I have a 1830 Post and beam house that has had all the beautiful beams covered with lath and plaster, then sheet rock over that. I plan to rip all the lath and plaster out and expose the beams in one of the rooms. I am trying to figure out how to get some insulation behind the sheet rock. The main beams are 8 inch wide but the cross members are 4 inches and are centered in the beams and I would like to have them exposed as well. I have 5 inches from where the face of the sheet rock will be to the outer sheathing. I have an unlimited supply of poly iso boards that are 4ft x 4ft and are 3 3/4 inch tapered to 3 1/4 in. They are aluminum faced. I thought about using liquid nails or another heavy duty adhesive and glue the Poly ISO to the sheet rock then cut the sheet rock/polyiso boards to fit. Do I need a vapor barrier between the poly iso and the sheet rock? How about between the poly iso and the outside sheathing?
Would I be better off buying XPS and gluing the sheet rock to that with the same process? Can I push the XPS or the poly iso against the outer sheathing or does there need to be an air space between the two?