Closed-cell spray foam on roof deck… shingle underlayment choice?
I’m planning on doing spray foam insulation on the underside of my roof deck.
I’m about to start shingling next week, and was planning on using synthetic roof paper instead of tar paper.
Then I read an article saying that on unvented roof assemblies (what this will be), most synthetic roof underlayments should NOT be used.
I did some digging, and the product I had planned to use called “Rhinoroof” has a perm rating of 0.5, aka it would create a vapor sandwich between my shingles and under deck roof insulation.
My sprayfoam guy did not seem to conceared about it, saying that if my osb moisture content was below 15% when the foam was applied, then it would no be an issue.
Its winter, i’ve got snow on my OSB now and 1/2″ of ice. I doubt, unless I leave the roof until summer, that I will get the moisture anywhere near 15%.
Whats your guys take on this? It never crossed my mind…but glad I stumbled across it. I’m thinking #30 tar paper might be the way to go…?
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