Spray Foam and Underlayment

Stephen Gass
We are putting a metal roof on the house we are building and I am trying to figure out the venting/insulation/vapor barrier requirements. From what I've read so far, I think we can spray a closed cell foam under the roof sheathing for insulation and that will provide insulation and a vapor barrier that will avoid the need for venting (at least between the insulation and the metal roofing). If that is correct, then can we put self-adhesive ice and water underlayment over the sheathing and under the metal or do we have to use a moisture permeable underlayment? It seems like the the ice and water barrier on one side and foam on the other side could create a layer of trapped moisture in the plywood sheathing, which seems like it might not be a good thing. Should I be concerned? Thanks. Steve
Todd Miller
Isaiah Industries, Inc.
I do not see any issue with an ice and watershield on the outside. That ill give you a vapor barrier on both sides which should not be an issue,
Eric Novotny
An informed customer is the Best Customer!
Its not really an issue as the plywood, ice/water, and spray foam are all in the same plane. What I do not hear discussed is the insulation value that is going into that assembly. What type of R-Value are you putting on the roof and have you considered rigid foam to the exterior roof deck? This, in combination with lower amounts of SPF, will give you a better performance roof assembly.

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