Optimal thickness of closed-cell foam in unvented cathedral ceiling
I live in climate zone 5, and am in the process of refinishing the third floor space of a very old house, around 1800 square feet of ceiling space. It is an unvented roof assembly, with relatively deep rafter bays, between 7-8 inches. I was planning the use closed cell spray foam and was leaning toward using 5 inches of foam which would give me around R35.
However, I’ve read some things lately that suggest very thick foam may be less desirable than ~2 inch foam with cellulose the rest of the way, which could give me close to the same R value if I fill all the way out the full depth of the rafter bay. Something about how with less foam depth the moisture would condense on the surface of the foam, and with more depth it would condense on the underside of the roof decking between wood and foam.
Also, I was thinking of using wood for the finish instead of drywall, and so I am unclear on whether I need an additional air barrier such as MemBrane, with either the full foam, or foam/cellulose options.
Will I be fine either way? Is there a preferable option? From a cost perspective it would be nice to avoid having to use the MemBrane, if possible, since multiple inches of closed cell foam over 1800 sqft is expensive enough…
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