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Do the usual minimum thicknesses apply to “R-Sheathing” products?

cykilbourn | Posted in Energy Efficiency and Durability on

Huber and RMax and maybe others offer insulated sheathing – OSB and polyiso fused together with the OSB on the outside. When building a wall assembly with this, do you have to follow the minimum thickness guidelines here:

even though the OSBis on the outside of the envelope and will be able to dry to the outside? Specifically, I’m wondering about a 1.5″ R5 insulated sheathing on a 2×6 wall in Southern Maine (Climate Zone 6, but close to 5).

The cavity insulation will either be unfaced batts or Low Density Spray Foam. Will the answer change depending on which of these is chosen?

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  1. GBA Editor
    Martin Holladay | | #1

    I say that the rules still apply. If you skimp on the foam thickness, you might get condensation on the interior face of the foam sheathing layer -- and that moisture won't dry easily to the outside.

  2. cykilbourn | | #2

    Thanks Martin, this was my understanding as well. I had another idea - if you used LD spray foam but only sprayed 3" in a 2x6 cavity for ~R10.8 could you then follow the minimum thickness guidelines for a 2x4 wall instead of 2x6 wall?

  3. GBA Editor
    Martin Holladay | | #3

    If you skimp on insulation in your stud cavities, the thin insulation will keep your wall sheathing warmer. That way you will avoid condensation problems. So you are right -- this technique will work.

    However, it's a shame to have to keep your sheathing warm by this inefficient mechanism -- by deliberately skimping on cavity insulation so that the leaking heat from your home warms up the vulnerable surfaces.

  4. cykilbourn | | #4

    Agreed. Although in this instance it's not the sheathing that is vulnerable (that's on the outside), it's the interior surface of the polyiso. It's a shame to waste cavity insulation space, but it may be the best option in this case. 2" rigid wrap is out of budget, and 2x4's are out for other reasons. So it's either this or full cavity insulation with no exterior wrap. I'll take the insulated sheathing for infiltration and thermal bridging reduction.

  5. Expert Member
    Dana Dorsett | | #5

    Note the IRC prescriptive values also spell out other asseblies that comply:

    The permeance of Huber R-Sheathing is 0.8-1.1 perms, on the Class-III & minimal Class-II vapor retardency boundary. Since it is semi-permeable the cavity can still dry toward the exterior at a reasonable rate, provided you include rainscreened siding, it would be fully compliant with IRC 2012 in 2x6 framing with a full fill of fiber or open cell insulation through climate zone 5, and would not present a problem climate zone 6 (but may be open to interpretation by inspectors.) But even in climate zones 7 and higher it would meet code using variable-permeance "smart" vapor retarders such as MemBrain or Intello Plus.

    RMax & Dow SIS are rated sub-0.3 perms, and would not meet code without an interior side Class I or II (or variable type) vapor retarder, since they're not semi-permeable.

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