EPS Foam behind fiberglass – vapor issues – GreenBuildingAdvisor

Hi folks – I’ve been racking my brain and searching the internet on this one but can’t find an answer I’m completely comfortable with.

I’m doing a vented cathedral ceiling in an existing garage loft (finishing an unfinished building) and I want to do site-built baffles made out of 1″ EPS behind fiberglass batts (faced) in zone 5 which is primarily a heating climate – faced batts are the norm here.

At first glance, this should work because EPS is vapor permeable. However, when I dig into it, I find that EPS is a heck of a lot less vapor permeable than fiberglass.

Will this get me into some sort of double vapor barrier situation, or is the EPS sufficiently vapor permeable at 1-1.5″ that I won’t have any issues putting faced batts between it and the drywall? Keep in mind that faced fiberglass batts are rarely installed perfectly.

If this *is* ok, the next question would be whether or not I can put 2″ EPS behind my knee walls to improve air sealing behind the batts (and pick up another R8 for cheap)