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Spray foam retrofit question

pbout | Posted in Green Building Techniques on

I’m in the middle of a big remodel where we gutted almost the entire house down to studs, and are building it back up. We left one full bathroom intact, and one wall of he bathroom is exterior. Today we removed the exterior sheeting and can see that the current wall system from inside out is 1/2″ drywall –> 1/2″ pressure treated plywood –> 2×4 studs with fiberglass batt insulation–> 1/2″ sheathing –> (used to be cedar shingles).

We are redoing the rest of the house to have the following wall system: 1/2″ drywall –> 2×4 studs with blown-in insulation (cellulose or fiberglass, unsure yet [or mineral wool if we can find it]) –> 1/2″ sheathing –> peel and sick WRB/ARB (vapor permeable) –> 1.5″ Rockwool Comfortboard 80 –> 1×4 furring strips –> exterior cladding.

For the bathroom, I don’t plan on opening it up from the inside, so I won’t insulate it at the same time as the rest of the house wall cavities. After some minor framing repairs, I want to insulate from the outside before we put the sheathing back on. 

Total area of the wall is about 100 sq ft, minus a 6 sq ft window. My first thought was to find an open-cell spray foam kit and do it ourselves now. But the pressure-treated plywood is giving me pause. Does that plywood act as a vapor barrier? Does it matter? 

I know I could put batts back in, but the framing is pretty cobbled, so it would be lots of patchwork. In general my approach was to use a blown-in/on product to fill all the voids. 

Any recommendations on what to do here would be great. 

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  1. GBA Editor
    Martin Holladay | | #1

    First of all, I've never heard of an open-cell spray foam kit. As far as I know, two-component spray foam kits dispense closed-cell spray foam.

    But from a building science perspective, either type of foam will work fine. You can install open-cell spray foam or closed-cell spray foam between the studs.

    Plywood is a smart vapor retarder, not a vapor barrier, and its vapor permeance in this situation doesn't matter much.

  2. pbout | | #2

    I thought the same thing as far as open vs closed cell, but then I found several kits online (such as Tiger Foam:, this one by Dap:, and another by Foam It Green:

    Good to know that I can use either product. I've been told by installers that using open cell is nice if you need to fill the entire bay, because it is easy to trim. But if using closed cell, I should only plan to fill the bay up to 3", because it's very difficult to trim any excess.

    If I use closed cell spray foam, would it matter if I left an air gap in this assembly, in this case in between the closed cell foam and the exterior wall sheathing?

  3. GBA Editor
    Martin Holladay | | #3

    I'm grateful to you for pointing out that it's now possible to buy two-component spray foam kits that dispense open-cell foam. I didn't know about this type of kit. Every week, I learn something new. Thanks for correcting me.

    Q. "If I use closed-cell spray foam, would it matter if I left an air gap in this assembly, in this case in between the closed-cell foam and the exterior wall sheathing?"

    A. No, it won't matter. The air gap will do no harm.

  4. pbout | | #4

    Thanks Martin, advice appreciated!

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