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 open cell foam. But is any of this not really helping and not worth the cost. Is the cost of the second layer of sheathing worth it to get a ventilated deck?