As most energy-conscious builders know, a tight house needs a reliable source of fresh exterior air. This is particularly true of houses with fireplaces, high-powered exhaust fans, and other appliances that tend to depressurize the living space. A house that doesn't have enough natural air leaks to make up for the loss will draw the necessary makeup air from somewhere else. That's a potentially dangerous situation because it can lead to backdrafting and fume spillage from furnaces, water heaters, or other atmospherically vented fuel-burning appliances.

Several strategies can be used to admit makeup air in a more systematic way. The simplest method -- leaving a window partially open -- is simple and effective, but it has significant drawbacks: It often creates chilly drafts that cause the homeowner to cut off the air supply by closing the window. And when the living space is under positive...

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