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Community and Q&A

Insulating an unvented roof

David Wittner | Posted in Energy Efficiency and Durability on

I’m looking to finish the ceiling (underside of roof rafters) in a workshop/barn that is only heated when occupied. I’m in IEEC zone 6 (upstate NY) so it gets cold. Can’t pull off the roof because it’s relatively new and in great shape. Any work has to come from the inside. Right now there’s a double layer (6″) of rigid polyiso insulation in each rafter bay. The insulation if faced on both sides with fiberglass impregnated black paper (nasty stuff). There’s also about 3″ of open space between where the insulation stops and the edge of the roof rafters.

I’d like to cover the ceiling with either plywood or drywall to keep the mice out and I’m not sure if I should keep the insulation against the underside of the roof deck or pull it down to be flush with the ceiling. Either way I plan to seal any gaps in the insulation with spray foam. I guess I could also pack the remaining space with fiberglass too.

I have had moisture issues, i.e., condensation at the ridge some winters. Right now there are gable vents at either end of the room which usually get closed off in the winter to keep in the heat (and hence my moisture issue I’m guessing). One thought I had was to have a gap between the insulation and roof deck so I could drill in through the soffit put in some small vents and then install a ridge vent.

Any thoughts would be helpful and much appreciated. Thank you!

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  1. GBA Editor
    Martin Holladay | | #1

    Right now you have cut-and-cobble insulation in an unvented roof assembly -- a risky approach. (You didn't mention whether you sealed the rigid foam in place with caulk or canned spray foam -- did you?)

    For more information on why cut-and-cobble is a risky approach for this type of assembly, see Cut-and-Cobble Insulation.

    The right way to do it would have been to install the rigid foam as a continuous layer of insulation on the interior side of your rafters rather than to cut the insulation into narrow rectangles. It's kind of a shame that you have already cut up your rigid foam.

    I'm not sure how you want to proceed at this point. A vented approach is definitely safer from a moisture perspective than an unvented approach. Here is a link to an article that describes all of your options: How to Build an Insulated Cathedral Ceiling.

  2. Expert Member
    Dana Dorsett | | #2

    Is the rigid polyiso in contact with the roof deck, or is there the code-required 1" air gap between the foam & roof deck? With the gap just adding soffit vents for soffit to ridge venting it should be fine to leave as-is, maybe adding some R13 batts between the foam and the ceiling plywood.

    If there is no roof deck venting gap, pull the rigid foam toward the interior at least an inch and air seal the edges. You can then fill the remaining space between the foam and your ceiling plywood with 2" of open cell foam (which seals very well), or with fiber insulation, or pull it the full 3", and give yourself a 3" deep venting space for the roof.deck.

    For an intermittently heated building the thermal performance difference between 6" of polyiso and another R15 bringing up to IRC type levels probably doesn't matter a whole lot.

  3. David Wittner | | #3

    Thank you Martin and Dana for your quick and helpful replies. As the insulation is pushed up against the underside of the roof deck, it looks like I'll be installing spacers and dropping in down, sealing the perimeter, and venting it. The venting will have to wait for the spring, already have 8" of snow on the ground. At the time we built the barn, couldn't afford the extra $10k for insulation. While I haven't even come close to that cost after the fact, it may have been the easier way to go in the long run.

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