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About that open bedroom window

by: Paul Kando

Lots of folks like to sleep with their window open. They like fresh summer breezes, not to mention a wake-up call by the song of a thrush. Ah, heady days of summer: no furnace, oil bills, dry winter air to annoy the nostrils! Ah, the luxury of being caressed by fresh air streaming through the open window as we sleep! Alas, when we trade long summer evenings and the early sunrise for longer dusks and dawns, our love affair with that open bedroom window must also end. Here is why:

Pressure in a house
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Pressure causes flow. Any flow - of heat and all forms of energy, of air and other fluids. By "pressure," I mean any difference, in temperature, in atmospheric pressure, in voltage, in the height of a dam, etc. Nature is the world's truest democracy. It abhors such differences. It does everything to eliminate them, to equalize. So heat moves from warm to cold until there is no difference in temperature; water flows from high to low until there is one pool of level of water, batteries discharge. Air flow in a house is no different. On a windy day, for instance, air pressure on the windward side is greater than downwind. So "the wind blows through the house," stopping only when the wind outside stops blowing and pressures are equalized. Similarly, when an exhaust fan is turned on in kitchen or bathroom, it lowers the air pressure indoors, so air will move in from the higher pressure outdoors to replace the air removed by the fan.

Another important way air moves through the house is by the stack effect, caused by the difference in density (hence weight) between cold air and warm. As warm air expands, it rises, drawing cool air in its wake, until the air temperatures equalize. In summer, indoor and outdoor temperatures are close to equal, so any air movement is driven either by differences in air pressure (caused by summer breezes) or by slight differences in temperature between two sides of a house. We cross-ventilate by opening windows on opposite sides. In cold weather the furnace, boiler, or stove are fired up indoors, creating convective air movement of their own, changing the whole game. The rising heated air is now continually pushed out of the house through leaks in the upper part of the structure, while cold outdoor air streams in through leaks in the lower part to replace it. (Figure 1.) We don't feel the warm outbound flow because the air is blowing away from us; we only feel "cold drafts."

An open bedroom window during the heating season functions like an additional big leak through which more warm air is lost. The fresh air that used to stream in during the warm months is replaced by an illusion during the heating season. Fresh air now enters only through cracks somewhere in the lower part of the building, often in the wet cellar or crawl space, filling the house with that faint musty smell as it filters upstairs.

A radiator in front of the bedroom window only make things worse, as its heat increases the outbound flow of warm air if the window is open. Moreover, there is that moisture in the air. We know from research that air moving through a small, one-millimeter hole can move as much as 13 ounces of moisture per day in the form of water vapor. When that vapor condenses or precipitates into liquid water, 60.6 Btus/oz. of heat energy is released to the surroundings. With an open window those surroundings are the great outdoors. Imagine how much air-borne moisture can travel through an open window and do the numbers: all that energy is removed from your house!

This is how, once the heat is turned on in our houses, that summer night's worth of fresh air streaming through that open bedroom window turns into a costly illusion. Keep that bedroom window shut during the heating season, except for brief periods to air out the place if needed. The real way to get fresh air in winter, short of freezing, is through heat recovery ventilation, our subject for another day.