Uncontrolled air leakage (infiltration) is one of the largest and most preventable energy wastes in the home. Infiltration results from a pressure difference forcing air through cracks, holes, and crevices in the home. The pressure difference results from temperature differences between the indoor and outdoor environment, and wind blowing against the side of the home producing a higher pressure than that within the home.

Since warm air rises to the top of the home by convection and then leaks out of cracks in the upper walls and roof, it creates a lower pressure in the bottom of the home than outside (causing air to leak in). Another pressure difference is created in the home's heating, ventilating and air conditioning systems.

Infiltration typically occurs around doors and windows, points where dissimilar materials are joined and penetrations through walls, ceilings, and floors for electrical wiring and plumbing fixtures. Another significant source of air leakage is through the home's ductwork.

The best way to begin reducing air infiltration is by knowing where it may be occurring. Studies show that leakage in a typical house looks something like this, with the majority occurring through the sills, walls and ceilings. The second largest offender is the windows, closely followed by the home's heating, ventilation and air conditioning systems. Openings for pipes, doors, vents and wall outlets complete the air leakage pie. Homes with fireplaces look a little different because 14 percent of their air leakage is related to the fireplace, which slightly reduces the percentage lost from the other areas.