Moisture must be coming from somewhere- either furnace or interior of
house(I'm assuming no leaks from outside). If furnace, check for leaks
again. No blockages in exhaust ducting/ chimney? If from inside of
house, you would probably see darkened areas on ceiling of 2nd floor,
condensation from where warm, moist air encounters gap in attic floor
insulation. Moisture is definitely inside attic, yes? North side of
roof could certainly show signs of moisture(eg lichen/ mildew) on
outside- put zinc strips under shingles for this.