Looks like I am a little late to the discussion-- at least as far as the ceiling project goes.
I suggested that a friend of mine try a housewrap underneath the siding of his pole barn. It was just outstanding in how the drafts shut down in the space. I am thinking that housewrap under rigid foam would provide a nice level of insurance and might eliminate the necessity to tape the joints of the rigid insulation.
Some guys place fan fold insulation beneath the roof to cut down on condensation,but unless you install bird stops on the rafters between the purloins, those little buggers will ruin it in a hurry
So if you are anticipating using rigid sheet insulation on a ceiling or are concerned about adding a vapor barrier to the cold side of the insulation,consider that the housewrap lets the moisture out and provides a superior barrier against air infiltration. The stuff is just as tough as nails and there is also joint tape available. A 9x150' roll is a little more than a hundred dollars and money well spent
I am also doing a bathroom in one of my rentals that will get rigid foam sealed with spray at the joints between the joists [treated] because of the high moisture conditions beneath the floor. Therefore, fiberglass insul is out. Then either tar paper or synthetic tar paper to protect the underside of the plywood from condensation,in the event it should occur. there is a ceramic tile floor and thick mud shower going in,so the floor is permanent with only one chance to do it right
It is not a perfect solution,but it will easily outlive the rest of the house