Insulating an Existing Unvented Cathedral Ceiling
My current plan is to install 4 layers of 1.5″ (published r-9.8 each) polyiso foam boards via the despised cut and cobble method. I’d put 2 layers in the current rafter bays, install more 2″x4″ rafters (offset to reduce thermal bridging), then 2 more layers of foam in the new bays. I’d spray foam around the boards to try to get the best air seal possible. I know this doesn’t get it to an R49 for my zone, but following the IRC N1102.2.2 provision it should be acceptable. I haven’t calculated it, but it might also be close to my zone’s 0.026 U-factor as well.
I have considered making the additional rafters 2″x6″ and doing cut/cobble with batts. But this starts to bring the ceiling down closer to the windows than I’d prefer. I also think offsetting the polyiso boards should help out with any cut and cobble shortcomings.
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Non-expert opinion but I think it's more important to air seal and prevent interior air from reaching the sheathing than to hit a code-mandated R number. GBA generally discouraged the cut and cobble method because maintaining the seal is difficult as the framing moves in response to seasonal conditions.
It's possible you could meet code with R-30 or R-38.4 (see https://www.greenbuildingadvisor.com/article/three-code-approved-tricks-for-reducing-insulation-thickness for more on that).
I'll give you a bump and maybe the experts will chime in.
Thanks for your feedback!
As an update to my original post, I've modified my plans slightly after reading more about other unvented cathedral ceilings projects. My current plan is still to cut and cobble two layers of 1.5" polyiso in the existing rafters, but now I plan to cut and install two more layers of foam under the rafters and tape the seams. The full sheets and tape (foil faced boards) should provide a reliable air seal. I'll then attached furring strips to the rafters so that I can install the finished shiplap ceiling.
What you're doing could work, but it's still a bit risky. I would strongly prefer closed cell spray foam in those rafter bays. If you do end up doing what you plan with the cut'n'cobble, be extra careful to seal the ENDS of each rafter bay, including the rafter tails. You don't want moist air to sneak in around the ends/edges of your taped inner layer of polyiso. That taped inner layer is critical to your assembly in this case. I would take care to seal the continous layer of polyiso using foil tape on the seams, and polyurethane sealant around the perimeter.
Note that you may be required to put a layer of 1/2" drywall up as a thermal barrier (for fire resistance) behind the T+G. A quick call to your local building department should get you a yes or no answer for that.