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Dense Packed Cellulose in Unvented Vaulted Ceiling?

megapointe | Posted in General Questions on

I am designing a home for Climate Zone 4A and am planning on having an eave-less roof and vaulted ceiling. Currently, I’m planning on roof rafters being 9-1/2″ I-joists so I can get an adequate amount of dense packed cellulose in the roof cavity of a vaulted ceiling condition (R-30 min.). I’d like to use a smart vapor barrier like the Intello Plus to hold the cellulose in before drywall is installed. I plan for the roof to be sheathed with 5/8″ Zip sheathing and asphalt shingles to be the finish layer. 

My big question is whether it would be important for there to be any ventilation space under the roof sheathing. I’ve seen instances where a similar roof construction was just sprayed on the underside for an air tight assembly but I’d like to avoid spraying if possible. Do you see any issues with not venting and packing the roof cavity completely with cellulose?

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  1. user-2310254 | | #1

    Not an expert, but R-30 in CZ4 is less than ideal. (Maybe it would be allowed if other parts of the roof system are at R-49 or higher.)

    In any case, you should read

    And let's see what the experts think about your plan.

    1. megapointe | | #3

      Thanks for the reply. Because the home (actually an addition) is under 500sf I'm following the IRC's N1102.2.2 section which states insulation can be reduced to R-30. Not necessarily the best but allowed. If I need to I may just increase the depth of the i-joist rafters for to meet an R-49 assembly.

  2. megapointe | | #2

    One thought I had was that the roof ventilation is probably most necessary for the longevity of the asphalt shingles. In this case, I wonder if I was to install baffles on the underside of the sheathing all the way up so I have a 1-2" air layer directly under the roof sheathing. I could intake ventilation into that baffle space with a Cor-a-vent product and exhaust with a ridge vent. Thoughts?

  3. Expert Member
    Michael Maines | | #4

    There is no safe way to insulate a cathedral ceiling with cellulose alone without using a vent channel. Variable permeance membranes reduce the risk but don't eliminate it. The temperature of the shingles is not a major issue, though their lifespan can be reduced by a few years; the bigger problem is that when vapor drive is from indoors to outdoors--as is typical when it's colder outside than inside--water vapor from the interior can accumulate at the sheathing. This vapor diffusion does not move a lot of moisture but enough to be a concern. In very warm climates you can use a vapor diffusion port at the ridge to mitigate the risk, but in zone 4 it's not a code-compliant option and not a safe one from a building science perspective.

    The IRC does allow open-cell foam to be used in an unvented assembly because it's airtight, but it's not vapor-tight so it's really not a foolproof option either. Your best bet in a foam-free assembly is to include 1-2" for a vent channel; the more the better. Or follow one of Martin's suggestions for safe assemblies that include foam.

    We discussed this situation with Kohta Ueno from Building Science Corp on a BS + Beer episode:

    1. Deleted | | #6


    2. megapointe | | #10

      Thanks so much, Michael. I think I'm convinced to include a vent channel but my concern now - as is noted in the BS + Beer Show - is that the baffles that are available may not actually be vapor permeable (Accuvent being plastic). I could use a cardboard baffle but would that collapse under the packing pressure?

      1. Expert Member
        Michael Maines | | #11

        I don't recall the conversation but my view, which I learned from someone on GBA--maybe it was Martin--is that the small amount of moisture that finds its way to the impermeable Accuvent (recycled PVC) will diffuse into the rafter and quickly out into the vent channel, so the rafter shouldn't get damp enough, or remain damp long enough, to lead to mold and microbial growth. If you want a vapor-permeable baffle anyway, I recommend using 1/4" plywood on furring strips. But when I need baffles, I usually spec Accuvent's 1 1/2" option. (Their standard product is 1", which is ok, but deeper is better.)

        1. megapointe | | #12

          I just read Martin's comment at the bottom of this article and I think it's exactly what you're referring to. Thanks so much, Michael.

  4. Expert Member
    ARMANDO COBO | | #5

    In CZ4, per Code you are required to install R49. Again, per Code, you can NOT install permeable insulation against the roof decking. You would need to install a 1.5" ventilated channel under the top flange of the I-joist, and 14" of cellulose, which means you need 16" I-joists. You could use rigid foam to form the ventilated channel and use 14" I-joists.
    If you are set on using 9.5" R33 I-joists filled with cellulose, you need to install 3" R15 min. rigid foam on top of the roof decking.... and that is to meet Code (pesky little thing, eh?)

    1. megapointe | | #7

      Thanks so much, Armando. Because my home (actually an addition) is under 500sf I was going to follow the reduced insulation provision as stated in IRC's N1102.2.2 which states insulation can be reduced to R-30. I could easily be convinced that this is a bad idea simply from a thermal performance standpoint - just trying to keep the assembly cost down. With that said, at R-3.5/inch and 7-1/2" of cavity space left over after a 2" baffle I would only have an R-26.25 assembly. Probably best to increase i-joist depth no matter the case.

    2. Expert Member
      ARMANDO COBO | | #8

      Physics will always win over chemistry. If you are comfortable with just doing the minimum to meet code, then R30 should be minimum, however, there is a good reason the ICC recommends R49 for your climate zone.

      1. megapointe | | #9

        You're definitely right... I'm reconsidering this roof assembly altogether. I'd like to get the minimum R-49 but now I'm wondering how to achieve that in as practical application as possible. Cellulose may not be the best bet here. Maybe I need to explore a rigid foam & batt assembly. It doesn't make it right/wrong, but I don't see any builders using foam above roof sheathing (or below roof rafters for that matter) so I was trying to figure out something that would work and be easy for these builders to understand.

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