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Wall Assembly in Portland, ME

Brad17 | Posted in General Questions on

Hey folks,
I was hoping to get some feedback on our proposed wall assembly for a new house in Portland, ME. 

Hardie Board shiplap
hydro gap house wrap 
Rockwool Comfortboard 80 exterior continuous insulation
OSB Sheathing
Rockwool Batt to fill 2×6 cavity
Gyp Bd.

Thanks!

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Replies

  1. Expert Member
    Michael Maines | | #1

    Brad, you'll need something to attach the Hardie Board to. With continuous exterior insulation, that's usually vertical 1x furring strips, though thicker wood strips also work. Portland is on the 2021 IECC so you'll need at least R-5 Comfortboard over 2x6 framing, though it would be best to use at least R-12 exterior insulation for long-term protection against moisture accumulation in the wall assembly. In theory you might be able to use long nails to support the Hardie through the Comfortboard but I don't believe Hardie provides a warranty for that approach, and I wouldn't recommend it.

    Where is your continuous air control layer? I usually place the WRB (house wrap) at the sheathing layer, or use the Zip system as both an air control layer and WRB.

  2. Brad17 | | #2

    Thanks, Michael. Roger that about the furring strips. Ugh the house wrap was out of order! Yes, it is on sheathing, then comfort board layer. I was hoping that I could avoid having to go to R-12 if we used mineral wool because of its moisture resistance. If that isn't adequate, would using blueskin on outside and/or intello on inside make the difference?

    1. Expert Member
      Michael Maines | | #3

      Because Comfortboard is vapor-open it's ok if it doesn't have enough R-value to keep the sheathing warm enough to prevent moisture accumulation. In other words, code-compliant R-5 (over a 2x6 wall) is ok; it's just better to have more insulation. The potential moisture issue is at the sheathing, not the exterior insulation, so the moisture resistance of mineral wool isn't important.

      This article and the related content might help you understand the various factors at play, even though you're not planning on using foam: https://www.greenbuildingadvisor.com/article/persistent-worries-about-exterior-rigid-foam.

  3. Brad17 | | #4

    Thanks again Michael. Great article, super helpful. To follow on your comment about the sheathing, in your opinion, would the general vapor permeability of my assembly allow the sheathing to dry in both directions, precluding the need to go to the R12 for exterior?

    1. Expert Member
      Michael Maines | | #5

      Brad, yes, the lowest-perm part of your assembly is the OSB sheathing, at somewhere around 1 perm which still allows some drying, and if the OSB gets damp its permeance increases to dry more quickly. In Portland, vapor drive is almost always from interior to exterior so there is not much need to dry to the interior, except when air conditioning is running which seems to be more every year.

      I missed that you weren't planning on an interior vapor retarding membrane. Unless your exterior insulation is at least R-11.25, which allows painted drywall (a class 3 vapor retarder) as the only interior vapor retarder, you need a class 1 or class 2 membrane at the interior. I recommend Siga Majrex (sold by Performance Building Supply on Diamond St) but other products will also work.

  4. Brad17 | | #6

    Great advice! Thanks again Michael.

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