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Community and Q&A

Feedback on Wall System Design

sethcooper94 | Posted in GBA Pro Help on

Hey guys!

Please let me know what you think about my wall system, will I be good to go or should I change something?? Were building a 1560 SF slab on grade house with a hard trowel finish slab for finished floor.  Climate zone 5a.

From interior -> exterior

1. plastered walls
2. plasterboard
3. rockwool r21 cavity insulation
4. zip sheathing (liquid flashed nail holes and seams)
5. 2” dupont xps rigid foam ( i know its not green, our family are Dupont XPS dealers so i get it cheap)
6. coravent rainscreen
7. cladding

The one thing I am considering changing is going from the 2” XPS to 2″ rockwool comfortboard 80 because rockwool is Vapor permeable. It is a little more money and a -1.6 R value compared to the 2” XPS but the vapor permeability seems like a big benefit in our climate.

thanks guys!

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  1. Expert Member


    That sounds like well performing wall system for your climate zone. I can't see any problems with it.

  2. Expert Member
    Akos | | #2

    +1 Solid wall.

    Walls are not roofs, WRB doesn't need to be liquid tight, filling the nail holes on Zip is a waste of time. Even without the foam, nail holes on walls don't matter. Any water that makes it to a nail hole can easily dry through the rest of the sheathing beside it. Think about how many nail holes a typical siding install makes in the WRB and those never cause issues.

    1. canada_deck | | #7

      Maybe they are doing that for air barrier vs WRB reasons?

      1. Expert Member
        MALCOLM TAYLOR | | #8


        Nails are self-sealing. It's not worth worrying about them when the sheathing is the primary air-barrier.

  3. walta100 | | #3

    I like you plan a lot I built almost the same walls for my house.

    You may find damp sprayed cellulose is a better value that fills the gaps and odd spaces better.

    Two layers of one inch foam with staggered seams may be slightly better.

    You were vague about your cladding be sure its manufacture has instructions for 2 inches of foam without strapping.


    1. sethcooper94 | | #9

      So I am going with a metal panel for my cladding, and the reason I am going with rockwool inside is just because we are rockwool distributors so i can buy it at cost. same with Dupont XPS.

      Just been going between the two.

      I am sealing for air leakage. In commercial buildings its always specd so im going to do it. Its not a lot of work or money for the belt and suspenders.

  4. sethcooper94 | | #4

    Thanks Guys! I got the chance to message with Travis (@catalystbuilt) on instagram and he brought up the fact that he is skeptical with rigid foam that it could not dry to the outside which has made me wonder if I should spend the extra money and go to rockwool.

    1. Expert Member
      BILL WICHERS | | #6

      While being able to try in both directions is a nice plus, it's not a requirement for a reliable wall assembly. As long as you can dry to at least one side, you're OK -- just be sure to air seal your wall well. Adding a smart vapor retarder (MemBrain, etc.) on the interior side under the plaster (unless you are plastering over lath, which would complicate things) or drywall is good for some extra insurance.

      As long as you do a good job with details like flashing on the exterior, and air sealing on the interior, your wall will be safe even if only able to dry to the interior.


  5. Expert Member
    Dana Dorsett | | #5

    Are the windows going to be mounted "innie" or "outie"?

    If innie, either use a drainable XPS (often used for foundations) to allow the ZIP to drain between the foam & ZIP, or add a crinkle type housewrap between the ZIP & foam. With either solution the ZIP can still dry to the outdoors with negligible loss of thermal performance.

    If outie it's fine to let ZIP dry to the interior, as long as the bulk water details/flashing at the windows properly sends water out to the rainscreen, never between the foam & ZIP.

    For a longer discussion, see:

    1. sethcooper94 | | #10

      Yeah, Dupont makes perimate. I actually thought about that. I haven't 100% made up my mind on the windows yet. I was originally going innie and just going to fast flash around my windows and then put like a 1x4 box on edge around the windows like Risinger shows in his videos but not sold on either way yet.

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