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Sandwiching Exterior Rigid Foam Between Two Layers of Ply Sheathing

quantumgirl | Posted in General Questions on

I plan on installing thin stone veneer on the first story of my house. Regular 2×4 wall construction with 1/2 plywood sheathing and 1.5” polyiso on the exterior of the sheathing.

The stone veneer company is telling me that I need to add another sheet of 3/4 ply to the outside of the foam, fastened into the studs with a specific length and amount of nails, to support the stone veneer assembly. Does this sound like the right approach? And if so, would I install and flash my windows onto that second layer of sheathing. I usually do innie windows, and integrate the flashing with the WRB that sits between sheathing and foam. but with that extra outer layer of ply, it may be more sensible to do outies? 

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  1. user-2310254 | | #1

    Foam requirements vary by region. Where are you located? In any case, you might be better off with ZIP-R as your sheathing.

    1. quantumgirl | | #2

      I’m in Houston. Climate zone 2a I believe. Exterior rigid foam isn’t really required here. I’m mainly doing it cause we’re using spray foam as cavity insulation and I want to offset the negative effect of thermal bridging with the exterior polyiso.

      I’ve never worked with zip system before but the zip-R certainly looks interesting. Thanks!

  2. user-2310254 | | #3

    You can build a high-performing house with minimal (maybe zero) use of spray or rigid foam. Just focus on air sealing with caulk and tape.

    Regular ZIP with 2x6 framing and R-23 rockwool batts would give you pretty good performance. You could use reclaimed foam to lower the environmental impact, but as you noted, it's not necessary in CZ2A.

    Keep in mind that wood products are very expensive in today's market. ZIP or plywood is running $50+ a sheet in some markets--if you can find these products at all.

    1. Expert Member
      BILL WICHERS | | #4

      I was just reading an article today that is expecing lumber prices to return back to normal, or close to normal, around the end of this year or very early in 2022. I think the article was in the Wallstret Journal, but I don't remember for sure -- it was one of those articles that my phone thought would be interesting to me and it popped up in the morning.

      If you need a lot of plywood for your project, you might be able to save a lot of money by waiting a few months if you can.


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