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Community and Q&A

Single vs Double Hung window EE

oldbungalow | Posted in Energy Efficiency and Durability on

Generally I understand that picture windows are the best for light if ventilation is not needed, and casements/awnings if ventilation is.

However I’m curious if single hung windows, which are somewhat of a mix of fixed picture window above a lower vertical slider are more efficient (better sealed) than double hung since the top unit is fixed, or is the difference negligible.

I know the ventilation benefit of cracking open both top and bottom sashes, but leaving that out of the equation.

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  1. Expert Member
    BILL WICHERS | | #1

    The weak spot are usually the sliding seals. It’s difficult to make such a mechanism that seals as well as a pressed seal such as on a casement window. If the window manufacturer provides real air infiltration numbers (an air volume and the pressure at which the test was done), you can use that information to compare how well the windows seal. Unfortunately, sometimes all you see is a “<number” which isn’t a real number, it’s just saying that the window performs under the minimum needed to pass inspection.


    1. oldbungalow | | #2

      Right, so in the case of a single hung only the bottom sash has sliding seals, and top sash is completely fixed? So essentially the single hung has 1/2 the sliding seal area of a double hung.. Then the only other weakpoint is the interface between top and bottom sash (but that's the same regardless double or single hung)..

  2. Peter Yost | | #3

    HI KT -

    It's a shame that the NFRC system does not require reporting air leakage (AL) numbers, because then we could evaluate the specific impact of double hung vs single hung windows. My gut is that since hung windows don't seal by gasket compression, the difference between double and single hung is substantially less than the difference between casement and any hung window.


  3. Expert Member
    BILL WICHERS | | #4

    Some manufacturers DO publish real air leakage numbers (I know inline fiberglass does, I’m sure there are others). It may be possible to at least get an idea of the relative difference between single and double hung windows.

    My guess is the difference between single and double hung will be minimal when compared to the difference between double hung and a casement though. It’s all about the difference in how the seals work between hung and casement windows.


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