Lindsey M. Roberts, Houzz
Houzz is the leading platform for home remodeling and design, providing people with everything they need to improve their homes from start to finish.
If you can’t save your historic windows, then it’s time to find energy-efficient replacements. And selecting new windows involves industry knowledge. It’s important to have a grasp of the alphabet soup: U-factor, SHGC, air leakage, VT and LSG. Then, after you understand all the different rating systems, you can choose windows based on your climate and the requirements for your house.
But who rates these things anyway? Can you depend on the manufacturer to be truthful? Just like food labels are monitored by the U.S. Department of Agriculture, window labels are certified by the National Fenestration Rating Council. And all Energy Star windows should have an NFRC label. (Note, though, that Energy Star bases its ratings only on U-factor and SHGC, which are explained below.) Windows that don’t have the Energy Star label may or may not be rated; then it’s time to check with the manufacturer.
How fast a window allows nonsolar heat to pass through it. The lower the U-factor number, the more energy efficient the window.
SHGC (solar heat-gain coefficient)
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. So choosing the right SHGC rating depends on your climate and the window’s location and any shading, says the U.S. Department of Energy.
The lower the rating, the slower air leaks.
VT (visible transmittance)
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 daylighting you have and whether you need to reduce glare.
LSG (light-to-solar gain)
The ratio between the SHGC and VT, basically how efficient the window is in allowing daylight in, while also blocking heat gain. The higher the LSG, the more light you get without the side effect of heat.
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