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I am building a new garage/ wood shop addition in Fairmont WV. My concern is with the roofing system. The structure is wood trusses with exposed 2x6 T & G decking. The garage and shop will be air conditioned or heated from time to time. I plan to install a metal roof over the wood decking. The question is what type of insulation and vapor barrier should be use under the metal roofing? I've been leaning toward a reflective foam core insulation (solid or perforated?). Also, should the vapor barrier be directly over the wood decking or just under the metal decking?
My concern is that, with direct contact between the metal panels and the roof deck, you will be at risk of condensation inside the building when it's cold outside and warm inside. I would suggest working closely with your roofing manufacturer to determine what type of insulation they are comfortable with. Right off hand, though, I'd suggest a premium synthetic roofing underlayment over the decking and then 2 -3" of poly iso insulation followed by the roofing. Again, though, work with your roofing manufacturer to determine what is best.
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