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WRB detail with exterior foam and strapping

user-1037041 | Posted in GBA Pro Help on


I’ve got 2 layers of 1.5″ polyiso with seams taped and staggered. The panels are currently being held up with cap nails and are awaiting strapping @ 16″ o.c. My problem is the house wrap and how it fits into my window/R.O. flashing. I’ve read the article “Where Does the House Wrap Go?” and it says to locate it over the foam (I’ll have “outie” windows) and then attach the rainscreen strapping. Do I then hold the WRB OVER the “picture frame” strapping? This would mean that the WRB would be under the strapping in the “field,” but then be over the strapping around the window. This seems kind of bizarre to me, but what do I know? This is my first “green” build. And apart from having to think hard about every single detail, it’s going great!

Anyway, I think I’ll have this nailed if I can figure out what the heck to do with my WRB and rainscreen around the windows.

I’m sure more than one of you have dealt with this. Any ideas?

Thanks in advance.

Portland, OR (4C)

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  1. GBA Editor
    Martin Holladay | | #1

    Here's the key to flashing windows: you are flashing the rough opening, not the window.

    Start at the rough sill. The rough sill needs a sill pan. It can be a manufactured pan or site-built. Ideally, it needs to slope to the exterior and it needs to include a back dam on the interior side.

    Then flash the jambs. You want to be sure that any water that dribbles down the rough jambs is directed to the sill pan.

    Then install your window. Your window needs flashing at the head. Remember, the nailing flange isn't flashing -- it's just an aid to installation.

    The WRB at the head needs to lap over the head flashing.

  2. user-945061 | | #2

    Here's what I've found so far.
    1. Standard (brick mold) trim that comes pre-attached to most doors and windows isn't deep enough. Our exterior casing is site-built.
    2. Use the exterior foam as WRB. Seriously. Abandon all hope of separately attached WRB. Abandon all hope of WRB beneath foam. Everything gets easier when you do this, eg. how do you flash the never-discussed small penetrations with polymeric wrb outside of strapping? What about kick-out flashings? If you want to improve the exterior foam as WRB, pick the best details from here:
    3. Don't picture frame your windows with strapping. Drain foam beneath on to a thick window head flashing.

  3. GBA Editor
    Martin Holladay | | #3

    There is more information on flashing windows in foam-sheathed walls, along with some useful links, in this article: How to Install Rigid Foam Sheathing. Look for the section under the heading, "How do I flash the windows?"

  4. user-1037041 | | #4

    Thanks for your responses. In the blog titled "", they use a "Dudley Box," which is kind similar to what I'm doing. I've attached a drawing of what I have going on so far. I understand window flashing basics (I've been successfully installing windows for a long time), but I'm not really getting here is at which plane the window should be installed. If I have a 3/4" rain screen, does the window get attached to the strapping or do I forgo the strapping at the window? The problem, as I see it, is how do I lap the WRB (which is just behind the rain screen strapping) OVER the rain screen strapping at the window flange? Or do I install the window at the plane of the second layer of polyiso? If that's what's recommended, the window trim will be very proud of the window and I'm worried about how that will look.

    I hope I'm being clear with this -- I'm finding it difficult to describe. Maybe these pictures will help.

  5. user-1037041 | | #5

    PS -- sorry my pics are so gul-durned huge!

  6. davidmeiland | | #6

    Jeff, what's useful to me is to think of the strapping as part of the siding. Install the foam, the WRB, the sill pans, the windows... you are now dried in. Then, install the furring and the siding.

  7. user-1037041 | | #7

    Thanks, David --

    I had a feeling today, while skiing the hell out of some Mt. Hood powder, that I'm overthinking it a little. You might have just confirmed that for me.

    And thanks again, Martin and Jesse for your input.

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