Single-sided flats versus cross-ventilation
Windows on one façade cannot flush a home the way opposite façades can. Here is how to tell which layout you have, and what to do with a fan in each case.
Look at where your windows are, not how many you have.
Single-sided: every openable window is on the same façade. Air has no natural path through the flat. An open window mostly exchanges air with itself. Common in studios, corridor-side flats, and anything that only faces the street or only the courtyard.
Cross-ventilation: you have openable windows on two different façades, ideally opposite ones. Air can enter one side and leave the other. Opening an interior door is often what unlocks it.
Neither is a lost cause. They are different jobs.
Single-sided: you create the pressure
With one façade you have to manufacture the draught.
- After sunset, once outdoor air is cooler, put a floor fan on the sill facing in, angled slightly upward
- If two windows on the same wall are far enough apart, use one as intake and put the fan in the other facing out
- Open interior doors so the moving air has somewhere to go
- Expect the flush to take longer. Start as soon as outdoor air is cooler, not at midnight
Cross-ventilation: you steer a flow
Here the air already wants to move.
- Open the intake wide: shady side, greener side, courtyard rather than a baked street
- Open the exhaust just a crack, usually the street side
- Place the fan 1 to 2 metres back from the exhaust, facing out
Narrowing the outlet speeds the air leaving through it, which pulls harder on the intake. Opening everything equally gives you a gentle drift instead of a flush.
More placement detail: where to put a fan. What sits outside each pane matters too: courtyard or street.
Timing still rules
None of this helps if the air you pull in is hotter than the air you already have. Layout is a Phase 2 tool. During the day the windows stay shut and the fan cools you. See the three phases of heat.