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Thermal bridging

kp_newbuild | Posted in Green Building Techniques on

Hello, I am looking for more information or to speak with someone about external foam insulation. My builder wants to put on 1/2” foam, but I am unsure this is the right thing to do. We currently have it framed as a 2×6 and are considering closed cell foam on the interior. My concern is the thermal bridging where he suggested the 1/2” exterior foam. We are in climate zone 6 with climate 7 in the county just north of us. Would it be okay to put on the 1/2” or put on none at all?

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Replies

  1. Zephyr7 | | #1

    1/2” foam is NOT enough in your climate zone, especially with 2x6 walls. 1/2” foam isn’t more than about R3 at best, regardless of what type of foam you use. See this article:

    https://www.greenbuildingadvisor.com/article/calculating-the-minimum-thickness-of-rigid-foam-sheathing

    You need between R11.25 and R15. Using polyiso, that’s about 2”-2.5”. Derating a little for cold weather puts you closer to 3” needed. Any other type of foam will be thicker for the same R value.

    You should also be aware that some consider exterior foam that is of a type and thickness to act as a vapor barrier to be risky when combined with interior closed cell spray foam.

    Bill

  2. User avatar
    Dana Dorsett | | #2

    With closed cell foam on the interior the assembly is protected from interior side vapor drives, and half inch foam on the exterior is sufficiently vapor open to dry toward the exterior, as long as it doensn't have low permeance foil or plastic facers.

    That said, the performance boost of that half inch of exterior foam exceeds the performance boost of that expensive closed cell foam in the cavity. It's much more cost effective to install R20-R23 fiber insulation in the cavities and boost the thickeness of the exterior foam to where the sheathing stays warm enough to not need an interior side vapor retarder, letting the assembly dry toward the interior. In zone 6 that would be R11.25 or greater on the exterior, which can be achieved with 2" of foil faced polyiso.

    See the math on closed cell foam between studs here:

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

    1. kp_newbuild | | #3

      Thank you, Dana. I am unsure if I can talk my builder and significant other out of closed cell spray foam as they believe it is the best. If my only option is adding 1/2” how would I do that properly in my climate?

      1. User avatar
        Dana Dorsett | | #4

        What, your builder & s.o. don't believe in ARITHMETIC? L'aritmetica non è un'opinione...

        A belief in the superiority of closed cell foam in stud cavities seems like a cult.

        I'ts hard to find half inch EPS without facers (which would created a potential moisture trap, but unfaced half inch XPS is sufficiently vapor open to allow drying toward the exterior, and can even be detailed as the weather resistant barrier. It's thin enough to long-nail siding through it. In zone 6 with a vapor open cavity fill you'll need a smart vapor retarder such as 2 mil nylon (Certainteed MemBrain) or Intello Plus, but with 2" or more of closed cell foam wasted on the interior the vapor retarder would not be necessary. Using a ship-lap edged foam sheathing keeps a full gap from forming as it shrinks with age. With XPS you can go as thick as 1" without creating too serious a moisture trap.

        Seriously, spending the foam budget on thicker exterior foam is by far the better option. In zone 6 as little as 2" of foil faced polyiso or 3" of EPS on the exterior is sufficient for keeping the sheathing warm enough in winter to not need an interior side vapor retarder tighter than standard interior latex paint.

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