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Window U-Factor Vs. Window R-Value – Replacement Window Reviews

Replacement Window Reviews – Window U-Factor Vs. Window R-Value. What is the real measure of a great replacement window? Well, truth is that both of these measures help your custom home windows retain energy in different ways.

What’s the Difference between U-factor and R-value?

The biggest difference between U-factor and R-value (or R-factor) is that U-factor measures the rate of heat transfer (or loss) while R-value measures the resistance to heat loss. R-value is a measure of conductivity. (National Fenestration Rating Council) A&A’s Sunrise Windows stack up to the average window style in a home (see picture below). Sunrise Windows (made in U.S.A) is a model for energy efficiency in the industry. Aluminum Windows have the lowest R-value. Here you can see the sunrise window has an R-value twice as high as a foam filled window.

So what about U-factor? 

The NFRC only requires that the U-factor be displayed on their certification sticker (picture on the right). The smaller the U-factor the more energy efficient the window. Almost every window manufacturer in the U.S. applies this label to their product. Here is an example of Sunrise’s Double Pane Window NFRC sticker. U-factor is on the top left of the sticker, and carries the most weight of any of the numbers on the sticker. Sunrise’s double pane window U-factor is .29, one of the lowest in the industry. Even if the window is energy efficient, custom home windows are always going to have a more airtight fit to the home.

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