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Why not stack windows in a thick wall?

AmiGanguli | Posted in Energy Efficiency and Durability on

I’ve been contemplating window details for a thick wall assembly, and reading about “innie” vs “outie” windows.

That leads me to wonder: why don’t people put two windows into the same opening?  What not an innie and an outie?  Wouldn’t that allow for the use of cheaper windows, while achieving higher r-values?

I’m sure I must be missing something, and there’s a good reason this isn’t done, but I can’t think what the reason would be.

Can somebody educate me on this?

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  1. freyr_design | | #1

    I think this would lead to condensation issues. There is a reason multiple pane windows are sealed units, and if it is not sealed (which no windows is perfect) then you would have a vapor sandwich

  2. AlexMcGlashan | | #2

    Historically where I live, this is what most homes once had in the form of storm windows that were permanently installed on the exterior wall. Condensation was dealt with weeping holes in the outer window.

    1. AmiGanguli | | #3

      That seems sensible enough. Why did people stop doing this?

      1. AlexMcGlashan | | #6

        If you do a search on GBA you'll find quite a few articles on storm windows. As for frost, we had some, but as long as the interior window was sealing properly nothing serious. Unlike the stories that my father would tell of waking up with snow on the bed that drifted in through the cracks :)

      2. Expert Member
        DCcontrarian | | #8

        I've lived in a few houses with storm windows. They're awful.

    2. AmiGanguli | | #4

      Answering my own question... storm windows are still a thing:

      This looks like more-or-less what I had in mind. Install a high-efficiency window as an "innie", and then have storm windows on the outside. Although I'm still not clear on the difference between a storm window and just having a second window with weep holes.

  3. DennisWood | | #5

    The current take on this is the suspended membrane window, like these folks are doing: I believe their thickest window is around R20. 5 membranes suspended between 1 layer of glass interior, and another layer of glass exterior.

    If you try to do anything else not 100% sealed, in cold climates the inside of the exterior window would be a wall of frost. I've been using acrylic inner storms (magnetic) that essentially do what you're suggesting, however any leaks are rapidly obvious by icing on the inside glass of the exterior window. These interior acrylic panels are very effective in stopping air leakage, knock transmitted sound down about 50%, and optically are more or less invisible. I've attached FLIR images taken in outside ambient temps of -22 C (windchill -34 C) of side by side double pane (non operating) units showing a difference of 13 F on the inside surface. One window was "bare", the other with an acrylic inner storm window. The windows are approx 24"x52".

    This makes a pretty massive difference if you're close to that window. My commercial building used triple pane (all casements, no double hung) and the acrylic panels as well. That was a hyper efficient building running under $200 of NG gas use to heat 9000 square feet, in a month that averaged -20 C.

    1. AlexMcGlashan | | #7

      As a fellow Canadian, I second Dennis' comment on litezone. The only caveat is that once you start going above about R16/17 the Visible Light Transmission drops below 30%. (Which may or may not be an issue for you depending on the application)

  4. jollygreenshortguy | | #9

    I stayed in a hotel in the Alps, a stunning old chalet with walls 30 inches thick and it had double sets of windows. I don't remember if each was double or triple glazed but it was impressive. The inner window was an inward opening casement. You could access the sill by opening it.
    The place was very comfortable.

  5. tim_william | | #10

    I'm in Maine CZ6 with single pane windows in a 2x4 framed 1950s house. We have window inserts made by a local nonprofit. The inserts are wood frames with 2 sheets of film plastic surrounded by a foam gasket. You shove them into the interior of the window. They are an improvement over nothing at all, but the glass on the shady side of the house ices up. They are really a stopgap before replacing the windows entirely.
    Those litezone windows seem really cool, are they available in the US?

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