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foam board WRB tie in with rough openings

cord12 | Posted in General Questions on

We have 1” of XPS tongue and groove foam board used as the water resistive barrier of our house. Existing windows are flanged mounted with tape flashing attaching the flange to the foam board. Under the foam board is OSB sheathing.

We are in the early stages of planning to replace the flanged mounted windows and for a number of reasons are considering to install non-flanged windows.

I’m looking for suggestions on how to best flash the rough openings. Specifically, how to best integrate the water barrier of the foam board with the inside surface of the window rough opening.

Climate zone 6.

Thanks, David

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  1. Expert Member
    Michael Maines | | #1

    David, why do you want to tie your WRB to the inside face of the rough opening? The outside face of the window is the WRB, with a sill pan to help with any leaks. In your case, that means sealing the window to the face of the XPS, just don't seal the bottom of the window so the sill pan can drain.

    Air control is another matter--what is your air control layer? I usually treat the sheathing as the air control layer. I've never tried making exterior foam airtight but I've heard that others have. Some builders prefer airtight drywall but I find that problematic in most cases. The inside of your window is a continuation of the air control layer so wherever it is, it needs to connect (with tape, sealant or canned foam) to the interior of the window.

  2. cord12 | | #2


    Sorry for the poorly worded question. Basically, I'm worried about the long-term integrity of the connection between the rough opening flashing and the XPS. Based on a few spot checks under the siding, I've already found places where the original tape (Dow Weathermate) has failed. I've looked at both Siga Wigluv and DuPont FlexWrap tapes and they seem like potentials, but at the moment I leaning toward just wrapping all the foam board in house wrap.

    The house is 17 years old with no air sealing. Through the years, I've done a decent job of air sealing from the inside, but there are places that I can't get to. So, I'm thinking of taking down the foam board in key locations to air seal the OSB, reapply the foam board, and then wrap it all in house wrap.

    Thanks for taking the time to respond, David

    1. Expert Member
      Michael Maines | | #3

      David, thanks for the additional information. I agree that XPS and tape does not make a reliable long-term WRB. While in new construction I prefer to keep the WRB at the sheathing, it can also go on the outside of the foam.

      I would prep the openings like any other: wrap the edges with flashing tape so you can tie the window interior into your air control layer, and use a membrane sill pan at the bottom.

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