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Why Single-Pane Windows Feel Drafty

July 6, 2013 | Posted by Mitt Jones
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We hear a lot that windows are drafty, and sometimes they really are: Every now and then the sashes in old wood windows are so loose that they easily rock back and forth, and these windows can really leak. Most of the time, though, once we crank up the blower door to measure and detect air leakage, even old single-pane wood windows don’t leak all that much. Most leak some around the pulleys and where the two sashes meet, but decades of old paint do a pretty good job of sealing other areas.

“Oh, yeah?” you say? “Then why do I feel so cold sitting in front of the windows? I mean, I FEEL the draft.”

Here’s what’s up: First, you may well be feeling a draft. Aside from the possibility that the windows really are leaky, it’s certain that the cold window surface is causing air movement. Remember the lesson from physics: Cold air sinks. On a cold day, the cold pane of glass cools down the air immediately next to it, which in turn sinks. Once away from the window, the air heats up again and rises. And this process continues in what's called a convection loop. Even this relatively slight movement of air can create discomfort.

Radiant heat loss also contributes to the chill. If you’re sitting in front of a cold window surface, your body is radiating heat to that window surface but is receiving very little heat from the window in return. Even if the temperature of the air around you is comfortable, you feel a chill. It’s the opposite of sitting in the sun or under a heater in an outdoor café.

The same phenomena also apply to cold, uninsulated walls. As with replacing windows or adding Indow Windows, blowing insulation into your walls also boosts comfort, by reducing convection currents and radiant heat loss.

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