Moldy roof decking

We have a conditioned partial attic off our master closet/bathroom. My wife noticed a musty smell coming from there the other day. Went in there and pulled the “vapor barrier” (plastic sheeting)🤮and insulation (fiberglass bats stapled to the rafters) down. And found a ton of mold growing on the roof deck. We live in a cold climate (Michigan) The roof is pretty good. Middle-aged. The decking is in good shape, sprayed a bunch of bleach up there 2 1/2 gallons in a 500 square-foot area and it seemed to have killed the mold. Now running a dehumidifier and space heater and can’t seem to get the place dried out to re-insulate. Was going to do R-21 flush to the roof decking unfortunately it’s not possible to vent the roof at this time. What would you guys do to keep the decking from getting moldy again besides no vapor barrier? 2 x 6 rafters. I could always scab on a few more pieces and make them 2 x 8.