Polyiso and roofing underlayment question
I am decking a roof with 2×6’s and then plan for one layer of synthetic underlayment, followed by 4.5″ polyiso, another layer of underlayment, purloins and standing seam roofing. Does either layer need to be vapor permeable? The base layer will be laid to protect the roof until I can add insulation, which may be some time. What are the best practices? Thank you in advance.
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