A glass rating—logically enough—will indicate how the glass (glazing) itself performs. But a window is made of more than glass. The framing material, the number of panes of glass and air spaces between the panes, any coatings applied to the glass...all these are components of the window, and affect how the entire structure performs. The NFRC window rating indicates the overall performance, and thus is a better guide in choosing one particular type of window product over another. Note that the NFRC label is found on all ENERGY STAR® qualified windows.

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