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New Home – Wall Assembly Review

Nethaniel Ealy | Posted in Plans Review on

I’m building my first custom home this summer and could use GBA’s collective wisdom to vet the wall assemblies.

Stats:
Zone 5a
Logix Platinum ICF basement (R-28)
2×6 advanced framing, blown cellulose, 7/16″ OSB, Tamlyn Drainable WRB, 1.5″ Terrafoan EPS, Fiber cement or steel siding.
Conditioned attic with min. R-38 closed cell spray foam
“Innie” window placement

Questions:
Will placing the drainable WRB between the sheeting and EPS cause air flow that will serve to defeat the EPS insulation?
Does this mean I should move to an “Outie” window placement?
Should I be concerned with air penetration through all the siding fasteners that go through the EPS and WPB? How can I mitigate this?
Other comments concerns?

Thanks in advance for your help to this newbie!
Nethaniel in Idaho

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Nethanial,
    Since you are posting your question on a green building web site, I assume that you don't want to skimp on insulation R-value.

    The minimum code requirements for ceiling insulation in your climate zone is R-49, so your proposal to install R-38 is less than the code minimum.

    If you have 2x6 walls, and you want to install rigid foam on the exterior of your walls in Climate Zone 5, then the rigid foam needs a minimum R-value of R-7.5. You are proposing using only R-6 foam. That's not enough. For more information on this topic, see Calculating the Minimum Thickness of Rigid Foam Sheathing.

    Q. "Will placing the drainable WRB between the sheeting and EPS cause air flow that will serve to defeat the EPS insulation?"

    A. No. The degradation in performance is very minor and not worth worrying about.

    Q. "Should I be concerned with air penetration through all the siding fasteners that go through the EPS and WPB?"

    A. No, but it is essential to know what materials you are depending on as your air barrier, and it's always a good idea to perform a blower door test some time during the construction so that air leaks can be located and sealed. I don't recommend that you fasten your siding through 1.5 or 2 inches of rigid foam; it's much better to install vertical furring strips on the exterior side of the foam to create a rainscreen gap, and then to install your siding to the furring strips.

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