Mold germinates in a crawl space when relative humidity holds above 60% or wood moisture content exceeds 16% for several days. In North Carolina, summer dew points routinely push unsealed crawl spaces past both thresholds, so the target is 50% to 55% relative humidity maintained by a dehumidifier.
The numbers that matter
Wood moisture content below 15% is safe. Between 16% and 19% surface mold becomes likely. Above 20%, decay fungi that actually destroy structural capacity can establish.
Relative humidity above 60% sustains Cladosporium and Aspergillus growth; above 70% it becomes rapid. Triangle summer outdoor dew points in the low 70s guarantee condensation on any surface cooler than that.
Why vented crawl spaces fail here
Ventilation only dries a space when outdoor air is drier than indoor air. From May through September in North Carolina, outdoor air is far wetter, so every open vent is a humidity supply line.
This is why the sealed-and-conditioned approach has replaced ventilation in Southeastern building science guidance and in the North Carolina residential code's closed crawl space provisions.
Verification, not assumption
Ask for a hygrometer reading before and 30 days after work. A properly sized dehumidifier should hold 50% to 55% relative humidity through August without running continuously.
If humidity still climbs, the source is usually liquid water intrusion that drainage should have handled, not an undersized dehumidifier.