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Air leakage, or infiltration, occurs when outside air enters a house uncontrollably through cracks and openings. Properly air sealing such cracks and openings in your home can significantly reduce heating and cooling costs, improve building durability, and create a healthier indoor environment.
It is unwise to rely on air leakage for ventilation because it can't be controlled. During cold or windy weather, too much air may enter the house. When it's warmer and less windy, not enough air may enter. Air infiltration also can contribute to problems with moisture control. Moldy and dusty air can enter a leaky house through such areas as attics or foundations. This air in the house could cause health problems.
The recommended strategy in both new and old homes is to reduce air leakage as much as possible and to provide controlled ventilation as needed.
Plug the leaks and keep the heat
Heating a house pressurizes the inside air and drives it through ceiling penetrations into the attic, which in turn draws in cold replacement air from outside. Simply plugging the air leaks into the attic eliminates problems such as drafts, ice damming and condensation. Effectively insulating ceilings and walls completes the thermal envelope, the boundary between inside and outside air.
 
Under natural conditions, pressures are always higher at the ceiling than at the windows. Although wind and mechanically induced pressures are sometimes stronger, hot air applies constant pressure upward toward the ceiling and the attic. As a consequence, ceiling bypasses, or holes in the thermal boundary, generate more significant natural infiltration through the heating season than do window leaks.
This doesn't mean that door and window weather-stripping or replacement isn't cost-effective, but it does mean that doors and windows may not need replacement. There are reasons to replace windows, like excessive moisture problems, mold and mildew, glass missing or a large gap between sash and jamb.
Another place to concentrate on is the common wall between the house and its attached garage, if there is one. Air leaks here always have the potential to vacuum car exhaust, solvent and weed-killer fumes, and fuel gases into the living space, so this is a spot that requires a NASA-grade air-seal. Caulking framing and sheathing joints down to and along the foundation makes a big difference.
Basements and crawlspaces should also get a thorough inspection. Musty odors are a sure sign that moisture and cold air are mixing and that wood is under attack. Crawlspaces are usually built to save money, and difficult access is often a reliable indicator of potentially significant building defects. We often have to saw our way into crawlspaces, where we can find bare soil, open concrete-block cores, no insulation, no sill seal, empty whiskey bottles, mold, decay, and lots of insect and animal debris.
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