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U value of double windows for sound proofing?

Griffin728 | Posted in Green Building Techniques on

Hello, I’ve just recently seen a soundproofing technique where and inner and outer window are installed in the same rough opening with a very large air gap between them (4-6″). What would the U-value impacts be of such an arrangement? Is there a way to calculate this outside of doing a labratory test? Let’s assume two double hung windows with U= 0.30 ea. Does the air gap do anything for thermal performance as it does for sound?

Also, if the interior space is as air sealed as can be with normal installation techniques, will these get condensation build up?


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  1. GBA Editor
    Martin Holladay | | #1

    The old rule of thumb, before low-e coatings were developed, was that a single-glazed window was R-1; a double-glazed window was R-2; a triple-glazed window was R-3; and a quadruple-glazed window was R-4.

    Low-e coatings complicate the calculation, of course, but if you assume that the air space between your two windows contributes between R-1 and R-2, you'll be in the right ballpark.

    The other old rule of thumb for multiple windows is to install the outer window to be a little leaky, and the inner window to be as airtight as possible, to minimize condensation problems. (Outdoor air is dryer than indoor air during the winter.)

  2. charlie_sullivan | | #2

    In case it's not clear you get a better estimate by simply adding the r values (1/U) than by adding the r values and adding 1 to that for the air space. Martins rough numbers illustrate that. Double gets you R2, not R3

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