Sometimes experience is the best teacher. Our experience with moisture have allowed us to gather a few facts.
If there is enough moisture present within the structure to cause visible problems, there is a source of water intrusion.
Duct work does NOT make water, therefore should not be considered a source.
Introducing warm moist air underneath the floor (fan) may hurt more than it helps.
Beware of folks who are quick to think they see the problem. You must do a thorough investigation when moisture is a problem. If someone diagnosis the problem without going into your attic, under your floor, and maybe even the roof, you would be better off considering someone else to help you.
Structures with excessive moisture problems will most likely have at least one of the following problems;