I’d rather put 2″ of rigid foam on the interior of the rafters than on top of the roof deck.
Seth Kelley| Posted inGeneral Questionson
I have 2×12 rafters and was planning on insulating with dense pack cellulose or rock wool for a cathedral ceiling. Then putting rigid foam on the inside , then drywall. Does this make sense? I was hoping not to vent. Will this work.
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Forgot to mention I'm in central Vermont
Venting that assembly is safer.
Seth,
Building codes require, and most building scientists recommend, that you include a ventilation channel between the top of the cellulose layer and the underside of the roof sheathing if you choose to install cellulose between your rafters.
For more information, seeHow to Build an Insulated Cathedral Ceiling.
I think most builders would say rigid foam on the inside of a cathedral ceiling would be preferable, as long as the ceiling is vented, which with dense pack cellulose means plywood rather than foam baffles. Non vented would be considered risky.