When air infiltrates through cracks in a fenestration products assembly, heat loss and gain can occur. The air leakage (AL) rating is expressed as the equivalent cubic feet of air passing through a square foot of the product's area (cfm/sq ft). The lower the AL, the less air infiltration.
For heavily trafficked buildings, air infiltration through doors is an important energy consideration. Revolving doors and double-doored entry ways should be considered in this case.