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Vented cathedral ceiling insulation and drying

Travis_G | Posted in General Questions on

I have a 12:12 2×12 vented cathedral ceiling with zip sheathing zone 6. My initial plan was to build a 2in site made baffle using 2in 1.8lb EPS foam, fill cavity with r30 rockwool, then fasten another layer of 2in 1.8lb EPS foam below the rafters with strapping. Cover with T&G board. Should I be worried about inward drying due to the layers of EPS? Maybe there is a better way to go about this?

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  1. BrianPontolilo | | #1

    Hi Travis.

    You wrote that you have a vented cathedral ceiling, but if you haven't installed any venting or insulation yet, it sounds like you are planning a vented cathedral ceiling, but still have options. Or maybe there is a reason why you have settled on a vented assembly? In any case, have you read this article? I think you'll find it very helpful: How to Build an Insulated Cathedral Ceiling

  2. Jon_R | | #2

    With unfaced EPS, moisture will be OK as specified - but would be better with plywood to create the vent.

  3. Expert Member
    Dana Dorsett | | #3

    Unfaced Type IX EPS runs just about 1 perm @ 2". Type II EPS would be between 1 and 1.5 perms at that thickness.

    In zone 6 you're really more concerned about drying toward the exterior than toward the interior. Dropping back to 1.5" of Type-II EPS (R6.3) on the baffle would give a bigger vent space and increase the permeance to 1.5-2 perms. Then using extremely low permeance 2" foil faced polyiso for the continuous layer below the rafters sets it up for an R-value a bit closer to code (R48.3, labeled, vs. R46.8 using 4" of Type IX EPS), an improved thermal break on the rafters (R12 instead of R8.4), and a much-impeded interior side moisture drive.

  4. Travis_G | | #4

    Thank you for the replies. I am rethinking my plan a bit now that i have a better understanding. I like the idea of using the polyiso as an air barrier layer under the rafters. Does polyiso need to be covered like EPS due to fire reasons? Is foam my best bet for a permeable vent baffle or is there a good wood alternative? Plywood seemed pretty low permeability, maybe 1/2in MDF?

  5. Jon_R | | #5

    Plywood is quite permeable (wet perms is what matters in this application).

    1/2" gypsum or 3/4" wood panels is the safe bet for a thermal barrier over any foam, but perhaps R316.5.2 applies and T&G is OK.

    I'd at least review the advice here, particularly "Avoidance of using vapor barriers where vapor retarders will provide satisfactory performance."

  6. Travis_G | | #6

    Plywood sounds like the way to go as far as cost and ease. If i was to use something like rockwool comfortboard below the rafters then i should be using a vapor retarder? I assume with the barrier being below the rafters the plywood baffle wouldn't require extensive sealing?

    1. Jon_R | | #7

      The same link has vapor retarder recommendations. Do air seal well, either on the interior side or both sides.

    2. Expert Member
      Akos | | #8

      You don't want comfortboard under rafters, it will be a lot of work to get your ceiling flat, stick to rigid foam.

      More than any of the vapor barriers, the important thing is to air seal. For a ceiling you want this air seal on the warm side, you can air seal on the cold side, but it is not buying you much. Ceilings that don't leak, need very little drying. For baffles, fiberboard is your friend, cheap, permeable and buys you a bit of R value.

      I would stick with foil faced polyiso with taped seams under the rafters. If you can go with 1.5" you might be able to get away with putting the T&G directly over it with 2.5" nails. I've done 1" no problem, you might want to mock up a small section to test.

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