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How do you insulate with rigid foam under decking in cathedral ceiling?

Larrygorept | Posted in Green Building Techniques on

I live in zone 3, building a new home, rustic, barn style. Large great room with high cathedral ceiling and would like to leave some portion of the 2×12 rafters exposed. Most of the rest of the house has vaulted ceilings also. Roof is metal on 1×4 purlins, non-permeable synthetic underlayment on OSB (foil face down), on 2×12 or 2×10 rafters (about 50% each thru-out the house). The plan is to use barn tin as the ceiling between the rafters in the great room. If I were to use 6 inches of closed cell rigid foam panels under the OSB, do I need a vented air gap above the foam or not? Do I need to air seal the ceiling, and how? I currently have a ridge vent in a simple roof design, have not installed soffett yet.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Larry,
    Q. "How do you insulate with rigid foam under decking in cathedral ceiling?"

    A. The two most common ways to do that are (a) to use structural insulated panels (SIPs) and (b) to install rigid foam sandwiched between two layers of roof sheathing. For more information on this approach, see this article: How to Install Rigid Foam On Top of Roof Sheathing.

    You wrote, "The plan is to use barn tin as the ceiling between the rafters in the great room." That's a challenging approach. If the metal roofing is a continuous layer, it won't be between the rafters (and it will be challenging to get an air seal above it). If the metal roofing is cut into narrow rectangles and inserted between the rafters, you are looking at a lot of work.

    Q. "If I were to use 6 inches of rigid foam panels under the OSB, do I need a vented air gap above the foam or not?"

    A. A vented gap above the rigid foam is optional. It is not necessary.

  2. Larrygorept | | #2

    Martin, thanks for the prompt response, my other option is venting above R38 fiberglass insulation with site made ventilation channel and barn tin on the bottom of the rafters, but if I air seal the bottom of the vent, above the fiberglass, do I need any seal under the tin, other than the kraft paper of the insulation?

  3. GBA Editor
    Martin Holladay | | #3

    Larry,
    Your description is quite confusing. Please describe your proposed roof assembly, with each layer described in order -- either from the exterior to the interior, or vice versa.

  4. Larrygorept | | #4

    Martin, sorry, but from the roof down, if using the fiberglass insulation idea: metal roof, on 1x4 purlins, on OSB, one inch air gap, half inch rigid foam board, R38 fiberglass faced batts, between 2x12 rafters, then a ceiling of barn tin below rafters
    If rigid foam: below OSB, 6 inches of rigid foam, no air gap, tin between rafters.

  5. GBA Editor
    Martin Holladay | | #5

    Larry,
    Q. "From the roof down, if using the fiberglass insulation idea: metal roof, on 1x4 purlins, on OSB, one inch air gap, half inch rigid foam board, R38 fiberglass faced batts, between 2x12 rafters, then a ceiling of barn tin below rafters."

    A. Your proposed assembly needs a few additional details: (a) You need roofing underlayment between the 1x4 purlins and the OSB sheathing. (b) The 1/2-inch rigid foam board (presumably, installed as a ventilation baffle) makes sense as long as it is stiff enough to prevent cracking, and as long as it is installed in an airtight manner. For more information on this aspect of the work, see Site-Built Ventilation Baffles for Roofs. (c) You need an air barrier between the bottom of the rafters and the corrugated steel; I would suggest that you install taped drywall as your air barrier.

    Q. "If rigid foam: below OSB, 6 inches of rigid foam, no air gap, tin between rafters."

    A. This is confusing. Presumably, you are intending to install two layers of OSB -- one layer above the rigid foam, and one layer below the rigid foam. Is that correct? You don't want to install the rigid foam directly above the rafters, because the rafters will dig into the soft rigid foam.

    With this approach, you will still need a layer of roofing underlayment above the top layer of OSB sheathing. As I wrote in my earlier response, trying to cut narrow rectangles of steel roofing to insert between your rafters is a lot of work. Good luck with that.

  6. Larrygorept | | #6

    Martin,
    We have non-permeable synthetic underlayment on top of OSB, under the 1x4s.
    I was considering 6 inches of rigid foam between the rafters, below the OSB. And yes the thought of cutting all that metal is awful.
    If I go with the fiberglass, I would seal the half inch foam as the bottom of the vent channel as per the suggested article, or go with styrofoam baffles, then R38 fiberglass, an air seal layer, Then 2x4 stripping perpendicular to the bottom of rafters to attach barn tin to, running parallel with the rafters. Would something like Smart Membrane or poly sheeting work for the air membrane instead of drywall? We really want to use this barn tin in the great room, I just need to figure out how to get it done and maintain a functional insulating layer. I'm open to suggestions.

  7. GBA Editor
    Martin Holladay | | #7

    Larry,
    Q. "I was considering 6 inches of rigid foam between the rafters, below the OSB."

    A. The only sensible way to install rigid foam is as a continuous (uninterrupted) layer above the roof sheathing. That way, the rigid foam reduces thermal bridging through the rafters. It doesn't make any sense to cut perfectly good sheets of rigid foam into narrow rectangles, and then insert the rectangles between rafters.

    Q. "If I go with the fiberglass, I would seal the half inch foam as the bottom of the vent channel as per the suggested article, or go with styrofoam baffles, then R38 fiberglass, an air seal layer, Then 2x4 stripping perpendicular to the bottom of rafters to attach barn tin to, running parallel with the rafters. Would something like Smart Membrane or poly sheeting work for the air membrane instead of drywall?"

    A. I'm not in favor of the idea of trying to use MemBrain or polyethylene as an air barrier. Both materials are fragile. On the job site, they get ripped, and it's hard to seal seams. Moreover, fastener penetrations introduce the chance of air leaks. Drywall works in this location, so that's what I recommend. (It's quite common for a cathedral ceiling to get damp due to air leaks, so I'm very conservative when it comes to air barrier details for a cathedral ceiling.)

  8. Expert Member
    Dana Dorsett | | #8

    With 6" of rigid polyisocyanurate foam above the roof deck it would be sufficient to meet code-min performance on a U-factor basis in most stackups even without any insulation between the rafters, in locations where R49 is code-min.

    For zone 3 the IRC calls out R38 OR U0.30, which is R33 "whole assembly R, with thermal bridging factored in, and with credit for the R-value of the roof decking and multiple air-films included. Even 4.5- 5" of continuous rigid polyiso would get you there.

    https://up.codes/viewer/utah/irc-2015/chapter/11/re-energy-efficiency#N1102.1.4

    https://up.codes/viewer/utah/irc-2015/chapter/11/re-energy-efficiency#N1102.1.2

    You could also get there with 2" of polyiso above the roof deck and low-density R30s fully filling the 2x10 rafter bays.

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