Cathedral Roof: To Vent or Not To Vent?
I’m building a vacation house on some rural property in Montana. It can get very cold in the winter and very hot in the summer. And we have a fair amount of wind in our area. The walls of the structure are going to be built using Faswall ICF blocks. The roof will have two sections with a single pitch roof design separated by a middle section with a more conventional two-pitch roof.
For the two single pitch sections, I’m planning on using 14″ TJIs to span roughly 20′ with a cathedral roof design. I can use 4″ of closed-cell polyurethane foam and 7″ of Roxul Comfortbatt to get close to an R-60 roof. But I’ve heard some people say they’d NEVER spray foam to the underside of their roof decking, anticipating what a nightmare it would be if the roof ever had to be torn off.
So I’m wondering why a guy couldn’t set the TJI joists then fit some 1/4″ plywood between the trusses attached to the upper web of the joist, spray your foam onto that material, fill in the remaining thickness of the web with the Roxul, and then the interior ceiling material. The actual roof decking itself could then be laid down and there would be about a 1.5″ open gap (the thickness of the flange) between the 1/4″ filler plywood with the foam on the interior side and the 5/8″ roof decking.
This way I could have a vented space below my roof decking, cover it with 30# felt, and put down my metal roofing. It seems like a pretty clean way of insulating the cathedral portions of my roof while protecting my roof decking and, should I have to ever replace or repair the roof, I could remove the decking without having to rip out all the sprayed foam insulation.
Does this make sense or am I missing something big?
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