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Sealing around windows

arnoldk | Posted in General Questions on


Is there a reason not to spray foam (low expending) the entire gap between the rough opening and the window?

Thank you,

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  1. kbentley57 | | #1


    It is typically done that way, and it's easy to overdo it, you don't really want to have the foam come past the interior plane of the window, or you'll have to trim it off for the finish.

    An alternative is closed cell filler rod, with caulking if the gap is small.

    1. arnoldk | | #2

      Hi Kyle,

      Is there any risk having the foam extending out flush to the exterior sheathing?
      Would doing that not allow any water that may find its way in to drain out from the bottom of the window flange?

      Thank you,

      1. kbentley57 | | #3

        I suppose in theory that if water got behind the top window flange, ran down to the side along the top of the window and hit the foam, it may not have anywhere to go, in a completely filled void space.

        I think there may be a blending of different concepts at play here though. The sloped sill, and the un-taped bottom flange serve as a method for water to escape, if that water were to ingress any part of the window frame itself. It's not a method for controlling bulk water entering from the top of the window behind the flange.

        At that point you're kind of hosed anyways, A flanged window should not only be sealed with a bead of some form of caulking around the top and sides, but also have head joint flashing, and be sealed to the WRB on the three sides. There's not really a method that's common to allow for bulk water intrusion around the edges and redirect it safely to the exterior under the bottom flange.

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