Closed and open cell foams on unvented cathedral ceiling
I am planning a cathedral ceiling over a great room that will have skylights, so based on what I have read on this site, it seems an unvented rafter assembly with closed cell foam is my best option. However, Martin’s recent “Musings” in Fine Homebuilding noted that closed cell foam is hard to trim, so most installers stop filling the bay a bit short, wasting R value.
I want to get the maximum insulation in the rafter bay, so I am wondering if it would be cost effective to use enough closed cell foam to seal the sheathing from moisture (2″ flash w.o batt), followed by open celled foam that can be trimmed a day or so later.
Also, would it be worthwhile to put foam board on the underside of the rafters after the trimming to address thermal bridging?
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