Depends. Conditions in Malone differ from Brooklyn. Mainly in that the
vent fans I've seen are controlled by t-stat, so they run when temps
are over set-point. Nothing to do with moisture- just keep from
charring rafters (DAMHIKT in CT) and frying shingles.
For moisture, you gotta let it, or help it, to go outside, and
absolutely prevent it from going where it can do harm. Even a small gap
in sheetrock taping on ceiling can let lots of water vapor escape
through and up. I'd check entire area below location of mold, and
really seal it properly. Been there.