If you’re researching options for replacement windows for your home, you’ve probably come across the term “ENERGY STAR® certified.” The name implies energy efficiency, but do you know what goes into earning the ENERGY STAR label?
The U.S. Department of Energy says replacement windows that are ENERGY STAR certified can lower household energy bills by seven to fifteen percent. But not just any replacement window will provide you with those savings. To meet the criteria, windows must meet specific requirements set by the ENERGY STAR program. And those requirements must be certified by a third party.
The ENERGY STAR program for windows, doors, and skylights originally began in 1998, with a couple of updates to the requirements since then. Today, windows must be independently tested and certified by the National Fenestration Rating Council (NFRC). ENERGY STAR sets the minimum performance standards, and the NFRC conducts the certification rating and labeling.
There are five categories that the ENERGY STAR program considers for their certification, with established ratings that must be met or exceeded. The ratings differ for four basic climate zones in the U.S.; all of Virginia, Washington DC, and most of Maryland (except the extreme north west of the state) fall within the North Central Zone. Once certified, the NFRC label shows the ratings for these five categories:
U-factor – This measures the heat transfer of a window, which means how well it insulates your home. The U-factor value is typically between 0.25-1.25. The lower the number, the better insulated the window. The North Central Zone requires 0.32 or lower.
Solar Heat Gain Coefficient (SHGC) – This tells you how well the window blocks heat from the sun. The SHGC scale is 0 to 1, and values usually fall within 0.20 to 0.80. As with U-factor, lower is better – the lower the value, the less solar heat the window transmits. The North Central Zone requires 0.40 or lower.
Visible Transmittance (VT) – This is a measurement of the amount of light the window lets through. The VT scale ranges from 0 to 1 with values generally between 0.20-0.80. When it comes to VT, a higher value is preferred because it indicates more light coming through the window.
Air Leakage (AL) – This value measures the rate that air passes through joints in the window. The value is measured in cubic feet of air passing through one square foot of window area per minute, or cf-m/ft2. The industry-standard value is 0.3 cf-m/ft2.
Condensation Resistance – This tells you how well the window resists water build up, which is typically in the form of condensation on a window’s glass surfaces. The scale is 0 to 100, so the higher the number, the less condensation build up a window will have.
Not all replacement windows meet the minimum criteria established by the U.S. Department of Energy to considered ENERGY STAR windows. When shopping for windows, look for the ENERGY STAR label to ensure that what you’re purchasing meets or exceeds the requirements.