Measuring Performance: Air Leakage (AL)

Heat loss and gain occur by infiltration through cracks in the window assembly. It is indicated by an air leakage rating (AL) expressed as the equivalent cubic feet of air passing through a square foot of window area. The lower the AL, the less air will pass through cracks in the window assembly. An AL rating is required for ENERGY STAR ertification. This rating must be 0.30 or less and can be displayed on the NFRC’s temporary label, the AAMA’s Gold Label, Keystone Certifications, Inc.’s NAFS Structural Certification Label, National Accreditation & Management Institute’s (NAMI) NAFS Structural Label, or the WDMA’s Hallmark Certification Label. This means that the AL value may not alway show up on the NFRC Label for this value may qualify for the ENERGY STAR requirements through another program. For code compliance purposes, however, air infiltration is often tested in accordance with the North American Fenestration Standard (NAFS), which produces similar results to the NFRC air leakage rating.

Select windows with an AL of 0.30 or less (units are cfm/sq ft).

Is my window leaking air?

The Air Leakage (AL) rating pertains to leakage through the window assembly itself. Air infiltration can also occur around the frame of the window due to poor installation or poor maintenance of existing window systems. Make sure windows are properly installed and maintained (caulking and weatherstripping). Cold glass can create uncomfortable drafts as air next to the window is cooled and drops to the floor. This is not a result of air leaking through or around the window assembly but from a convective loop created when next to a window is cooled and drops to the floor. This air movement can be avoided by installing high-performance windows.