Air infiltration is the unintended leakage of outdoor air into buildings through cracks, gaps, and openings in the building envelope. This passive process occurs due to pressure differences created by wind, temperature variations, and stack effect. Uncontrolled infiltration accounts for approximately 25 to 40 percent of heating and cooling losses in typical residential buildings.
Sources and Causes
Common infiltration pathways include gaps around windows and doors, deteriorated weatherstripping, cracks in foundations and walls, and unsealed utility penetrations. Stack effect causes warm air to rise and exit upper levels while cold air enters through lower level openings, particularly in homes exceeding 20 feet in height.
Testing and Control
Blower door testing measures air infiltration rates, quantifying leakage as Air Changes per Hour at 50 pascals (ACH50). Modern standards like IECC 2021 require residential buildings to achieve 7 ACH50 or lower. Reducing infiltration through proper sealing, weatherstripping, and caulking improves comfort and reduces HVAC runtime requirements.