Fresh air does not enter a room simply because a window is open. Air moves from areas of higher pressure to lower pressure, and it needs a way in and a way out. That is the principle of cross-ventilation.
Two forces move air
Wind pressure. When wind blows against a building, it creates higher pressure on the windward side and lower pressure on the leeward side. Openings on both sides allow air to flow through.
The stack effect. Warm air is lighter and rises. If there is an opening high up — a ventilator, clerestory window or courtyard — warm air escapes, drawing cooler air in through lower openings. This works even when there is no wind, which is why traditional Indian homes with central courtyards and high ventilators stayed comfortable.
Rules of thumb
- Openings on opposite or adjacent walls ventilate far better than a single window.
- A smaller inlet and larger outlet increase the speed of air through the room.
- High outlets let hot air escape; low inlets bring cool air to where people sit and sleep.
- Keep the path clear. A tall cupboard in front of a window, or a closed internal door, can block the flow.
- Single-aspect rooms can still ventilate using a door to another room with a window, or by opening a window at both its top and bottom.
How Vastu fits in
Traditional Vastu emphasises openness on the north and east, light and air in the north-west (the zone of Vayu), and an open centre. These ideas encourage open, through-ventilated plans. The building-science lens adds one important point: what matters most is the pair of openings and the local wind direction, which varies from city to city and season to season.
Try our Light & Air questionnaire to see how your own home is doing.