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Comments on Wall Assembly

Howard Klein | Posted in General Questions on

Hi there,
I’ve been reading a lot on this board and have been working my architect to design a well insulated, energy efficient house. I would like comments on the proposed wall assembly;

The house is ~3000 square feet. It will be heated with Geothermal either via forced air distribution or radiant in floor heat.

It is a two story house consisting of the following

5″ stone masonry on the 1st floor and stucco over 4″ concrete block on the 2nd floor
1″ vented air space
2″ closed cell spray foam
5/8 exterior grade plywood
2×6 wood studs 16″ OC
Cavity filled with blown in cellulose or blown FG
membrain vapor barrier by certainteed
5/8 drywall

I am curious why the community thinks about the membrain vapor barrier… the builders here all insist it is needed.

Will this wall assembly keep dry by being able to breathe to the interior?

Thanks.

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Replies

  1. Howard Klein | | #1

    Forgot to mention - house located in Montreal, which I believe is zone 5/6.

  2. Dick Russell | | #2

    First, I wonder if instead of "2" closed cell spray foam" you mean a 2" layer of rigid foam board. I would think that this would be cheaper than spray applied cc foam, and allow a uniform gap under the stone/stucco. Is this foam layer, whatever it is, the WRB?

    As to the MemBrain, that's your required vapor retarder layer, it seems, and it could be used as part of the air leakage barrier as well, if properly detailed and integrated with other parts of that barrier. I used that on my house (superinsulated). If I had decided just to use the Airtight Drywall Approach (ADA), with a vapor retarder primer on the inside, I would have skipped the MemBrain. However, I used it as the air barrier, with edges taped to small strips of poly that had been placed and sealed under the bottoms of exterior wall plates and to the upper level ceiling sheet of MemBrain. The ceiling sheets were taped to similar strips of poly over the tops of interior partitions. Along with a few other details, I found that using MemBrain let me achieve a very tight house, about 0.8 ACH50. There are other ways of doing an air barrier, but in my mind if MemBrain isn't integrated with other parts of the assembly to be part of the air barrier, then it's just an overly expensive vapor retarder; VR primer is far cheaper.

  3. Howard Klein | | #3

    Hi,
    I do mean 2" of spray foam on the outside. It is believe (at least here in Montreal) that doing this ensures the house is air tight, and using just foam board would result in hundreds of taped seams that would allow air infiltration.
    It is though that using this with a 1" air gap between the foam and the masonry will be the WRB.

    Thanks for the comments on the membrain, will keep this in mind.

  4. Dick Russell | | #4

    Then how is the 1" drainage gap provided between foam and stone/stucco? Strips of something imbedded in the foam or applied before foaming?

    I have to wonder if the SPF is going to be applied so well and stay stuck, without separating here and there over time, that it will provide a better air barrier than rigid foam sheets with properly tape sealed joints. Will the SPF be used also at the edges of the plywood sheathing?

  5. Howard Klein | | #5

    According to the structural drawings the stone will be held in place with galvanized metal ties by dur-o-wall ?

    Why would there be any fear of the spray foam loosening up? I thought that stuff sticks to anything and is water proof.

  6. Expert Member
    Dana Dorsett | | #6

    Most stucco & masonry guys hate working with more than 1" of exterior foam, and it may require semi-custom masonry ties to get there (have you verified the dur-o-wall lengths?) but it's a decent stackup, and it'll dry OK.

    Canadian code will still require an interior vapor barrier- I'm not sure if MemBrain will be a code-compliant substitute for poly, (even though it will function better than poly overall), since the sheathing will still dip below the code-prescriptive design dew point of the interior air at the 99% outside design temperature. Some inspectors/municipalities are pickier than others. Even with poly, as long as the vent cavity behind the brick/stucco is vented a the top & bottom, the vapor retardency of the spray foam is low enough to protect it from those summertime "sun on wet masonry" peak drives, but clearly MemBrain would offer more resiliance.

    FWIW: A friend of mine recently tested his 3 story deep energy retrofit project house at 464cfm/50pa with a combination of some spray foam for air sealing on some parts, taped foil-faced iso over cellulose cavity filled stud bays on some others. It's definitely possible to get there with rigid foam rather than standing back and blasting the entire exterior with closed cell foam. (He still used way more closed cell foam than I was advising him to, but it 's his project, his budget.)

    If sprayed with an improper mixture or at too cold a surface temp there can be issues of shrinking or weak bonding to wood, but I wouldn't sweat that aspect here.

  7. GBA Editor
    Martin Holladay | | #7

    Howard,
    I'm having a hard time visualizing this wall assembly.

    On the first floor, you have 2x6 studs. I assume that the 2x6 wall is load bearing.

    To the exterior of this 2x6 wall, you have a 5-inch-thick layer of stone masonry. That's thick -- but not thick enough to be load-bearing, I assume. So this 5-inch-thick stone wall is probably supported by a concrete footing or foundation, and is probably connected to the wood-framed wall with masonry ties. Correct?

    Then on the second floor, you switch to 4-inch-thick concrete blocks. OK -- a little odd, but I think I get it. This concrete block wall is bearing on the stone wall below. Is that correct? And, presumably, on the second floor you have another 2x6 wood-framed wall, which is the bearing wall. Is this correct?

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