Rigid foam above roof sheathing: can I use a synthetic underlayment?
I have read warnings about using a vapor-impermeable synthetic underlayment (e.g. Grace Tri-Flex) over an unvented space (which I do not assume to mean a roof with absolutely no ventilation channels).
However, in the “How to install Rigid Foam On Top of Roof Sheathing” article, a roof system is described where taped foil faced polyiso is installed above the roof sheathing and below the ventilation channel–despite the fact that foil faced polyiso is also vapor impermeable.
Why is it that this is permissible with such a roof? And in such a case would it therefore be OK to use vapor-impermeable synthetic underlayment overtop the foil faced polyiso?
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