REMARK: This Q&A was part of the AIVC special COVID-19 newsletter published in July 2021. To subscribe to the newsletter please click here.

The air change rate depends on the size of the window relative to the room volume, the opening mechanism and position, the window height, the indoor temperature, and the weather conditions such as outside temperature, local wind speed and direction. It also depends on other possible window openings in the room in adjacent or opposing walls, and on leakages in the wall and ceiling constructions, for instance cracks around a closed door.

Generally, the air change rate as a result of window airing is normally higher than provided by the standard ventilation system for IAQ purposes. The windows are normally sized for airing to achieve large flowrates for instance to get rid of heat during periods of too high temperatures inside, or to quickly get rid of incidental smells.

Assuming single sided ventilation the situation is always a result of the combination of the driving forces temperature (indoor-outdoor difference), wind speed and turbulence.

In case only temperature differences play a role the air velocity in the window opening can be calculated. But also wind and turbulence play a role in flow through open windows. It is a rather complex phenomenon. There are several publications available in which one can find a method to estimate the flow through open windows.


Willem de Gids, VentGuide 


  1. Investigation of the consequences of opening one window on the internal climate of a room. J.C. Phaff and W.F. de Gids. AIVC  AIR November 1982
  2. A Guide to Energy Efficient Ventilation AIVC 1996 
  3. CIBSE Applications Manual 10 Natural Ventilation in Non-Domestic Buildings, CIBSE 2005