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Insulated roof question

user-270695 | Posted in Energy Efficiency and Durability on

I have a cape and have torn off the roof, added eaves and overhangs, moved the thermal boundary from the knee-walls to the gable walls and now am tweaking my approach on the roof/rafter bays.

We will be adding 3″ rigid foam to roof deck before installing aluminum shingles. To achieve something close to r-50 and a good air-seal, I could clad the underside of the rafters (in the small attics of which there are 9)with sheetrock or rigid foam and densely pack cellulose into the cavities. Having spent many days in these attics moving the thermal boundary, I am not interested in this approach because it is SO DIFFICULT to work in there and thus my question:

I want this roof assembly to be able to dry to the interior. Can I install open cell foam alone or in combination with un-faced batts, aka flash and batt, into the rafter bays in the small attics and still have the perm I need? If so, what thicknesses would achieve this?

Many Thanks!

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  1. GBA Editor
    Martin Holladay | | #1

    Either open-cell spray foam or fiberglass batts are permeable enough to use in this application without any worries, even if you achieve a total of R-50.

  2. user-270695 | | #2

    Thanks Martin-

    Then I could install 6" open cell in bays and in combination with 3" rigid foam on deck achieve about R-50 and still dry to interior, right?

  3. GBA Editor
    Martin Holladay | | #3

    I disagree with your math.

    6 inches of open-cell foam = 6 x 3.7 = R-22.2
    3 inches of XPS = 3 x 5 = R-15
    Total R-value of the assembly = 22.2 + 15 = R-37

  4. user-270695 | | #4

    I mistakenly thought open cell had a higher R value. I have a 2x8 bay that if full of OC would yield R-25. At that depth, do i compromise perm to the point of concern?

    As an alternate, 3 inches of OC plus an un-faced R-19 batt would yield R-30. Would this approach perform as well? I imaging it would cost a lot less. I'm not a fan of fiberglass, but would it perform better in combination with the OC?

  5. GBA Editor
    Martin Holladay | | #5

    As I said in my first answer, any of those choices is fine. Don't worry.

  6. benalbright | | #6

    If you are putting the 3" of rigid foam on top of the roof deck, does it even make sense to use foam under the roof deck. Open cell foam's R/inch isn't much different than fiberglass. Is the rigid foam on the roof not enough to seal the assembly?

    Fiberglass in an 2x8 rafter should get about R-25 +/-, plus the R15 from the rigid foam would be in the R40 range and I would think would be cheaper than spraying the foam.

    I'm seriously considering doing this on my home when I do a roof replacement next Spring and I am very curious if I am not putting enough thought into sealing from the inside (or some other reason to drive me to spray foam from the underside). For that matter, would a much lower thickness of open cell foam on the underside do the trick for sealing (I'm thinking a small enough volume that I could utilize the diy type tanks you can buy that contain around 600bdft and avoid having to mobilize another contractor onsite).



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