In coastal North Carolina, crawl space vents should be closed and the space conditioned. Venting assumes outside air is drier than crawl space air; on the Cape Fear coast it is not for most of the year, so venting brings humidity in rather than removing it.
Why the old advice existed
Crawl space vents were required on the reasoning that moving air dries things out. In a dry inland climate with a cool crawl space, that reasoning broadly holds.
It fails in a humid coastal climate, because the mechanism is not airflow — it is dew point. Warm humid outside air entering a crawl space where the framing and ground sit cooler will condense on those surfaces. The vent is delivering the moisture.
What the code says now
IRC R408.3 permits unvented crawl spaces provided the space is conditioned — either supplied with conditioned air or served by mechanical dehumidification — and provided a Class I vapor retarder covers the ground with sealed joints and terminations.
In other words, closing the vents is permitted, but only as part of a system. Closing them and doing nothing else is neither compliant nor a good idea.
The North Carolina research
Advanced Energy carried out closed-crawl-space field research in North Carolina specifically, comparing vented and sealed crawl spaces on real houses in this climate. The finding that sealed and conditioned crawl spaces outperform vented ones on moisture is not a marketing claim — it is a body of work conducted in this state.
Seasonal opening and closing does not work
Opening vents in winter and closing them in summer sounds reasonable and is not practical. It requires someone to do it on the right days, and the humid periods on this coast do not align neatly with seasons. In practice vents get left in whatever position they were last in.
The bottom line
Close them — as part of sealing and conditioning the space, not on their own. If you are not going to condition the crawl space, leaving the vents open is the lesser mistake.
For a number against your own crawl space, use the cost estimator, or call (910) 886-2018 and we will come and measure it.