With the help of sites like GBA, a few years ago I removed all of the fiberglass insulation in our attic, foam sealed all of the penetrations, installed Accuvent soffet vents and blew in about 16” of new insulations in our New England home. I created a small platform at the top of the attic stair hatch directly on the joists, and a raised platform next to it to store some season stuff. I recently pulled up the flooring next to the hatch to do some renovations to the ceiling below and discover a small amount of mold on the underside of the plywood. Is there a way to prevent this from happening? Between the drywall ceiling and the plywood flooring is just old fiberglass batt insulation. As part of this recent renovation, I did significantly increase the seal around the attic hatch and the 2” foam insert that I fabricated to go over the attic hatch opening. The ceiling below also go another layer of drywall and paint. There is a full bathroom nearby that does have a fan for venting, but I assume is the biggest culprit for water vabor, spilling into the hallway, getting into the attic and hitting the underside of the cold plywood in the winter.

Would it be better to install paper backed fiberglass between the ceiling and plywood, or 1” foam on the drywall surface, spray foam the edges, and insulation on top of that? Or keep it semi-permeable and hope the new sealing does the trick. 

I included a sketch of the platforms.
Thanks for any input or suggestions.