Cut & cobble above existing ceiling? – GreenBuildingAdvisor

I have one additional energy, efficiency, and vapor barrier question for the expert community.

One story 125 year-old house in a cold climate in Northern Michigan..  We are removing old worn out Fiberglass batt insulation in the rafters above the master bathroom so that we can blow in loose fill cellulose at a significant level. I’ve realized that there is no vapor barrier between the drywall ceiling and the underside of joists in attic.   This home was traditionally not used during the winter so they only insulated it for the shoulder seasons.  We are updating and insulating as a Zone 5 full time residence.

I don’t want to tear out the drywall ceiling and I can’t go over the top of the attic joists due to mechanical systems that they have run above those joists.  Is it advisable to cut and cobble rigid insulation on the top side of the drywall in between the joists using van foam to seal the edges to provide a vapor resistant barrier?  I know that will be a lot of tedious work but I’m concerned about water vapor pushing up into the new insulation during the winters above that bathroom.

As always, thank you for your insights.