They did this on the rehab of the A frame across the street from me. They stripped off the existing roof shingles and used 2" foam insulated 4x8 sheathing right over the old roof sheathing.
I'm not sure what the R value is but they also wanted to keep the original beamed ceiling. The original sheathing was 2" tongue and groove pine boards, so they ended up with a 4" thick roof.
It passed the building code inspection so it must have been enough of an R value?
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