Question for the

4 inches poly iso @ 6.0/ inch. is being liberal...+ all the other stuff you mentioned. drywall, osb, shingles, roll roofing, felt, air space, etc...

http://www.arcat.com/specwizard/07222hun/

as far as GAF and icynene.....none of the manufacturers approve open cell poly foam., but with closed cell, they have no problems, that is as far as I know from an inquiry to my rep last July. I discussed it with roofdude and it seems that they approved it then, but I agree that a vented panel makes most sense when putting composite shingles down on sheathing.>>>

That all makes sense. What they all seem to hate is field-assembled systems. Filled grids used to be the norm. If they are not done meticulously though, they rot. I did one on my own home fifteen years ago and it is functioning perfectly. Back then if you said 'vent chute' they'd look at you like you were dingy. They said complete fill obviated any need for air space. Basically they were wrong. Anyway, just to be doubly sure, I installed ridge vent (without any vent chutes of course) and choked it down a bit. Vapor drive will send any moisture out, I guarantee it. There is not a drop in there. I took it apart recently to check for mice and ants. Lack of moisture confirmed.>>>