The R value of all closed cell foams as far as I can tell is the same. I wouldn't worry about 10% differences in reported value. Supposedly the R value decreases with time. Perhaps that is the reason for differences.
I would strongly suggest sticking with code to prevent ice dams in your area.
No, foamers aren't like, say plumbers or electricians. I am not sure of your codes where you are but up here some code violations have to be corrected where they impact on safety for example, an electrician has to rewire a plug if that plug is not currently up to code and regardless if the HO asked him to or not (I mean he couldn't just put a plug on bad wiring). But in the case of a foamer, I say that the code is a minimum guideline and not considered violated if not met. The foamer will spray whatever you want so if R38 can't be met in the space provided, he'll do what he can.
Inspectors have enough to do without being a Big Brother. "Live in ND and want R12 in your ceiling, do ya?"
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