We live in s.e. PA and have an Alladin package house, c. 1919. Siding is vinyl over 1/2" ply board, probably from the 80's, nailed to the original sheathing. The upstairs bath is down to the studs for complete renovation. Studs are full dimension 2x4. Is this a good application for iso board?---this would be one outside wall, two interior, and interface with the attic. With poly board on the outside, should there be an interior vapor barrier under the new drywall? Should there be any airspace left in the outside wall/ceiling bays? In other words, if using iso board should it be the full 4" or a lesser thickness?