Article #1479 - Keys to Improving Window Performance
The terms "fenestration", "window", and "glazing" are often used interchangeably. However, window actually describes a system of several components. Window is the term given to an entire assembly comprised of the sash, glazing, and frame.
Definition of U-value : A measure (Btu/h ft2 °F) of how well a material or series of materials conducts heat. U-factors for window and door assemblies are the reciprocal of the assembly R-value. The smaller the number, the less the heat flow.
Window U-factors typically range from 0.2 to 1.2, while the U-factors of other building assemblies (e.g. walls, floors) typically range from 0.02 to 0.10 when insulated. Using efficient windows decreases the energy consumption of the structure and helps the project comply with the code.
Definition of SHGC : Solar heat gain coefficient (SHGC) is the glazing''s effectiveness in rejecting solar heat gain. SHGC is part of a system for rating window performance used by the National Fenestration Rating Council (NFRC). SHGC is gradually replacing the older index, shading coefficient (SC), in product literature and design standards. If you are using glass whose performance is listed in terms of SC, you may convert to SHGC by multiplying the SC value by 0.87.
The solar heat gain coefficient (SHGC) for your window may be found on the National Fenestration Rating Council (NFRC) label affixed by the window manufacturer prior to sale.