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When installing “innie” windows, how do I keep water from getting between the WRB and the exterior foam?

Ry38men8ZA | Posted in Green Building Techniques on

When installing an “innie” window (the window and WRB are directly on the sheathing), then applying 2″ of foam over the WRB followed by rainscreened siding, how do you keep water from getting between the rigid foam and the WRB? Or does it matter?

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    F.B.,
    Q. "How do you keep water from getting between the rigid foam and the WRB?"

    A. Good flashing details.

    Q. "Does it matter?"

    A. Not really -- especially if you have a wrinkled WRB and a path for drainage at the bottom of your wall.

  2. Ry38men8ZA | | #2

    Where can I find some good flashing details? I understand how to flash the window and WRB to the sheathing, but the details I have seen do not show how to keep the water from getting between the foam and WRB when using exterior jamb extensions.

  3. GBA Editor
    Martin Holladay | | #3

    FB,
    The critical area is obviously the sill. The best material for flashing the sill area would be 16-oz. copper, formed in a brake, with vertical legs on each side. The exterior "jamb extensions" could be made of metal flashing, cedar lumber, or composite (plastic) trim.

    If you aren't familiar with roofing and flashing details, you might need to hire a good sheet-metal contractor or roofer to help you.

  4. Riversong | | #4

    Does it matter if water gets between the exterior foam and the WRB?

    I strongly disagree with Martin. Not only could such water freeze, depending on your climate and the in/out R-value ratio, but it can find it's way past flashings or extensions into openings or joints. Once saturated, sheathing and framing tend not to dry with exterior foam and also remain warm enough for long enough periods to culture mold and decay.

    And I also disagree that the sill is the most critical point. Window head flashings are the most likely to leak of not properly detailed, since water comes top down.

    The purpose of the WRB is to keep the weather (rain and wind) away from the thermal envelope and the structural members. So it makes little sense to bury it between layers - it should be "to the weather". If you want a drainage path between foam and sheathing, you can create that with another 3D membrane, but that somewhat decouples the exterior foam from the interior thermal layers. Also, a wrinkled WRB may not be sufficient for drainage and drying, since there will be insulation on both sides of the sheathing to limit the heat flux necessary for drying to occur.

    These are some of the many reasons that I oppose the trend toward exterior foam board. I think there are strong arguments for the case that there will be too many unintended consequences.

  5. user-869687 | | #5

    I never understood how a weather barrier could still be considered a "drainage plane" when there is foam board sandwiched over it, unless there were enough of a gap behind the foam board for water to truly flow through that gap--and then this would seem to undermine the thermal value of the foam. Or, maybe the point is that if gaps opened up between sheets of foam (say as a result of shrinkage over time) then water could flow back to the WRB and drain along that plane. But either way it seems better to have a shingle-lapped water shedding layer at the exterior of any rigid foam and face that into a rainscreen gap, which can clearly function as both a barrier to weather and a surface for vertical drainage.

    To answer the original question, 1a above still applies.

  6. Daniel Ernst | | #6

    You might be interested in reading an article on the Building Science website, authored by John Straube and Jonathan Smegal, titled "Modeled and Measured Drainage, Storage, and Drying Behind Cladding Systems."

    http://www.buildingscience.com/documents/reports/rr-0905-modeled-measured-drainage-thermal-x

    One of their interesting discoveries was that a small drainage gap between layers (as small as 1mm) may actually improve drainage performance, and decrease the water storage capacity of an assembly, when compared to a single drainage surface. The authors speculated that such a small gap reduced the water bead size, lowered the surface tension for easier drainage. See p. 6 for details.

    Perhaps that's why BSC recommends the crinkled housewrap behind foam board insulation?

    Whether or not residual moisture between the housewrap and foam causes further problems, or how fast it dries . . . those are different questions.

  7. J99aAMQzYo | | #7

    JLC (Journal of Light Construction) has an article with detailed pictures and guidance on how to do exactly what you're asking. It's in the May 2009 issue if you subscribe. If not, you can go to http://www.jlconline.com and subscribe to their online archives for a few dollars and then download the full article in PDF format. Search for "Installing Exterior Insulation in Cold Climates"

    Basically it will require specialty sealers, tapes and custom bent metal. Not an easy process, but also not one you want to take lightly and get wrong, so this isn't an area to look for the "easy" solution.

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