Let’s focus this discussion on relative humidity or the moisture content of the air as a percentage of how much moisture it can hold. High relative humidity initiates issues of mold, mites and wood rot.
A major culprit to high relative humidity is an open vented crawl space. Unless your crawl space is home to your heater needing fresh air intake, then there is no reason to vent your crawl space. Basements and crawl spaces are generally cooler spaces due to being constructed underground this means when warmer air from outside the vents enters the crawl space relative humidity goes up.
Cooling air by one degree increases the relative humidity of the air by 2.2%. Once it reaches 70% mold will grown on organic surfaces such as wood, cardboard, drywall and carpet.
There are two ways to reduce relative humidity of the air: heat the air or take water out of it by dehumidifying.
Dehumidifiers work by cooling the air to remove the water and the heated air back into the room. Dry air = healthy air!