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Few homeowners purchase new windows more than once. If this process is new to you, you may have some questions about the replacement window terms you’re hearing. These terms also apply to new windows if you’re planning an addition or custom build.


The National Fenestration Rating Council (NFRC) independently evaluates windows for their insulation rating. This is called the U-factor. The ratings are between .20 and 1.20 with a lower number meaning the window is more insulative.


This is another rating by the NFRC. SHGC stands for solar heat gain coefficient. The rating refers to how well the window prevents sunlight from heating up the room or home. The range for this is 0 – 1 and again, a lower number is better than a high one.

Visible Transmittance (VT)

All windows block some of the natural light. The VT rating allows you to compare windows to each other in an apples to apples fashion. Again, you’ll find a 0 – 1 rating, but the higher the number the more natural light.

Air Leakage

This is the last of the new and replacement window terms that are part of the NFRC evaluation. It speaks to amount of air that leaks in or out of a window. Look for a number less than or equal to .3.

Low-E Glass

Low-E glass is the standard today. In order for a window to be ENERGY STAR rated, it must have Low-E glass. Low-E isn’t a special glass, but rather, a coating on the glass that reduces the amount of UV light that gets in through the window. This reduces solar heat gain and fading of carpets and furnishings.

Double Hung Windows

There are may window styles, however, the most common replacement window today is the double hung window. Instead of the window only opening by pushing up on the lower sash, both the upper and lower sashes move allowing for increased ventilation and easier cleaning.


Although technically these terms have slightly different meanings, they do refer to the same thing – the dividers in a window sash. Some people even call this the window pane design.

Looking for Replacement Windows?

Call Cooley Roofing. As an independent window company, we offer high quality vinyl replacement windows. We will answer any questions you have about the replacement window terms that aren’t clear to you. Call us today for you no obligation quote at (336) 766-6488 or fill out our contact form. We offer financing on all new windows, siding and roofing.

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