Martin and friends:   Can you give an update on the issue of vapor retarder coatings for unvented open cell foam roofs, per IRC 2015 806.5.4 where the builder does not intend to apply wallboard?  Here in the Boston area, the issue is coming up more often.  I see material at GBA from 2015 that indicates that conventional vapor retarder paints do not actually reduce the vapor permeance of the foam.

What about applying a final coat of closed cell foam to the assembly, and how thick would it need to be?