I'm installing a new bathroom fan in a finished ceiling. The instructions are vague with respect to a finished ceiling. Should the hole in the ceiling be smaller than the fan housing so the fan rests (attached to the joist) on top of the ceiling / drywall? Should the hole be large enough so the housing can slide through? If the hole is large enough for the housing to slide though, should the edges be sealed some how as not to allow attic drafts and prevent moisture entering the attic. Should the entire housing be sealed with a vapor barrier of some kind?