My original plan called for multiple layers of foam sheathing with seams staggered, taped etc. Having been directed on this site to an insulation reclaimer I can purchase the necessary r value reclaimed and at a better value but at the 2.5" thickness a single layer is all I'll need. I've not detailed xps in this regard and wonder if sealing the seams on the unfaced foam can be done effectively. Any advice on approach and materials greatly appreciated.

I suppose that an advantage of reclaimed foam is that almost all of the age related shrinking is done. Consider taping the joints with Siga Wigluv (good adhesion and vapor permeable, which might deal better with any moisture being deposited from the back side of the joint).