Metal roof underlayment
I am building a very tight house borderline zone 3-4. The walls are 2×6 probably going with dense pack cellulose. Zip sheathing with foil faced 1/2″ polystyrene facing out to 1/2″ air gap rain screen.
The roof is metal over an OSB deck. Part of the roof is over an insulated attic (12″ cellulose ) the other part is over a vaulted ceiling. The rafters are 2×8. The vaulted ceiling is going to be closed cell foam.
The roof will be metal (white) screwed to 2×4 purlins. This gives me plenty of ventilation under the metal.
My question is what underlayment should i use under the metal? I thought initially I would use a reflective foil to keep the roof deck cool. I think that might be overkill. However I am looking at Tyvek Protek 120 and the cost would be comparable to the foil. Is there a problem over the vaulted ceiling with condensation under the underlayment? I have never had a problem with metal roof leakage so am not too worried about that. I don’t want to trap moisture under the underlayment on the deck though from condensation. My gut tells me I have plenty of air flow under the metal.
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