*Another* unvented cathedral ceiling insulation question – Zone 5B
Hello ladies and gentlemen, happy new year. I’ve scoured this site and others for the past 3 months looking for the proper way to make our finished attic more livable, and there are numerous articles (How to build an Insulated Cathedral Ceiling – https://www.greenbuildingadvisor.com/article/how-to-build-an-insulated-cathedral-ceiling; Over-roofing – Don’t do stupid things https://buildingscience.com/documents/insights/bsi-063-over-roofing; There’s Rot in the Ceiling – https://www.greenbuildingadvisor.com/article/theres-rot-in-the-roof; Cut-and-Cobble). I know what is theoretically correct but the old construction of our house presents enough quirks I felt like I needed to post my own questions.
Our house is in zone 5B, a few hours southeast of Spokane, WA, and was built in 1895. It has a “finished” attic with sloped ceilings and wood floors. Everything is 2×4 construction with 9″ shiplap on the interior and 5″ wood siding, tar paper and cedar shingles on the exterior of the sidewalls. The roof is skip sheathing, tar paper, and D-rib metal. I believe the pitch is 10/12 or similar. There is no duct work as the only heat comes from a small woodstove and a small propane stove downstairs (we are hoping to install ductless heat pumps in the future). There is no insulation in the entire house and drafts abound.
So, if I have read the documents correctly, an unvented cathedral ceiling in my zone needs R-20 of impermeable insulation to prevent condensation, which in my 2×4 rafters I could get with spray foam alone if it wasn’t so expensive. So then I was considering the “peanut brittle” method of cobbling in reused rigid foam and spraying the last 2″ with spray foam. However, should I be worried that the rigid foam won’t ever fit evenly against the roof because of the skip sheathing? Aka, should I spray 2″ of foam first so that it gets in between the skip sheathing and then put the rigid foam on the interior, or would that lead to condensation between the spray foam layer and the rigid foam?
I should probably note at this time that I realize that 4″ of foam isn’t going to meet code for total insulation of an attic/cathedral ceiling, but at this point the roof is fine and I don’t really want to rip it off to put on 6″ of rigid foam. Maybe that’s a future project? Anyway, if I can just stop the drafts with the spray foam it will make a huge difference.
Thank you for the help ahead of time. Perhaps someone could even comment on if they know of any rigid foam scrap dealers in the eastern Washington/northern Idaho area.
p.s. – uploading pictures failed
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