Air Infiltration

Air infiltration is the uncontrolled leakage of outdoor air into a building through gaps in the envelope. For garage doors, it describes how much air moves through or around the door when it is fully closed. The unit is cubic feet per minute per square foot of door area (CFM/sq ft), measured at a standard test pressure.

A door with lower air infiltration keeps the garage warmer in winter and reduces dust and exhaust from outdoors. For attached garages, a tight garage door also lowers the load on the house heating and cooling system. The garage acts as a buffer between outdoor air and the conditioned living space.

What affects air infiltration:

• Astragal (bottom seal) - The flexible rubber or vinyl seal on the door's bottom rail is the largest single source of air leakage. A worn or compressed astragal stops sealing the floor gap properly.
• Stop molding and jamb seal - The stop molding on each jamb and the header provides the surface the door presses against. If the stop molding is loose, warped, or the weatherstrip on it is worn, air bypasses the seal on three sides.
• Section joints - On older doors with worn shiplap or tongue-and-groove joints, gaps between sections allow air movement across the full width.
• Door construction - A solid polyurethane-core sandwich door with tight perimeter seals outperforms a single-skin pan door with no weatherstripping.

DASMA publishes test procedures for measuring air infiltration on residential and commercial doors. Manufacturers that test to these standards publish the results in product data sheets. A well-sealed residential insulated door with tight perimeter weatherstripping achieves substantially lower infiltration rates than an unweatherstripped door at the same test pressure.

Air infiltration is related to but distinct from thermal bowing. Thermal bowing is a structural deformation caused by temperature differentials; air infiltration is a fluid flow measurement. Both affect comfort and energy use in a garage.