Roofing membrane above deck + foam below deck = double vapor barrier?
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If double vapor barriers are so bad, why are unvented roofs with foam below and ice-and-water shield above a common practice? (Why is it okay?)
Edit: I found one related discussion on GBA: https://www.greenbuildingadvisor.com/question/standing-seam-ccspf-self-adhered-double-vapor-barrier
I’m not sure any of the comments directly answer my question, but at first glimpse I’m seeing comments saying to get vapor-permeable membranes and other comments saying self-adhering membranes may be safe from vapor penetration (and thus I suppose having a double vapor barrier is considered acceptable).
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The general idea is that the fully adhered membrane on top, and the fully adhered spray foam layer underneath, make the chances of moisture ingress small enough that you don't have much chance of failure. With a wall assembly, it's almost impossible to really seal everything 100%, so there is always a way for moisture to get in, even if only very slowly. With fully adhered vapor barriers, you end up with a sort of manufactured sandwich that is a much safer assembly when compared to something like an insulated studwall.
Note that it is really important to get the details right around the edges of a roof assembly like this. This means extra care in valleys, and especially around skylights. You have to avoid any sneaky paths for liquid water to get in where it can wet the exposed edges of the sheathing.
Bill
Thanks!
Ecological,
This article addresses your question: "Sandwiching Roof Sheathing Between Two Impermeable Layers."
Thanks! I read the article and most of the comments.