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Fact or Fiction?

Are "R-25 Window" Claims TRUE?

In a word, "NO!" May the buyer beware of misleading claims that appear to be making the rounds locally, prompted by an out-of-state manufacturer. An R-25 window equates to a .04 U-factor (they are reciprocals), and web searches reveal no windows on our planet yet with such a low U-factor. Have they used some "fuzzy math" to simply "add up" an R-25 thermal performance from separate performance estimates, one for foam and the other for vinyl? Not too many years ago, some window companies used to report only the center-of-glass U-factors while others reported total window performance. The resulting discrepancies were a motivating factor for the feds to threaten regulation of our industry that, in turn, led to the creation of the NFRC and its label system. The truth is foam-filling does slightly improve U-factor (confirmed by NFRC simulation from 0.30 to 0.28 on our single hung vinyl window, for example), but not to the massive degree implied and up-charged! They estimate the thermal value of a part of a window and then seem to report that its performance represents the total window's thermal value.

Simply Stated:

R-25 is achievable in microscopic "Portions" of the window frame only. The frame makes up maybe 5% of the total opening space, therefore it is unreasonable to expect the microscopic area to be a noteworthy performer.

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