How best to insulate a 12″ rafter-framed cathedral ceiling?
After reading (thoroughly, several times!) Martin’s article, “How to Build an Insulated Cathedral Ceiling,” I still have a number of questions.
First, my situation:
I’m building a garage with loft living space in central Maine (climate zone 6). The cross-section is a cathedral ceiling with rafter ties (image attached). The roof assembly is 12″ rafters, synthetic roof underlayment (a product called “Elephant Skin” by Warrior Roofing) and screw-down metal panels with a ridge vent. All of that is already done. The eave fascia is not yet completed; the existing plan is for soffit vents.
I had planned on insulating with fiberglass batts in the rafter cavities (vented by vent baffles) plus a layer of polyiso underneath the rafters, but because of the problems that that poses regarding nailing surface for electrical junction boxes, I’ve abandoned that plan.
My reading of code suggests that minimum ceiling R-value in my area is R-49 (please correct me if I’m wrong there).
I got in touch with a local spray foam contractor who recommended 5″ of foam sprayed to the underside of the sheathing. But as far as I know, 2lb. foam is only R5-R6 per inch, meaning 5″ would only get me to R25-R30, which is well below even the code minimum. I have an email in to him asking about this (in our conversations he didn’t say anything about code minimums or about including any other insulation in addition to the foam… so am I misunderstanding something? Or was this guy really going to leave me with less insulation than required, and not mention anything about it???), and I’ll update this post when I hear back from him.
After reading Martin’s article on insulated cathedral ceilings above, I’m unclear on a few things. I know that if you install foam on top of the roof sheathing that there needs to be at least R-25 (in my area) of it in order to keep the sheathing above the dew point and therefore prevent condensation that will rot the sheathing. But the article was less clear about foam sprayed underneath the sheathing. I think I understood that the required R-value is the same (R-25, in my area), but I’d love clarification of that.
Second is the question of venting. I understand from the article that using an air-impermeable insulation like spray foam allows you to use an unvented attic approach. I have a couple of questions about this though. First, my lumberyard specified (and my builder used) the “Elephant Skin” underlayment mentioned above. I was unable to find any vapor permeability information about that product, so I called the manufacturer and reached an individual who didn’t really seem to have any specific info but said he would not recommend using spray foam underneath sheathing with the Elephant Skin applied on top of it. What are people’s thoughts on that? I’m a bit nervous about sandwiching the sheathing between two different vapor-impermeable layers which would seem to prevent it from breathing, but I seem to remember reading where Martin said that wouldn’t be a problem. So I’d love clarification on that point.
Finally, regarding the total R-value of the assembly:
If I have 5″ of foam applied to the inside of the rafter cavities, do I then fill the rest of the cavity with fiberglass/rock wool to maximize my total R-value, essentially using a flash-and-batt method? Are there any problems with this?
(This is the first time I’ve built a building from scratch, and while I had thought I’d planned the build sufficiently in advance, it is dawning on me more and more that that is not the case. It seems just about the only thing I’ve done right is to avoid using recessed lighting cans, instead going with shallow boxes and those awesome new Philips flush-mount LED fixtures…)
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