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A common belief is that opening two windows or doors inside the home can create a strong and steady flow of air (a "draft") that a single window would not create. This seems somewhat intuitive, yet I have not been able to find a rigorous explanation.

Is this true? If so, how do the mechanics of this work? How does the effect depend on the positioning of the windows relative to each other? Additionally, how is it changed by the addition of more than two open windows?

Are there any other factors?

(Note: If I am asking this on the wrong stackexchange site, please tell me where to ask it instead.)

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  • $\begingroup$ It is just the continuity equation for the system. If there is air flowing into the room, the same amount of air has to flow out. $\endgroup$
    – alephzero
    Commented Feb 22, 2021 at 17:51
  • $\begingroup$ And on hot days with no breeze don’t count on magically making one by opening two windows. $\endgroup$
    – Jon Custer
    Commented Feb 22, 2021 at 17:54
  • $\begingroup$ Some people have answered how this works if one window is on the upwind side and the other is on the downwind side. As a follow-up, what if the two windows are opposite each other but the line between them is perpendicular to the outdoor windflow? And how about if the windows are in different rooms (with an open door but no straight line of flow)? Can this create any effect? $\endgroup$
    – bob
    Commented Feb 22, 2021 at 20:01

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Usually people who open a window are interested in replacing their inside air with outside air. If you only open one window, all of the exchange between inside and outside is happening thanks to turbulent mixing near the window. But if you open two windows, you may be able to arrange them so that air is mostly entering at one window and mostly exiting at the other. In that case there is a path from the enter-window to the exit-window, along which you indoor air will be replaced with outdoor air fairly quickly; that path can have much more volume than the turbulent mixing region near either window.

You get the biggest effect if you have one window open on the upwind side of your house and another open on the downwind side.

If your indoor air is much warmer than your outdoor air, you can also get a bit of a "chimney effect" by opening some downstairs windows and also some upstairs windows.

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Wind blowing against one side of the house will create higher air pressure on that side and relatively lower air pressure on the opposite side. so opening windows on those opposite sides will maximize internal air flow. Even opening a side window that is parallel to the wind can create some air flow from the lower air pressure due to Bernoulli's principle. If there is no wind, setting a fan to blow in or out of one of the open windows will create air flow. And as warm air rises, having open windows at different heights can also cause air flow. Even on a calm day with no wind, there can still be some air exchanging through open windows due to kinetic motion of air molecules.

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