I have a 1948 building in South Central PA with a flat, rubber membrane roof. There is a no attic space. The roof sheathing is directly on top of the joists. The joists are 2×12 with existing fiberglass batts installed between them. I am going to be lathing out the ceilings and hanging new drywall to cover up existing plaster damage and provide easy access for electrical contractors to rewriting the unit. I want to add some insulation while we are doing this work. I thought adding 1/2″ of rigid foam board directly to the plaster, then 2×4 lathing, then 1/2″ drywall would be the easiest method. However, I’m worried about creating a condensation issue between the rigid foam board and rubber membrane. What is the appropriate way to handle this situation? Can I/Should I add the rigid Polyisostyrene? This is a rental property. I don’t want to sink a ton of money into it. I simply want to help out the tenant’s on their energy bills, but not create problems for myself down the road.