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Structural support for windows/door with exterior insulation

Joshua Rogers | Posted in Green Building Techniques on

I am doing a large renovation that will involve adding 3″ of rigid foam insulation and I’m installing triple pane Inline Fiberglass windows that are quite heavy and I’m concerned that the methods that I’ve seen will not have enough structural support for the windows and sliding glass doors. What I have seen is having 3/4 plywood bucks that span the entire opening (from face of exerior insulation +/- furring strips to the face of the interior framing). My concern is that if I have a window unit that weighs in excess of 1000lb, will screwing through the window flanges into the framing support the window or sliding door enough given that the window is essentially resting on a cantilevered piece of 3/4 plywood? Maybe this should be a question for a structural engineer, but so far in all of the videos I’ve seen, people are installing relatively light weight double hung windows.



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  1. Raff Winks | | #1

    for a 1000lb window I would suggest going with the "innie" install..
    Sounds like people in the vids you're watching are cutting corners to save a buck/make a buck.
    The Remote manual suggests using 2x material for bucks, and is what I used. The 1.5" thick tempered glass, double pane, patio door I still mounted as "innie". My biggest window hanging off 2x bucks is a 48"x100" double hung Pella.

  2. Aj Builder, Upstate NY Zone 6a | | #2

    2x6 pushed out 3". Then 1 1/2"x1 1/2" foam, then same in wood. Now your window has solid nailer and the made up buck has thermal break. 2x should hold all the weight of any normal rough opening.

  3. Ethan ; Climate Zone 5A ; ~6000HDD | | #3

    Raff and AJ and Joshua... Do you think the Inlines can be supported by 2x cantilevered bucks? Are you concerned about the thermal bridge of this 2x buck? What would be the max cantilever you can imagine using with one of these 2x bucks Joshua what did you end up doing?

  4. Robert Opaluch | | #4

    A 2x face-nailed against the exterior of the wall sheathing or studs, below the extended out 2x6 sill or window buck, would extend support for the window 1.5". Not much thermal loss, given its small size vs. the wall.

    A few steel brackets below the extended sill or window buck, aligned with the window cripples, could provide even more support, but metal conducts excessive amounts of heat, though in a limited area.

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