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Debating cavity insulation

Chris B. | Posted in Energy Efficiency and Durability on

I’m starting a house build in New Brunswick (Climate Zone 6A). My planned assembly has 3″ of EPS foam on the exterior (3″ Silverboard XS). I’ve already bought the Silverboard, so there’s no going back now.

To please the local code officials, I will be installing Membrain on the interior.

I’m debating installing open cell spray foam in the cavity for improved air sealing. However, local spray foam contractors felt I should install closed cell even with the 3″ of exterior foam so that I wouldn’t need the Membrain. I have concerns about a ‘foam sandwich’ if I installed closed cell sprayfoam with 3″ of exterior EPS, as my sheathing would have no way to dry. Is my concern with closed cell valid?

Would open cell foam in my cavity with 3″ of EPS on the exterior be a safe wall, even with the Membrain on the inside, or should I stick with a fluffy insulation, like blown fiberglass?

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Chris,
    You are correct. If you use spray foam, open-cell makes more sense than closed-cell in this application.

    Blown fiberglass will also work.

    For more information on this topic, see How to Design a Wall.

  2. Expert Member
    Dana Dorsett | | #2

    With either a 2x4 or 2x6 wall and 3" exterior Silverboard XS of you don't need the MemBrain for anything but inspector-appeasement.

    Providing the inspector-appeasement function with closed cell foam would be MANY times more expensive than with MemBrain. There would be a very modest (negligible, really) improvement in net thermal performance, but at a cost that's truly not "worth it". See:

    http://www.finehomebuilding.com/2017/07/10/closed-cell-foam-studs-waste

  3. Chris B. | | #3

    Thanks for the input. I can't find anyone around here who still does open cell anyway - they all want to do closed. When I asked AMVIC about it, they responded that I could install any cavity insulation in the walls with Silverboard, including closed cell foam. This is based on the foam having a claimed permanence of 3.48 perm for the XS. I'm skeptical though If it would be permeable enough at 3" to allow me to safely install closed cell foam on the other side of my OSB sheathing.

    What had me debating this was there wasn't a substantial cost difference between closed cell spray foam and blown in insulation when I factored in the cost of the Membrain. I could justify the expense if it gave me significantly better air sealing then my current strategy.

    My air sealing strategy right now is continuous exterior insulation from footing to roof with all seams taped and then the Membrain on the interior as a secondary barrier. I am getting the critical leakage areas spray foamed - rim joists and a 2" flash coat on the attic ceiling. My goal is 1.5 [email protected]

  4. Expert Member
    Dana Dorsett | | #4

    If it's 3.48 perms @ 1" it will still be over 1 perm @ 3".

    Dry OSB or CDX is less than 1 perm, so it doesn't much matter. Moisture arriving on the interior side of the sheathing takes longer to get through the sheathing than it takes to leave through 1+ perm foam on the exterior- it's still a "reasonable" drying path. Installing closed cell foam reduces the total amount of moisture that reaches the interior face of the sheathing in winter, but it also prevents drying toward the interior.

    A bead of caulk under the bottom plate of the studwall as well as between doubled-up window framing can still be important, even with the seams of the sheathing taped. Be sure to tape over the seam between the sheathing and the top of the top plate too.

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