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Community and Q&A

Exterior foam and fluffy inside

pmwatson | Posted in Energy Efficiency and Durability on

Building a new house in climate zone #5 and was planning on 5″ of exterior foam and R30 on the inside of a flat, non-vented roof. The two story part is still good for this set-up but the one story portion – garage and heated workshop is now messed up as the EIFS installer only allowed for 2″ of foam on the garage and workshop roof. I spoke to the EIFS installer and he said it would be quite a job to correct and not his fault as the builder did not tell him 5″ of foam. I can tell the builder to fix but wanted to know if there is anyway to install 2″ of foam on the deck and combine with something else on the inside – open cell, closed cell…? The workshop will be heated with a mini-split and will probably be on only when is use.

Another question, builder wants to add EPDM roofing material over the 5″ of foam without a second layer of OSB on top of the foam, is this correct? Thanks in advance for the replies.

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  1. GBA Editor
    Martin Holladay | | #1

    If I are aiming for an R-49 roof (and it sounds like you are), and we assume that the 2 inches of rigid foam on the exterior side of the wall sheathing amounts to R-8 or R-10 (depending on what type of foam you used), then you could install R-39 to R-41 of open-cell spray foam on the interior side of the roof sheathing. That means about 11 inches of open-cell foam.

    It's somewhat of an unsatisfactory solution, for a variety of reasons, but it would work. The open-cell spray foam is air-impermeable (a code requirement for this type of unvented assembly), so you are covered in that department.

    The open-cell spray foam is somewhat vapor-permeable, so the roof sheathing will be able to dry slowly to the interior.

    The only remaining worry concerns the fact that unvented roof assemblies insulated with open-cell spray foam sometimes develop moisture problems. To learn more about these cases, see these two articles:

    Open-Cell Spray Foam and Damp Roof Sheathing

    High Humidity in Unvented Conditioned Attics

    To address the worries about moisture accumulation, you are probably going to want to tell your builder to install a "smart" vapor retarder (for example, MemBrain) on the interior of the assembly, followed by drywall (required for fire safety).

    The alternative, of course, is to tell your builder that this error is his problem, not yours, and to insist that the specified 5 inches of exterior rigid foam be installed.

    -- Martin Holladay

  2. pmwatson | | #2

    Thanks, Martin for the reply. Any advice on the second question for applying EPDM directly over the foam?

  3. GBA Editor
    Martin Holladay | | #3

    My understanding is that the consensus among roofing experts is that it is best to specify a cover board above the rigid foam. For more information on cover boards, see these articles:

    Insulation Cover Boards: When and Why We Use Them

    What’s the Deal with Cover Boards?

    -- Martin Holladay

  4. Jon_R | | #4

    This source has some info on selecting a cover board. Glass mat gypsum and cement board score the highest.

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