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Community and Q&A

Minimum R-values for exterior foam for 2×6 walls

Craig Steinman | Posted in Energy Efficiency and Durability on

I am building an Arctic entry with 2×6 walls, what is the minimum R-values for exterior foam over my OSB sheeting and house wrap to reduce the chance of moisture accumulation in the walls. I live in Wasilla Alaska, which is zone 7 with about 10,500 Annual Heating Days.

After completing the Arctic entry plan on removing siding and retrofitting all outside walls.

Thanks, all help/info will be appreciated

Craig Steinman

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  1. GBA Editor
    Martin Holladay | | #1

    Here's the short answer: R-15.

    For a detailed discussion of the issues surrounding exterior rigid foam on walls, see Calculating the Minimum Thickness of Rigid Foam Sheathing.

  2. Aj Builder, Upstate NY Zone 6a | | #2

    Can you see Russia from your place too?

  3. Expert Member
    Dana Dorsett | | #3

    The relevant code on that lives here:

    It's important to note that at AK winter extremes you can't rely on the labeled R-value for the exterior foam, and be particularly wary of polyiso, which performs quite poorly when the average temp through the foam is below 25F. The cheapest and possibly best solution would be to use 3,5" of 1.5lb density (Type-II) EPS, which while labeled at about R14.7, it's R/inch increases to about R4.7/inch or higher during the coldest weather, performing at about R16.5 then, and delivering an annual average performance of about R15.5-R16 in your location & application.

    A layer of 2" with the seams taped with housewrap tape and sealed over with duct mastic, for adhesion insurance, with a second layer of 1.5" staggered with those of the other layer by a foot or more (with it's seams treated similarly) helps keep it air-tight over the long haul, and over seasonal thermal expansion/contraction.

    This presumes you'll be putting R21 fiberglass or R23 rock wool in the 2x6 cavities, with only latex paint on wallboard as the interior vapor retarder.

    Better than EPS would be to use a similar depth of rigid rock wool, which would offer vastly more drying capacity toward the exterior, and doesn't experience thermal expansion/contraction issues to nearly the same degree as foam board.

  4. Craig Steinman | | #4

    Thanks Martin & Dana for the info.....

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