Poly iso isn't my favorite. I've used it, but once water gets into it, it never dries out. If you want to use sheet foam use the pink or green stuff XPS I think. Water doesn't effect it and R value is about the same. In fact as poly iso get colder and older the R-Value drops.
You can also use Comfortboard by Roxul, great sound barrier but harder to find. It doesn't hold moisture, burn, melt or mold. It just does its job.
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