more than likely you're going to have a pre-insulated box/register. I had the same prob. I took off the vent from inside the house, filled in the little gap between the sheetrock and box/register with sealant. I then sprayed killz primer and so far so good. one of my registers was pretty bad so I wiped down the best I could with clorox wipes and it seemed to kill off that stuff.P.H. Dexippus said:
Pull off register and clean. Best cleaner is Concrobium to prevent regrowth. To fix the source of the problem, from the attic side, air seal the where the register meets the ceiling drywall with foam and make sure there is sufficient insulation around and above the register.
