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Double interior rock wool

Scott Mangold | Posted in Energy Efficiency and Durability on

Working on plans for my next home. After lots of research on here and others I am considering the following wall assembly. From the exterior… wood siding, tar paper, 1/2″ plywood with taped joints, 2×4 ,16 on center wall stud with roxul, uniterupted layer of 1 1/2″ comfort board, layer of a variable perm vapor barrier (taped seams and such), then a layer of 2×3 installed 2 ft. on center as battens for electrical with shallow boxes , second layer of comfort board fit between the battens, drywall finish. I am in zone 5 in northern ct.
should yield a r30 ish wall, thermal break and a tight construction.
Thanks for any thoughts…

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  1. Expert Member
    Michael Maines | | #1

    Add a rain screen and pay attention to airtightness at transitions, and you'll have a decent assembly.

  2. Jonathan Lawrence CZ 4A New Jersey | | #2


    Any reason you don't want to put the ComfortBoard on the exterior? It will be held up by the rain screen and keep your sheathing nice and warm. If I read your post correctly, it appears as if you have the ComfortBoard between the studs and the service cavity battens, as well as in the service cavity, which could make for uneven drywall.The concern with ComfortBoard is usually with its somewhat less than rigid nature making it more time consuming to get a truly flat surface. Personally, I would rather place the potential risk for uneven surfaces on the exterior than the interior.

  3. Brendan Albano | | #3

    To elaborate on Jonathan Lawrence's comment, why not use the standard 2x6 with exterior insulation stackup?

    Wood siding
    1x4 furring strips
    2" Roxul Comfortboard (R-8)
    Tar paper (or other house wrap)
    1/2" Plywood with taped joints
    2x6, 24" on center, with fiberglass batts (R-19)
    Drywall finish

  4. Scott Mangold | | #4

    Jonathan and Brendan . I feel that detailing with flashings at dormers, windows and the like adds quite a bit of labor to the exterior finishing. These details all get focused inside. I am not sure why this would create a wavy surface as I would be screwing 1 1/2 " thick spruce material through the realativly dense comfort board. The stiffness of this thickness I believe will limit the wavy effect dramatically. I also feel that with the detailed vapor barrier set 1 1/2 " in behind the drywall it will keep moisture from condensing on the cold plywood. I could be wrong with this ? Thank you all for your responses. Michael , seems good advice for a rainscreen. Bumping the siding out on 1/4" p.t. Lath would be pretty economical and not very time consuming...

  5. Scott Mangold | | #5

    Maybe you mean the 1 1/2" Roxul would stick in proud of the 2xs... I could make the inner layer 1 1/4"..

  6. Expert Member
    Malcolm Taylor | | #6

    Once you have identified a number of wall assemblies that would work fairly well for your project, I think you are right to pick one based on your own preference for how the work will be performed - especially if you are doing a lot of it yourself. As Michael said, with the addition of a rain screen you end up with a pretty good wall. If it's what you are comfortable with, I'd do it the way you are planning.

  7. Jonathan Lawrence CZ 4A New Jersey | | #7


    Good point on the detailing. It does complicate things a bit with the insulation on the outside. Plus those 6"-7" screws needed to attach the furring strips to the studs are not cheap.

  8. John Clark | | #8

    IMO you'll go mad trying to even out the wall with the layer of Comfortboard between the 2x4 stud and the individual 2x3 battens.

    Have you actually handled the Roxul product in order to get a feel of compression?

    Why not throw up something like insulated nailbase such as albeit thick R12 ZIP-R on the outside and insulate the cavity as you wish?

  9. Scott Mangold | | #9

    I have only handled the comfort board at the JLC show in RI. The lumber yards I deal with only seem to be able to get it in full pallet quantities. I am avoiding using foam as I prefer something a bit more natural .

  10. Jonathan Lawrence CZ 4A New Jersey | | #10

    If I recall correctly, they had the old brand name ComfortBoard IS at the show and I think in just the 8 lb density. Now they sell ComfortBoard 80 (8lb) and ComfortBoard 110 (11lb). The 80 is bit squishy as Chris is hinting to. The 110 is very rigid, almost as rigid as Type II EPS, but of course more expensive than the 80. They market the 110 for commercial jobs, but you can get it residential.

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