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Windows installed over 2 inch rigid foam insulation

Salesi | Posted in General Questions on

After sifting through many documents, videos and different opinions, I am leaning toward installing our new windows on top of 2″ polyiso. I would install 1″ x 4″ polyiso and then a 1″ x 4″ board around the window opening to provide a thermal break. The window flange would be attached to the 1″ x 4″ board. The fill in everything else with 2″ polyiso to level the plane. I was considering using Henry Blueskin VP100 on top of the polyiso foam and cut and wrap it on the sides of the window opening to seal everything up. Flash the bottom sill over the Blueskin so water drains away. Any flaws in my plan??? Almost forgot, zone 5, rain screen on top of polyiso, siding on top of rain screen. Thanks for anyone’s assistance.

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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi Salesi.

    There are a number of ways to install windows in walls with exterior rigid foam. If your polyiso is foil faced, you don't need a water-resistive barrier over it. You can flash the windows to the foil. (Some builders use a drainable housewrap behind the rigid foam for a bit of drainage and drying potential should water get back there.)

    The only other thing I see in your post that could be a "flaw" is flashing the bottom of the window. It is best practice to install the windows above a sloped sill, backdam, or both and leave the bottom flange of the window open for draining and drying. The window is air sealed from inside, near the inside edge of the frame with canned spray foam, backer rod and caulk, or tape.

    You may find these articles helpful:

    Installing Windows in Thick Walls
    Dudley Boxes for Windows
    Innie Windows or Outie Windows?

    1. Salesi | | #2

      Thanks for the response. The bottom of the window will be on a piece of beveled cedar siding that will have a flashing tape over it and lapped to the outside drainage plane. Then the window will be set in place so any water that make it to the window opening will drain away to the outside. On the drainable house wrap, I am considering using Benjamin Obdyke HydroGap wrap that would put a small gap between the sheathing and insulation board.

      1. GBA Editor
        Brian Pontolilo | | #3

        Hi Salesi.

        Both of those details sound good to me.

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