In this climate, closed — but only as part of sealing and conditioning the space properly. Simply shutting the vents on a damp crawl space and walking away can make things worse rather than better.
Why Vents Exist At All
Building codes required foundation vents for most of the twentieth century, on the reasoning that air movement dries things out. In a cool, dry climate that reasoning holds.
It does not hold here. Venting only dries a crawl space if the outside air is drier than the air underneath. For roughly four months of the year in the Tennessee Valley, the outside air is considerably wetter.
The Physics, Briefly
Warm air holds more moisture than cool air. When June air at a low-seventies dew point enters a crawl space where the ground and framing sit around sixty-five degrees, it cools. Cooled air cannot hold what it was carrying, so the moisture condenses onto the coolest surfaces available — your joists, your subfloor, your ductwork.
Open vents in August are not ventilation. They are a moisture delivery system.
Why You Cannot Just Close Them
If you seal the vents but leave the ground exposed and no dehumidification, you have trapped the crawl space with its own moisture source. Bare soil under a house can release several gallons of water a day by evaporation, and now none of it has anywhere to go.
That is the version of this advice that gets people into trouble, and it is why “just close your vents” is bad advice on its own.
The Full Answer
Vents sealed and a heavy liner over the ground and the seams and walls sealed and humidity controlled, usually with a dedicated dehumidifier. At that point the crawl space is inside the building envelope and behaves like a very short basement.
Drainage first if liquid water enters, because a sealed space with a liner over standing water is worse than an open one.
What About Code?
Modern codes explicitly permit unvented, conditioned crawl spaces built to a specification, and have for years. This is not a workaround — it is the recognized approach for humid climates. A contractor who tells you vents are mandatory is working from a code cycle several revisions out of date.