The Mississippi climate is very similar to S.C. Generally, in the coastal plains area from Virginai to Texas, unless you are going to have a completely sealed and conditioned crawl space, you had better leave those vents open. Closing the vents in a hot and humid climate will not keep water vapor from entering, even with a good vapor barrier. Better to have the air moving. How one handles this problem is dependent to a large degree upon one’s climate. If winters are rather mild but extremely wet, closing off vents may be a good idea. One solution that works very well is to install a fan with a continuous duty motor at the access.
When it comes to crawl space ventilation, you will never find full agreement among home inspectors. But the best solution for most climates is the sealed crawl space. This involves some changes, some expense, and may not be practical in all cases, especially if the home is without central heating or air conditioning. In a sealed crawl space, the vapor barrier is extended over the inside of the curtain wall from the top, and draped over the ground vapor barrier. All seams are taped. A supply duct is installed through the floor, and the return air is whatever squeezes through the cracks. If the crawl space is large, the volume of free air space may dictate a larger air conditioning/heating unit. This is where upgrading a home to a sealed crawl space can become expensive or cost-prohibitive.