Effectiveness of Roxul “flash and batt” – irregular closed-cell spray foam causing air gaps?
I have a spray foam crew coming Thursday to convert my attic to “unvented” (2×8 rafters). They want customers to do 5″. My plan though was to do 4″ and then 3.5″ Roxul batts to fill followed by a layer of the Roxul board to reduce bridging on the rafters.
I’ve spent way too much time researching and cannot find anything decisive on this. Specifically the question is about the air gaps that would be present between the ccSPF and the Roxul. I’ve seen this question come up several times in comments here, but it’s never addressed. The closest I found was Martin referencing someone to Joe L.’s “Mind the Gap” article on the “foam sandwich”, which states that a 1/8″ gap would reduce the thermal effectiveness of the outer foam sheathing by 5%. That seems reasonable.
The “flash and batt” assembly is documented by Building Science and they don’t call out this issue as a warning. However, flash and batt is mostly documented with fiberglass which seems like it might contour to the irregular ccSPF better than Roxul.
I am nervous now because more often than not when this assembly is discussed in forums/comments someone calls out the issue of the irregular shape of ccSPF and that the batts will be less effective. For the cost of my Roxul materials I could upgrade from 4″ to 5″ and let them flash coat the exposed rafters to reduce bridging. Less R than my flash and batt, but it’s a “proven” approach. Any advice/opinions?