Air Infiltration

Air infiltration is the uncontrolled flow of outdoor air into (and conditioned indoor air out of) a building through unintentional openings in the building envelope. In windows, infiltration occurs through gaps between the sash and frame, failed weatherstripping, cracked glazing compound, and open weep holes. It is measured in cubic feet per minute per foot of crack length (cfm/ft) at a standard pressure difference.

How Much Energy Does Air Infiltration Waste?

The US Department of Energy estimates that air infiltration accounts for 25–40% of a home’s heating and cooling energy use. Windows and doors are the primary culprits. A single double-hung window with worn weatherstripping can leak as much heat as leaving a 4-inch hole in the wall open 24 hours a day.

Air Infiltration Ratings

Windows are rated by the NFRC for air leakage in cfm per square foot of window area at a standardized pressure (1.57 psf / 75 Pa). Lower is better:

• < 0.1 cfm/ft² — excellent (Energy Star threshold)
• 0.1–0.3 cfm/ft² — good
• 0.3–1.0 cfm/ft² — typical older windows
• > 1.0 cfm/ft² — poor (replacement warranted)

Detecting Air Infiltration

Simple tests to find window air leaks:

• Hand test — on a cold windy day, slowly move your hand around the window perimeter; you’ll feel cool drafts at leak points
• Incense/smoke test — hold a smoking stick of incense near the perimeter; smoke disturbance indicates air movement
• Blower door test — a professional test that depressurizes the house and uses an infrared camera to map all infiltration points

Reducing Air Infiltration

In order of cost-effectiveness: replace weatherstripping ($10–$30 per window), re-caulk exterior joints ($5 per window), install interior window insulation film in winter ($5–$15 per window), replace failed windows (last resort). For most windows built before 2000, weatherstripping and caulk replacement alone reduces infiltration by 50–70%.