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Community and Q&A

Flashing a window in concrete block (no ext foam insulation, no housewrap)

user941025 | Posted in General Questions on

I’m looking around for good detail for flashing a window I’m installing in 12″ concrete block (with a 2×4 buck). There’s no exterior foam or housewrap there. Do I just extend a layer of Grace Vycor across the gap from the buck to the concrete on every side below the nailing flange, lapped for positive drainage, tape again at the sides and top and leave weep at the bottom, then double up using heavy beads of watersealing adhesive behind trim boards, and foam from inside?

That’s a whole lot of tape by the time I’m done….

That corner of the house exterior has never seen a drop of water, so I’m not otherwise worried in the least about rain.

Am I on the right track? Advice appreciated. Thank you.

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  1. GBA Editor
    Martin Holladay | | #1

    More details, please. You call the building a "house," so I assume this is a basement window?

  2. user941025 | | #2

    Yes, it's going in the basement of my house (1925 bungalow). The sill here will be about a foot above grade, and it's well-sheltered.

    There is stucco directly above (vertically) where the window is going, and that stucco comes out about an inch and a half from the face of the concrete block where the window will go.

  3. davidmeiland | | #3

    Unless you're going to extend a flashing material down from behind the stucco, your only real option is to use sealant. All you're trying to do is keep water out from behind the window flanges, especially the top--you're not keeping water out of the wall as you would with a wood-framed wall. I don't think the Vycor you describe above would do much. I would install the window to the buck with sealant behind the flanges, then install the trim with sealant behind it as well. If the window is not wood there isn't much that can go wrong. I would expect an install like this to require maintenance of the sealant periodically, in MN with the temp swings it might be more often than here in the balmy NW.

  4. user941025 | | #4

    Excellent. If that's sufficient, that sounds about as easy as it's going to get. Thank you.

  5. GBA Editor
    Martin Holladay | | #5

    Most basement windows are not flanged windows. If you know that the window will be installed in a wooden buck inserted in a concrete-block basement wall, it's best to order windows without flanges.

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