Hi. I have an opportunity to purchase some Rmax branded polyiso sheets directly from the factory for about 1/3 of the retail price. Obviously, I am excited about this opportunity but I want to make sure that this product will work for my application. The house, with a full basement, is located in Clemson, SC. It was built in 1968 with a full walkout basement. The basement walls are constructed of concrete block. I would like to eventually finish the basement. Dana previously indicated that one inch of extruded polystyrene against the wall would be a good start. I am wondering if polyiso will work in this application or is the vapor permeability going to create an issue. My source indicated he can get me roofing polyiso that is faced with a tar paper. The rmax website indicates the permeability of this product is <1.3 perms. Will this work or will I experience moisture issues? Thanks!

Is it really paper, or some other fiber? (Papery-looking fiberglass is often used as facers for roofing iso.) Under no circumstances would you want to put mold-food-paper directly against the foundation, but it's OK as the interior-side facer.  If it's sold as roofing insulation, mold shouldn't be a problem.

But at ~1-1.3perms you'd be good-to go, maybe even as a double-layer.  Were it 0.5 perms you want to leave at least ~12" up from the bottom of the slab sans-foam, so that moisture wicking up from the footing or slab can dry toward the interior.  Under 0.5 perms could be risky, but it's probably at least 0.8 perms if advertised to be <1.3 

With 1" iso + an interior 2x4 studwall with unfaced batts you'd have a whole-wall R of R17-ish.

So I need to identify exactly what the facing is.  My source was unsure of the exact makeup of the facing.  I suspect it is some sort of carbon-based organic compound.  It certainly appears to be brown paper.  The rmax website indicates they have products with both organic and inorganic facing.  I asked my source for the product (by name) with the inorganic facing but he said they didn't have any.  As such, I am going to assume that it is organic and will provide a food source for mold.

Thanks again for your help.