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What’s the best WRB option when using foam to insulate your structure / sheathing?

GBA Editor | Posted in Energy Efficiency and Durability on

I’m most concerned about the critical details around windows. I plan to use OSB or plywood sheathing coated with StoGuard as the building’s air barrier. This will be covered by Tyvek Drainwrap, followed by two layers of taped foam insulation (XPS or Polyiso). Finally, I’ll have an airgap between the foam and siding (i.e. furred fiber cement). What is the safest option for windows: 1) Use Drainwrap as WRB, flash windows to wrap, then use exterior trim? 2) Use foam as WRB, extend window box flush with foam, install and flash flanged windows to foam? 3) Install building paper on top of the foam, flash to this layer? (Note: Picton Brothers used this third option on their LEED house).

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Replies

  1. Daniel Ernst | | #1

    I forgot to provide a context for the project:

    The location is the Shenandoah Valley, VA in a mixed-humid climate. It's in Zone 4, but near the border of Zone 5. The closest climate data point is Staunton, with 5447 HDD and 622 CDD (65 F base). The annual rainfall places it in the High exposure category.

    I would like to hear from builders and designers that have hands-on experience with exterior foam insulation. I'm leary about the water management details with foam, leary about condensation potential without foam! I know these are no easy answers and the installation details can have a dramatic effect on whether any of these systems will be successful. That said, I would like to approach the project with the best possible strategy.

  2. Daniel Ernst | | #2

    Looks like I may have found part of my answer here (this site has A LOT of material to sift through)!

    https://www.greenbuildingadvisor.com/blogs/dept/musings/innie-windows-or-outie-windows

  3. User avatar GBA Editor
    Martin Holladay | | #3

    Daniel,
    I'm glad you found a helpful blog.

    Any one of your listed options will work, as long as you get the details right. It sounds like you're on your way to building a wall that will perform well. Good luck.

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