100% of all crawl spaces today in NC have high, above 80% moisture, for at least 6 months out of the year! May-October. Many are above 80% year around due to added moisture from inside, water coming through foundation walls and open pipe penetrations in foundation wall, and not just NC’s standard 90% warm season humidity levels.
Why is this bad? Over 80% humidity makes the wood floor members in the crawl space over 16% wood moisture content allowing them to grow mold.
Has North Carolina always had high moisture in the crawl spaces. NO!
Before 1973, the North Easterly winds blew in one side of our crawl spaces through our foundation vents and out the other side causing an EVAPORATION effect, similiar to your bathroom fan after a shower. So this breeze of warm moist air wisking through the crawl space before 1973 never came across a chilled crawl space ceiling or the cold metal on the crawl space AC unit nor the colder ducts. Yes the AC metal is insulated on the inside but that in NO way stops all moisture against the metal and the ducts but it does slightlyt minimize the effects.
AFTER 1973
North Carolina got central air. Window units were available in 1950’s but most residence of NC did not chill their entire first floor like we do today. Before 1973 the inside temperature, the outside temperature and the crawl space temperature were all the same. After 1973 we ARTIFICIALLY chilled our homes to 72F-74F. Thank God! BUT it made the ceilings of our crawl spaces also chilled. So when warm air, which holds more moisture, comes into contact with a cold surface condensation happens. Like a cold glass of sweet tea in your kitchen. 
The above picture is a 30 day old home that its ceilings condensated so much it exploded mold throughpout the entire crawl space ceiling. Again, this home was 30 days old, and it was a builders personal home. We do all his homes sealed crawl spaces but he built in a flood zone and was NOT allowed per NC Building Code to seal up the foundation vents. He told us to come back after final home inspection to give him a sealed crawl space. And he got busy and a month later called saying mold exploded in his crawl space. So sometimes it can happen that fast.
Inside your home your AC makes it 50% humidity in the warm season and doesn’t allow much condensation to go un evaporated inside the home. Put the glass of sweet tea outside and in condisates so much that it leaves a ring of water on the bottom of the glass. Keep putting ice cubes in the glass and this puddle under the glass gets bigger and bigger as long as the glass is chilled.
This is whats happening to everybody in NC crawl space ceiling in the warm season. It condensates and then gravity pulls it down to the bottom of the wood floor joists making the highest wood moisture content at the bottom of the joists. This is why you see mold growing at the bottom of your floor joists first. And each year it climbs up the beams. Each day the mold eats at the cellulose, which is the glue/strength, in the wood. Cellulose are the tree rings appearing on the wood. Slowly the bottom of the joists no longer have tree rings on the wood and the wood becomes weaker.
SOLUTION:
Stop warm moist air from coming into the crawl space making everything chilled condensate. That means:
1. Air seal and insulate the inside of the foundation vents
2. Air seal and insulate the crawl space door(s)
3. Trickle in 1/10th of 1% of the AC’s already 50% humidity air ONLY when the crawl space needs it, which is usually 90 seconds each hour during the warm season. The amount of air is literally as lightly as you can blow on your hand. Insignificant to the AC capabilities to cool your home. In fact, air sealing the vents and door makes the AC run 18% – 22% less. Making it last 20% – 25% longer saving you big $$,$$$.
There are other things that lead to high humidty, but none of them compare to the condensation effect of warm air in the crawl space coming through the foundation vents and coming in contact with the chilled ceiling, AC metal and ducts.
Other reasons:
1. Water coming through foundation wall pipe penetrations that are under ground. Usually the sewer pipe is the biggest culprit.
2. Water coming through builder designed weep holes in bottom of crawl space walls.