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Is it OK to put rigid foam on two sides of cellulose in a sloped roof?

LPjR97u3BM | Posted in Energy Efficiency and Durability on

We are planning to insulate at the roof for a shed portion of our home. We are planning on using densepak cellulose in the 2×12 rafter bays with a ventilation chute at the top. We would like to also add 2″ of rigid at the underside of the rafters to reduce thermal bridging and bring the R-value up for the assembly. I believe that I have heard the technique of using rigid insulation to create the ventilation chute at the top of the rafter bays, which would help us as we try and reach an R-49 for this assembly; But is it a mistake to have the rigid on both sides of the cellulose?…

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  1. GBA Editor
    Martin Holladay | | #1

    As long as your rafters and cellulose are dry when the ceiling is assembled, I think you'll be fine.

  2. LPjR97u3BM | | #2

    OK. How about if we strap the rigid below the rafters and put up a wood plank ceiling? Does that change things?

  3. GBA Editor
    Martin Holladay | | #3

    Just be sure that your ceiling has an air barrier. (Wood planks don't count.)

    Some people believe that taped rigid foam is an air barrier, but I'm not so sure -- rigid foam can shrink over the years. The best approach is to cover your strapping with gypsum wallboard with taped seams, and then install your board ceiling under the gypsum wallboard.

  4. LPjR97u3BM | | #4

    Thank you Martin.
    But wouldn't fastening the board ceiling through the gyp bd to the strapping put a bunch of holes in your air barrier?...Or does that not matter?...
    Thanks again.

  5. GBA Editor
    Martin Holladay | | #5

    The nails that penetrate the gypsum wallboard do not create substantial air leaks. But if you want a belt-and-suspenders approach, tape the seams of your rigid foam before installing the strapping. Then you'll have two interior air barriers.

  6. LPjR97u3BM | | #6

    OK. One final question: If we were to move the rigid above the roof sheathing, could we denspak the rafters? If so does the minimum rigid thickness table apply similar to wall assemblies?
    Thanks again.

  7. GBA Editor
    Martin Holladay | | #7

    See the discussion of roofs in my blog, Calculating the Minimum Thickness of Rigid Foam Sheathing.

    I wrote, "Section R806.4 of the 2009 IRC allows builders to insulate unvented rafter bays with a combination of air-impermeable insulation (for example, closed-cell spray polyurethane foam) and air-permeable insulation (for example, cellulose), as long as the thickness of the air-impermeable insulation that is “applied in direct contact with the underside of the structural roof sheathing” meets the minimum R-values required for condensation control shown in Table R806.4. These values are:
    * Climate Zones 1-3 — R-5
    * Climate Zone 4C — R-10
    * Climate Zones 4A and 4B — R-15
    * Climate Zone 5 — R-20
    * Climate Zone 6 — R-25
    * Climate Zone 7 — R-30
    * Climate Zone 8 — R-35."

    Although this code provision refers to a combination of spray foam and air-permeable insulation installed under the sheathing, the same logic (and R-values) would apply if you install rigid foam over the roof sheathing and dense-packed cellulose under the roof sheathing. Remember, a roof with rigid foam on top of the sheathing shouldn't have ventilation channels between the roof sheathing and the cellulose.

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