Aim for crawl space relative humidity between 50% and 60%. Above roughly 60%, conditions support fungal growth on wood; below about 50% you are spending energy for no additional benefit. A sealed, conditioned crawl space in Wilmington should hold 50–55% year-round.
Why 60% is the threshold
Fungal growth on wood is governed by the moisture available at the surface, which tracks the relative humidity of the surrounding air. Sustained relative humidity above roughly 60% brings wood moisture content into the range where growth is supported; below it, growth stops being viable.
The wood moisture content equivalent is around 16%. Above 20% you should assume growth is either present or imminent.
Measuring it properly
A cheap hygrometer left in the crawl space for a week tells you more than a single reading, because humidity there swings with outside conditions. Place it away from the access door and off the ground.
Take readings in summer. A vented Cape Fear crawl space can look acceptable in February and sit at 75% in July.
If yours is above 60%
In a vented crawl space, that is expected here and no amount of dehumidification will fix it while outside air is free to enter. The fix is sealing first.
In a sealed crawl space, high humidity means one of three things: the space is not actually sealed, the dehumidifier is undersized, or water is entering that has not been dealt with.
The bottom line
Buy a hygrometer before you buy anything else. It costs very little and it turns a vague worry into a number you can act on.
For a number against your own crawl space, use the cost estimator, or call (910) 886-2018 and we will come and measure it.