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Opinions on This Wall Assembly

algoodwin | Posted in Green Building Techniques on

Hi all,

I want to build a wall as follows but I know it doesnt respect some rules and i would love a breakdown of my problems/mistakes:

1/2 drywall

2×4 wall 16″ c/c with R12 fiberglass (structural wall, wiring and plumbing chase)
Vapour barrier (if i can avoid Membrain, id like to)
2×4 curtain wall 24″ c/c with R14 roxul (offset studs relative to the 1st wall)

2.25in EPS sheathing with laminated air barrier (R9.1 and 1.2 perms…. They claim we can ignore the interior/exterior foam ratios because it is “permeable”…)
1×4 diagonal furring (horizontal i would prefer) nailed in with 4.5in nails (not screwed as i read it should be…)
Vertical wood siding (no gable overhang)

I appreciate any insight!

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  1. Jon_R | | #1

    > EPS sheating is too thin ... ignore the interior/exterior foam ratios

    I agree.

    > it is “permeable”


    No idea what your code says, but that's little exterior R for Montreal and high R walls - so I'd use only the interior/exterior perm ratio to assure moisture safety. Info is in Table 2A, which recommends a Class I on the interior if the exterior is 1 perm (apparently about what you have). The EPS helps, which means you aren't designing to minimums. So your plan can work.

    Why not make the wall thicker and skip the EPS? This would allow much better drying to the exterior. So would exterior mineral wool - and like your EPS, it would warm the sheathing somewhat.

    1. algoodwin | | #4

      Not sure about code restictions for exterior foam, i will look into it, this place is my first stop in my deaign questioning.
      Standard wall assemblies here are built 2x6 w/r20+r4 sheathing (R1 wood fibre laminated to R3 foam) and poly vapour barrier inside.

      I'm planning on using only EPS as sheathing (wind braces for racking strength), no osb, therefore as thin a wall as i can to not increase my bad interior exterior ratio. And i find exterior mineral wool too expensive (especially since i need to use sheathing of some sort in addition). Thanks for that table, i will read the whole article later!

      1. Jon_R | | #9

        It does seem that exterior rigid foam with no OSB/Zip/plywood sheathing is an underappreciated wall. Maybe the builders can say why it isn't more popular.

  2. Expert Member
    BILL WICHERS | | #2

    It's safest to stick with the ratios as a minimum, but the permeable foam does help. 2.25" of EPS isn't very permeable though, so it offers only a very small amount of additional drying potential.

    If those diagonal 1x4s are supposed to be for racking resistance, they have to be nailed directly to the studs. Nailing through 2.25" of EPS isn't going to work here.


    1. algoodwin | | #5

      Not for racking strength, wind braces for that (steel let into the wood framing). I was wondering why they always talk about screwing the furring instead of nailing when over foam.

      1. Expert Member
        BILL WICHERS | | #8

        Screws are more secure and don't loosen when you have squishy materials (relative to the framing) involved. You also tend to hang things from the furring strips (siding, etc.), and you're not supposed to hang things from nails in a way that would try to pull the nail out. The threads of the screws resist that pull out force.


  3. kyle_r | | #3

    What about putting the EPS between the walls?

    1. algoodwin | | #6

      I want to use the eps as weather/air barrier sheathing, thus outermost layer (it is laminated with a tyvek type membrane and seams are taped)

  4. Deleted | | #7


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