Understanding Window Energy Ratings

Buying new windows for your home can be expensive and intimidating.   Understanding the Energy Performance Ratings can make the process less painful while adding comfort and energy savings.  There are four main measurements to look for when shopping for new windows: U-value, solar heat gain coefficient, visible transmittance, and air leakage.  But what do these numbers mean?

U-Value is a measurement of the heat flow through the window.  It can be compared to the R-value (resistance) in insulation though it is the opposite.  A lower U-value means less heat is transmitted through the window.  U-value = 1/R-value.  A window with a U-value of 0.33 would have a R-value of 3.  A lower number provides more insulation.

Solar Heat Gain Coefficient is the ratio of solar heat gained by the window compared the amount of solar energy hitting the window's surface.  The measurement is a number between 1 and 0.  A SHGC of 1 indicates 100 percent solar gain while 0 indicates no solar gain.  It is best to develop a window strategy with different SHGC for different windows.  West-facing should be avoid but when they are used,  the SHGC should be very low to avoid over heating the home in the afternoon.  South-facing windows should have a high SHGC to allow low angle winter sunlight in to heat the home passively.

Visible Transmittance is the measure of how much light the window transmits.  The number is also between 0 and 1.  The higher the number, the more light allowed to pass through the window.

Air Leakage is the measure of cubic feet of air leakage per minute per square foot of window.  Windows with air leakage less than 0.3 cfm/ft2 is recommended.  Casement windows generally preform better than sliding windows in air leakage while fixed windows perform the best.

"Air leakage siphons about half of an average home's heating and cooling energy to the outdoors each year.  Air leakage through windows is responsible for much of this loss."  -Paul Fisette Energy-Efficient Building