Low-Tech Tips: Tune Your Windows

The windows on your home are no just holes in the wall that you open or close, they are actually part of a sophisticated ventilation machine. It is another "Oldway"—People used to take it for granted that you tune them for the best ventilation, but in this thermostat age we seem to have forgotten how.

For instance, everyone knows that heat rises, so if you have high windows and open them when it hot inside, the hot air will vent out. But it can be a lot more sophisticated than that. When air passes over your home, it works the same way as it does over an airplane wing: the Bernoulli effect causes the air on top and on the downwind side of the house to be at a lower pressure than on the upwind side. So if you have double hung windows, you can open the bottom section of the upwind side of the house and the upper section of the downwind side, and the low pressure will suck the air through your house. Make the outlet openings larger than the inlet opening, it increased the draft. That is why I love double hung windows; they offer the most flexibility and options. Others say that casement windows are best because they can open up to 100%; double hungs can never be open more than 50%. However I have seen studies (which I cannot find) that show that double hung windows actually work better because of the many options in setting them.

You may have seen old houses with cupolas on top; these provide more than just light. The air passing over them causes a venturi effect, essentially sucking air up through the cupola. You can also do this trick if you have a two-storey house, effectively getting the cross-ventilation on the second floor, which may be more exposed to the breeze, suck air up from the ground floor through the stairwell.

Just as the funnels on a boat face to the stern and not the bow, you want to maximize the suction from the low pressure side of the house. If you are designing your house from scratch, there are many more sophisticated tools and techniques you can use, such as thermal chimneys.