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What window assembly has the highest U-factor?

Eric Lechuga | Posted in Energy Efficiency and Durability on

I’m trying to buy windows and would like to know which assembly has the highest U-factor.

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  1. James Morgan | | #1

    U-value is the inverse of R, so I think you should probably be looking for the assembly with the lowest U-factor, not the highest. You'll find that information in the manufacturers' data sheets.

  2. GBA Editor
    Martin Holladay | | #2

    For operating windows, it's going to be hard to find a window with a lower NFRC U-factor than 0.17 (that's argon-filled triple glazing with 2 low-e coatings). You can find a lower U-factor for a fixed window, of course. Be suspicious of U-factors lower than that for operating windows -- they are probably center-of-glass U-factors, not whole-window U-factors.

  3. Jan Juran | | #3

    Eric, Serious Windows offers its 925 Series of Picture Windows with a U-factor as low as 0.11 and R-factor as high as 9.1. This window series features dual pane glass and 2 suspended low Solar Heat Gain Films with krypton gas fill. However the Solar heat Gain Coefficient is below 0.25 and the Visible Transmission is 0.4 or lower. Keep in mind that south facing windows with good solar access in heating climates may perform better with a higher SHGC and lower R-factor (higher U-factor) on a net annual basis, because they transmit more useful solar heat gain into a house during the low sun angle winter months even though they are less effective re insulation value.

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