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Comparing Two Wall Assemblies

PeterVi | Posted in General Questions on

Hi – working on a project in Climate zone 4 and trying to compare two 2×6 16″OC wall assemblies for thermal performance & also feasibility:

Version 1
In Cavity: 5in closed cell spray foam (assuming they can’t fill the entire cavity w ccspf)
Seating + WRB: Zip (without insulation)
Siding: Hardie Board with rainscreen

Version 2
In Cavity: 3.5in open-cell spray foam + 2in closed cell spray foam
Sheating+WRB+continous insulation: Zip-R6
Siding: Hardie Board with rainscreen

Thanks for your insights!

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  1. Expert Member
    BILL WICHERS | | #1

    Skip your first option entirely.

    With the second option, instead of using spray foam, detail the Zip as your air barrier, and use batts or dense pack cellulose in the stud cavities. you don't need spray foam. You don't need any spray foam here, and it doesn't really add anything performance-wise aside from air sealing, and you can air seal in other ways. Use the savings from not using spray foam to upgrade to Zip R9 or Zip R12 instead, which will give you much better overall wall performance.

    The reason to skip option #1 entirely is that spray foam alone within a wall doesn't perform significantly better than high density batts or dense pack cellulose because of the thermal briding of the studs. You're essentially spending money on an expensive insulating product and using it in an application where you won't fully realize it's benefits, and it doesn't really add anything to your project for the extra cost.

    The reason to skip spray foam in option #2 is that it doesn't add much R value relative to it's cost, and the money would be better spent on thicker exterior rigid foam that is continous insulation, which means no thermal bridges. If you really want to use spray foam, use open cell spray foam with option #2. The reason for open cell is that it will be overfilled and trimmed flush with the studs after it cures. Closed cell spray foam is underfilled, so you end up with about 1/2" to 1" (or more) unfilled space on the interior side of the wall where the spray foam is doing nothing at all -- since it's not there! This is why open cell performs as well or better than closed cell spray foam in walls: the extra empty space with closed cell spray foam cancels out the slightly lower R/inch for the open cell spray foam.

    I wouldn't bother with spray foam in a wall myself. You're better off with thicker exterior rigid foam, high density batts or dense pack cellulose in the stud bays, and do your air sealing the "old fashioned way", with a bead of sealant around the perimeter and canned foam in any holes. I'd install the interior side drywall airtight too as some extra redundancy.


    1. PeterVi | | #2

      Thanks Bill - this is very helpful! I've sent my contractor version#2 (he suggested the spray foam initially) and your recommendation (version#3) for pricing.

      Version 3
      In cavity: high density batts/dense-pack cellulose
      Sheating + WRB: Zip, taped seams (without insulation)
      2" rigid foam exterior insulation
      Siding: Hardie Board with rainscreen

  2. user-5946022 | | #3

    Version 4:
    2x6 top & bottom plate.
    2x4 @ 12" oc staggered. This eliminates the thermal bridge from the vertical framing, and even though it is more board feet 2x4's may be less expensive than 2x6's
    cavity: Damp applied cellulose which will get in every corner - less chance of poor install than batts, less chance of settling and gyp bulging than dense pack.
    Sheathing - taped zip with ALOT of detailing from the inside to seal it up
    Rigid foam exterior on top of zip
    Hardi with rainscreen siding.
    You might look into the cost effectiveness of the foam/rainscreen combined products.

    1. Expert Member
      MALCOLM TAYLOR | | #4

      If you are using exterior foam to reduce thermal bridging, is there any appreciable advantage to staggering the studs?

  3. walta100 | | #5

    I see large areas of spray foam as a red flag on new construction plans.

    There is almost always a better and lower cost way to insulate than spray foam.

    Spray foam is the most expensive way to get an R value and should be reserved the very few locations where it is the only real option like the band joist.

    Have you planed your HVAC systems in a way that will keep your ductwork and equipment below the ceiling flat ceiling covered in low costs fluffy insulation?

    I built my walls 2x6 -16 filled with damp sprayed cellulose sheathed with Zip covered by an inch or exterior foam clad in vinyl siding but the ZIP+R was considered.


  4. user-723121 | | #6

    You could also consider a 2" x 8" plate with staggered studs and (2) R-15 batts, superinsulation in Zone 4. Use a conventional OSB or plywood exterior sheathing with a rain screen. There is no magic to using exterior foam of any kind on an exterior wall, it just complicates the siding and trim installation.

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