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Should you insulate the exterior of a house with rigid insulation if you have spray foam inside?

Jed_Koljonen | Posted in Energy Efficiency and Durability on

A customer of mine wants rigid foam on the outside of his house to stop thermal bridging, and then wants to have either open or closed-cell foam blown in the interior.

I was wondering if that would be a bad situation for the OSB trapped between the two. What would be good ok around?

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  1. GBA Editor
    Martin Holladay | | #1

    I would use cellulose between the studs if it were my house. But if the customer insists on spray foam, make sure that it's open-cell foam (1/2-pound foam), not closed-cell foam (2-pound foam). That way the OSB will be able to dry to the interior.

  2. homedesign | | #2

    Do you have enough info?
    how do you know Jed's Climate?
    What is the cladding?...brick veneer?...Stucco?
    If Jed goes sans-sprayfoam where is the Air Barrier?
    ADA? Airtight Sheathing?

    If Jed chooses Rigid Foam...should he "Mind the Gap"?

    And Martin "if it were your house" in Vermont....would you include Outside Foam?

  3. GBA Editor
    Martin Holladay | | #3

    Q. "Do you have enough info?"

    A. I think I had enough info to answer Jed's question.

    Q. "How do you know Jed's Climate?"

    A. I don't, but I still stand by my answer. No matter the climate, OSB wall sheathing should be able to dry in at least one direction.

    Q. "What is the cladding?...brick veneer?...Stucco?"

    A. If the cladding is either brick veneer or stucco, then Jed's plan to use exterior rigid foam makes sense. What's your point?

    Q. "If Jed goes sans-sprayfoam, where is the Air Barrier?"

    A. Although Jed didn't ask me a question about air barriers, my advice has been consistent over the years: every wall needs at least one, and often two, air barriers. Again, what's your point?

    Q. "ADA?"

    A. That would work.

    Q. "Airtight Sheathing?"

    A. That would work, too.

    Q. "If Jed keeps the Rigid Foam...should he Mind the Gap?"

    A. Most types of siding perform better with a ventilated air gap between the siding and the sheathing.

    Q. "If it were your house in Vermont....would you include Outside Foam?"

    A. Yes, I probably would.

  4. homedesign | | #4

    I have noticed several BSC projects that include Closed Cell foam, OSB and Rigid Insulation.
    I was thinking of Joe Lstiburek's "Mind the Gap, Eh!" Insight

    My take from the article is that Closed Cell cavity foam can be used (co-exist) with OSB and Rigid Exterior Foam ....
    As long as you "Mind the Gap" ...(the gap in this case is crinkled-housewrap between the OSB and Rigid foam)
    And that Sometimes....the increased thermal resistance may be desireable.

  5. GBA Editor
    Martin Holladay | | #5

    You're right that Joe Lstiburek sometimes recommends sandwiching OSB between rigid foam and close-cell spray foam, as long as there is a small gap between the rigid foam and the OSB.

    I'm not as comfortable with that method as Joe. In ten years, after we all have more experience, I may come around and accept Joe's position.

    In the article you linked to, Joe wrote, "A wall assembly has to dry in at least one direction and with that foam on the exterior the only practical drying direction is typically inwards." Although you get some hygric redistribution with grooved foam, it isn't really drying.

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