U Factor vs SHGC

With the ENERGY STAR® window, door and skylight certification program,  residential windows have a variety of energy efficiency measurements associated with them that ensure both quality and function for the consumer.  Two of these measurements are designed to tell consumers about the energy efficiency of that particular window.  These measurements are, “U factor,” and “Solar Heat Gain Coefficient,” (SHGC). If a window qualifies for the ENERGY STAR® label, the National Fenestration Rating Council (NFRC) will have rated the window’s energy efficiency based on U factor and Solar Heat Gain Coefficient. The National Fenestration Rating Council is an independent council that rates and labels the overall quality of windows, doors, skylights and their auxiliary products for energy performance.

So what do these indicators mean? U factor looks at the rate of heat transfer and shows how well the window is insulated with respect to heat. If a U factor is low, meaning there is little heat transfer, the window has higher quality insulation. For Solar Heat Gain Coefficient, the measurement indicates how well the window blocks heat coming from the sun.  Like U factor, if the rate of solar heat allowed in by the window is low, the higher the window performs under those conditions.

If you’re interested in finding out this information about windows you are choosing for your home, there will be an NFRC label which gives the test results for that specific brand and model of window.  Currently the NFRC label is the only reliable way to determine how energy efficient a given window is when you are trying to compare apples to apples.

Measurements on this label will vary by type of window as well as the type of climate you live in.  For example, if you live in a Northern climate you should look for windows with a U factor of 0.35 or less if you have air conditioning.  If you don’t use air conditioning you can purchase a window with a higher U factor such as 0.40 with minimal impact on overall energy efficiency. Similarly, if you live in a Northern climate and want to keep your home cool choose a window with a high SHGC rating like 0.60. This way, you can leverage the winter sun to offset your home heating bill. Conversely if you prefer a cooler home, choose a lower SHGC rating to keep the heat out.

No matter what type of window you choose, be sure that you’ve taken the time to be informed about what you can expect in terms of energy efficiency and overall product performance.  Understanding the NFRC label is one of the first steps in this process. Work with your contractor or window manufacturer to learn more and find the right window for your needs. Read about the specific information contained on the NFRC Label by visiting the About Perfomance Ratings home page.