The solar heat gain coefficient is the fraction of external solar radiation that is admitted through a window or skylight, both directly transmitted, and absorbed and subsequently released inward. The solar heat gain coefficient (SHGC) has replaced the shading coefficient (SC) as the standard indicator of a window's shading ability. SHGC is expressed as a number between 0 and 0.87, SC as a number between 0 and 1; i.e., the relationship between SHGC and SC is: SHGC = SC × 0.87. The lower a window's SHCG, the less solar heat it transmits, and the greater its shading ability. SHGC may be expressed in terms of the glass alone or may refer to the entire window assembly.

 

To reduce the SHGC, manufacturers can apply a spectrally selective low-E (low-emissivity) coating to glazing. This type of low-E coating can reduce heat loss in the winter as well as solar gain in the summer. Reflective coatings and tinted glass can also help reduce the SHGC.

 

In passive solar design, south-facing windows with high SHGC ratings might be needed to provide a building with heat in the winter. But a properly designed roof overhang is typically used to reduce the solar heat gain from these windows in the summer.

 

Some window coverings – shades, blinds, mesh screens, and awnings – can also be used to reduce solar heat gain in the summer or as needed.

How to find the SHGC for your windows

In the United States, the solar heat gain coefficient (SHGC) for windows may be found on the National Fenestration Rating Council (NFRC) label affixed by the window manufacturer prior to sale.

 

Most residential windows are rated by the National Fenestration Rating Council (NFRC), and most new residential windows should have an NFRC label. If a window does not have a label (either because the window has not been rated or because the label has been removed), you can either ask the window manufacturer to supply you with this information or look for your specific window in the NFRC products directory at the National Fenestration Rating Council website.

 

If you have literature from a manufacturer indicating the shading coefficient (SC) value for a window, you can, as indicated above, easilier convert this to an SHGC value by multiplying it by 0.87.