You guys know there are two kinds of rigid insulation at HD and Lowes, right? Correct me if I'm wrong but I think R-tech is EPS, vs. R-Max is polyiso and has the foil layer on both sides. I also think the latter is designed for fire resistance, and should be light tight with proper seams. Polyiso is much stronger and is $20 a sheet for 1" and $30 for 2" at HD.
I visited a (non inspected) warehouse where 2" R-max was used for several freestanding structures (tall tree rooms mostly). The wall studs were on 4' centers with the foam glued between. I only saw the outside surface because it was during dark hours and we were there on other business, so I don't know how or if the interior was covered or just bare.
I'm considering similar construction for a new project this summer, and wondering if we can paint the Rmax foil and exposed wood studs with a good latex paint with mold inhibitor.