highest R-value spray foam
I'm looking into my using spray foam to replace batts for my roof. I have 2x8's for my roof rafters and I think I'm going to lose ~1 inch for venting, so I have 6 inches left for the spray foam. In order to get to the minimum 38 R value I need a high R-value closed cell foam. I'm told some types of Polyisocyanurate spray foam have an R-value of 8 per inch, does anyone know where I can buy these? I checked the normal places but had no luck.
Also, I've heard if you get foil faced Polyisocyanurate boards that the foil gives you an extra R-vale of 2 since it reflects one of the types of heat emission, is that true? if so I could use it for my framing of the roof venting.
Thanks for the help