmest air in your home collects at the highest point: under the roof, at the top of the stairwell, against the bedroom ceiling. With every window shut, that hot layer just sits there.
A roof window is an exhaust valve placed exactly where the heat is. Open it, and the buoyant hot air escapes upward. That escaping air has to be replaced, so cooler outdoor air gets pulled in through whatever low opening you provide. Physicists call this the stack effect; your fireplace has used it forever.
Why it works when wind doesn’t
Classic cross-ventilation needs wind and openings on opposite façades. The stack effect needs neither — it runs on two things you often have on summer evenings:
- Height difference between the low inlet and the high outlet. A ground-floor window paired with a roof window two storeys up beats two openings on the same level.
- Temperature difference between inside and outside. The warmer your home is compared to the evening air, the harder the chimney pulls.
That second point is why the stack effect shines exactly when you need it: after a hot day, once the outdoor air has cooled, the indoor–outdoor gap is at its largest and the draft is at its strongest — the same window of opportunity as night airing.
Set up the chimney properly
To get the most out of a roof window:
- Open it wide, not tilted. A small gap throttles the whole circuit.
- Open a low inlet on the coolest side of the house — typically the north or shaded façade, or the ground floor. That’s the air you’ll be pulling in.
- Open the doors along the path so air can actually travel from the low inlet to the high outlet. A closed staircase door cuts the chimney in half.
- Let it run. Unlike a quick shock-airing, evening stack ventilation can stay on for hours while conditions outside remain favorable — it keeps flushing out the heat your walls release. For short daytime air swaps, see how long you should air a room.
When to keep the roof window shut
The chimney doesn’t care which way “better” is — it moves air whenever inside and outside differ. So the usual rule still decides everything: open only when the outdoor air is cooler and carries less water in absolute terms than your indoor air.
During a hot afternoon, keep it shut and shaded: the space right under the roof is often the hottest in the house, and an open roof window then mostly invites hot air and sunshine in. The moment to open comes when outdoor temperature drops below indoor — the same crossover logic as the morning rule, in reverse.
Let the app watch for you
The tricky part isn’t opening the roof window — it’s knowing when the evening air has become cooler and drier than your home. Open/Shut watches both numbers for your location and, if you’ve told it about your openings, tells you which ones to use. Free, no account, no guesswork.