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Community and Q&A

Can you apply spray foam over exterior rigid foam?

Howard Klein | Posted in Energy Efficiency and Durability on

We are planning to build a house located in Montreal where the weather in the winter is relatively cold and swings to hot and humid summers. We wanted to build a wall system of R40.

A lot of builders here in Montreal are spraying the exterior of the building with 1″ of spray foam over plywood in order to prevent air infiltration and then filling the 2X6 studs with normal fiberglass.

From what I have read the 1″ of exterior foam is not enough to prevent condensation issues on the plywood.

Question: is it possible to install 2″ of rigid foam such as XPS or EPS and then add 1″ of spray foam for to seal the building? I would then install normal fiberglass batts in the studs.

Thanks

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Replies

  1. Cramer Silkworth | | #1

    I don't know that you can or can't put spray foam over rigid, but why not just use more rigid (have you considered polyiso?), maybe 4-6" of it, and tape the seams? From what I hear there's a pretty significant base cost just to get the spray foam guys on site, regardless of how much they're installing. And rigid foam installation is pretty simple (in theory anyway).

    What's the cladding system going to be? Rainscreen?

  2. Cramer Silkworth | | #2

    Also - unless that spray foam wraps the edges of the rigid to meet the sheathing, foundation, whatever, you're not guaranteed a continuous air barrier with the spray foam.

    Nope, don't like it. Don't like it at all.

  3. Howard Klein | | #3

    Hi,

    The cladding will be a mix of stone and stucco. I am not sure about the rainscreen.

  4. Cramer Silkworth | | #4

    Have you considered EIFS?

  5. Jerry Liebler | | #5

    Have you considered "double stud" walls? With mineral wool insulation, Roxul, I'm about r43 with 10 1/2" cavity in doubled 2x4 24" OC stud walls. Roxul is about 1/3 the material cost of foam. Stuco and the "thin brick" I'm using require a good vapor retarder under them to protect plywood sheathing. I'm using an XPS rain screen product , Greenguard DC14 over a WRB to protect the sheathing.

  6. GBA Editor
    Martin Holladay | | #6

    Howard,
    Like the other respondents, I don't see any advantages of spray foam on top of rigid foam, and many possible disadvantages.

    If you want exterior foam, I would choose one product or the other -- either rigid foam or spray foam, but not both.

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