Installing the right energy-efficient replacement windows for your home involves industry knowledge. There are a lot of abbreviations and numbers involved, but, by understanding the different rating systems, you can accurately choose windows to meet the needs of your climate and your home.
Window labels are certified by the National Fenestration Rating Council and all Energy Star windows should have an NFRC label. Windows that don’t have the Energy Star label may or may not be rated – if that’s the case, check with the manufacturer.
U-Factor: This represents how fast a window allows non-solar heat to pass through it. The lower the U-factor number, the more energy efficient the window.
SHGC (Solar Heat-Gain Coefficient): This represents how much solar radiation a window allows through it. The lower the SHGC, the less solar heat it transmits, which is good on warm days; the higher the SHGC, the more solar heat it allows, which is good on cold days. Choosing the right SHGC rating depends on your climate, the window’s location and any shading, says the U.S. Department of Energy.
Air leakage: The lower the rating, the slower air leaks.
VT (Visible Transmittance): This represents how much visible-spectrum light transmits through a window. The higher the VT, the more visible light. Deciding how much visible light you want depends on how much daylight you have and whether you need to reduce glare.
LSG (Light-to-Solar Gain):The ratio between the SHGC and VT, or how efficient the window is at allowing daylight in, while blocking heat gain. The higher the LSG, the more light you get without the side effect of heat.