Yesterday I was able to pick up a bunch of 1.5" polyiso boards for $50. I would like to just put them against the metal siding in my polebarn by cutting it to fit between the purlins. Then put sheetrock over it. I am located in south jersey. Do i need to put up some kind of vapor barrier? Should I spray foam beetween all the purlins and siding? It just seems like overkill. The insulation will cover most of the wall and then be covered with the rock anyway.