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What is the best approach to insulating an unvented cathedral ceiling?

cbiek | Posted in Green Building Techniques on

This project consist of a small ceiling area equating to roughly 80 sf. There is no form of venting currently as it was a 3 season sun roof converted from a back porch so perhaps it wasn’t intended to be used as room full time. I’m in the process of changing that though, insulating what I can of the room with studs exposed. Through my journey reading over the various methods on insulating cathedral ceilings, spray foam seems the be the weapon of choice to attack an unvented ceiling space and this would be an option for me if it was affordable ($700+). But I have to ask, what options does that leave me? I would think a combination of foam board and roxul batts would offer similiar reliabilty as the spray foam. Labor intensive? Yes. My weekends are free. As of now, my thoughts are to seal 2” of foam board in the bays against the decking, beneath the foam r-23 roxul batts. We’re at r-33 now, if your counting. Right before the drywall, 1” of foam board attached to the studs. the final r would be r-38, minimum I believe in zone 5. This is all on the interior side in case it wasn’t clear and the rafters are currently 2 x 6 though I plan on furring out the rafters by 2″ or 3″. Tearing up the roof and putting down foam board on the exterior side of the decking beneath the shingles is not an option for me. I have no experience in that department.

So it can’t be done reliably, then would somehow adding venting be cheaper/faster?

Anyways, I’d like to hear different thoughts on this. I know there will be many. Thanks.

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  1. GBA Editor
    Martin Holladay | | #1

    It is possible to insulate an unvented roof assembly with a combination of foam insulation and air-permeable insulation, as you propose, but there is one caveat: the foam insulation has to be thick enough to keep the rafter bays above the dew point during the winter.

    In your climate zone (Zone 5), the minimum R-value for the foam layer (not the entire assembly) is R-20. That's a code requirement. So your proposed thickness (2 inches of foam) is insufficient. You'll need at least 3.5 inches of polyisocyanurate, 4 inches of XPS, or 5 inches of EPS to make this approach work.

    If you want to use rigid foam, the best place to put the foam is above the roof sheathing, not between the rafters. The method you are proposing is sometimes called "cut-and-cobble." The method isn't really addressed in the code, since the method is never used by insulation contractors, only by homeowners. It falls into a gray area of the code.

    For more information on all of these issues, see How to Build an Insulated Cathedral Ceiling.

  2. cbiek | | #2

    Martin, from the guide you recommended my system is similar to the "flash and batt" but with rigid foam board instead. I suppose even better would be to fill the rafter bays fully with foam board though I would guess 8'' or so of XPS for 80 sq would get pricey enough for me to reconsider the spray foam. I did feel that the 2'' under the sheathing wasn't quite enough but I wasn't quite sure how much. You answered that question. Thanks for taking the time to answer my question. -Charles

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