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Exterior Wall Assembly Questions

VTLiving | Posted in General Questions on

We are in the final stages of designing our above-grade exterior wall assembly for what we hope will be a “pretty good house” in central Vermont, and would be grateful for additional advice (previous post responses have been so helpful).  Our builder is not familiar with high-performance installations.  We currently have in mind the following layers, from the outside in:

1) Metal or composite board-and-batten style siding (likely vertically oriented)

2) Rainscreen

3) 1/2″ ZIP sheathing, taped

4) 7.5-inch “composite” studs, comprised of 2×6’s with ripped strips of 2″ ZIP-R tacked to the inside, to provide an R10 thermal break, with 7.25-inch rockwool batting in cavities [this is basically the Bonfiglioli wall, but using Zip-R instead of the homemade strips — our architect uses this a lot]

5) Intello Plus smart vapor barrier

6) 2×4 studs, with 3.5-inch rockwool batting in cavities.  Our architect typically specs strapping here, but the extra wall protects the Intello, gives us extra insulation, and also to make electrical, plumbing, drywall easy, so it seems like a good idea (?).

7) drywall

8) ordinary paint

Our primary question is whether we should worry about the air gap resulting from having 7.25-inch rockwool batting in 7.5-inch deep cavities — and, if so, what we might do about it?  I would expect we would place the air gap immediately behind the intello.  If it is a problem, one possible solution we considered is to use 1.5″ ZIP-R to make a 7.0-inch cavity, and put two layers of 3.5″ rockwool in there.  

A secondary question: what would be the best way to construct the rainscreen in this case?  We’ve heard diagonal stripping can be used for vertical siding.  Cor-A-Vent will, of course, be substantially more expensive, but is on the table.  The 1.5-inch bidirectional strapping behind the siding, seems like a lot of extra work over diagonal or Cor-A-Vent, and also makes the wall much thicker and adds some complexity to doors/windows.

Thank you all very much!

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Replies

  1. Expert Member
    Akos | | #1

    Small gaps like that on the warm side of insulation are inevitable and generally don't effect performance.

    With doublestud wall, the thermal break comes from the offset studs of the service cavity. Doing the Bonfig stud doesn't change the assembly R value all that much, not sure if it is worth the effort.

    Generally you do one or the other, not both. That is bump up the studs 2x8 with foam strips and insualte with batts and no service cavity or 2x8 +2x4 stud double stud wall.

    Most metal panels have enough grooves as part of the profile for some amount of air flow. The ones I've installed before, I used horizontal strapping only.

    For regular board and batten, you can also put vertical strapping under each batten. You can also use larger cement board panels over vertical strapping and finish them with battens for a B&B look.

    You can also do reverse board and batten as now the battens becomes the rain screen strapping. Fussier install as you have to get the spacing even, no hiding uneven gaps under battens.

    1. VTLiving | | #5

      We were actually thinking of not offsetting the studs, to make things easier for the builder, but if we are insulating the cavities in 2 passes (behind Intello and in front of Intello), perhaps it's not a big deal to offset. My understanding is that the offset studs are primarily tough for blown-in.

      The idea of a reverse board and batten is a good one! Those have a pretty high open surface area for drying. I guess one could tuck Cor-A-Vent (or the sign material) in the gaps at the very bottom for airflow.

  2. FrankD | | #2

    A 7.25" batt will easily fluff up to fill a 7.5" space or compress into a 7" space. I agree with Akos on the Bonfiglioli approach. By itself it is a strategy worth considering,. Buried in a double-stud wall, the marginal gain over 2x8 studs will be very very small.

  3. matthew25 | | #3

    That is a ton of expensive rockwool. Why not use blown cellulose or fiberglass? With the depth of wall you have, that is going to be a high-performing and low-cost option. Also way lower embodied carbon if you care about that kind of stuff.

    1. VTLiving | | #6

      While I know that the studies indicate that moisture is not really an issue with cellulose, we still have a lot of hesitation on that front. I was hoping we could consider blown-in mineral wool, but the odds of finding an experienced installer in our neck of the woods is about zero. I haven't read much about blown-in fiberglass -- I'll look into that as a potential option, at least for the outer wall. Thanks!

      It's sad that a product like natural wool is so much more expensive than melted and spun stone. :-/

      1. begreener | | #7

        Don't forget right here in New England:

        https://www.timberhp.com/

        1. VTLiving | | #8

          I saw their products some time ago, but the Maine factory was still ramping up. I'll have to see if I can find some pricing and availability. Thanks for the reminder of the product!

          1. begreener | | #9

            9 locations in VT

  4. begreener | | #4

    As others have suggested, I think using staggered studs to deal with thermal bridging is the way to go here ...

    The only other thing you might consider for the service cavity is potentially using a lightweight steel stud that already has electrical knockouts

    They come in a variety of dimensions:

    1 5/8-inch width
    2 1/2-inch width
    3 5/8-inch width

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