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Look for the highest R-value per inch. That way you lose the least head room. But to get R-40, you will need more the 3 1/2". And once you are spraying the upper 4', you are probably better off doing it all, rather then trying to figure how to vent half way up the roof. But it can be done.

Is it important to have a ventilated deck for a metal roof? Or can you spray open cell foam insulation on the back side of the deck? I am thinking the optimum is to have a metal roof, plywood sheathing with radiant barriar backing, vetilated airgap, second sheathing layer, and then the...