This is indicated by an air leakage rating, expressed as the equivalent cubic feet of air passing through a square foot of window area. Heat loss and gain occur by infiltration through cracks in the window assembly. The lower the AL, the less air will pass through cracks in the window assembly.

Additional Resources

UF IFAS Energy Efficient Homes: Windows and Skylights - http://edis.ifas.ufl.edu/FY1045

 Building a Safer Florida, Inc., University of Florida's Program for Resource Efficient Communities, and the Florida Department of Community Affairs. May 2006. Window Systems http://www.dca.state.fl.us/fbc/publications/Fact_Sheets_0307/WindowSystems061506revised.pdf

 Efficient Windows Collaborative. September 2007. Fact Sheet: Selecting Energy Efficient Windows in Florida.  http://www.efficientwindows.org/factsheets/florida.pdf