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What are R factors and U factors for windows?

The u factor represents the total insulation that the window provides. Previously, the r factor was used. R stands for resistance to heat transmission. 

The definition of r 1 is the amount of insulation provided by a 1″ thick board of pine or spruce provides. 

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So r13 is the same as a 13″ thick wall of wood’s insulation. The reason that this format was a problem, is that the warmest part of modern insulated glass is the center. This is because there needs to be a spacer that is glued to both panes of glass.

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To make double pane windows: These spacers are most commonly made of metal, which is a good conductor. A bad insulator.

So the edges of the window pane are much colder than the center. Manufacturers published center of glass measurements for years, for the sole purpose of misleading buyers in order to make sales. 

Products like intercept spacers are also made of metal, and are marketed as warm edge technology, even the insulation gains are minimal. So the u value accounts for that by measuring the insulating value of the entire window. That’s what you get with NFRC


Notice : At Maryland Windows Doors Roofs we install windows with super spacer. It’s a silicon foam insulating spacer, and not made from metal. A side benefit is that it keeps the argon gas in the window dramatically longer than metal spacers

Keith Pearcy

Maryland Windows Doors Roofs

23 West All Saints Street, Frederick , Maryland 21701

Phone: 301 712-5228

MHIC #106619
email: info@marylandwindowsdoorsroofs.com