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Closed-cell spray foam or polyiso board on rafters under open-cell-foam-filled rafter spaces

MrDIY | Posted in Green Building Techniques on

This is a follow-on from my recent post:

Horizontal furring to break thermal bridging on wall studs?

I could add a perpendicular layer of 2×4’s and spray closed cell foam in the cavity. Or I could use polyiso board over the existing 2×6’s. Either way I have to do some more nailing (or screwing) up there.

I have a couple options for sourcing the polyiso board, I’ve found it locally in 2″ and 250 miles away in 3″ thickness (both recycled).  I need to add about R19 so 3″ would be OK. But 4″ might actually cost less in this case.

I’d like to know if anyone here has mounted drywall on a slightly sloped  ceiling over 4″ of foam board. Was it worth the hassle? What fasteners did you use?

Or any other wisdom I could benefit from here?


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  1. GBA Editor
    Martin Holladay | | #1

    Mr. D.I.Y.,
    Q. "I’d like to know if anyone here has mounted drywall on a slightly sloped ceiling over 4 inches of foam board."

    A. After installing the rigid foam, you should strap the ceiling with 1x3 or 1x4 strapping, 16 inches on center. The strapping is fastened to the rafters with long screws that extend through the rigid foam.

    Once the strapping is up, you can fasten your drywall to the strapping.

  2. MrDIY | | #2

    got it, thanks. I think I'll go with the closed cell spray foam, seems like way less effort, even though and a couple hundred more $. I plan to run 2x4's perpendicular to the rafters, on 24" centers - any problem with 24" ?

    1. Expert Member
      Michael Maines | | #3

      Some drywall is not rated for horizontal use on 24" centers, so check the specs before installing.

  3. MrDIY | | #4

    Thanks! I'll check to make sure I get the right drywall. I feel far more confident about this with the help I've gotten here, thanks to all who have sent comments!

    1. Expert Member
      Zephyr7 | | #5

      5/8” drywall will probably allow you to use wider spaced strapping. The manufacturers give you this info in the datasheets for their products.


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