The crawl space is one of the most neglected areas of a home, even though it is literally the foundation of the home. Why is that? Well, even when a company builds a home the crawl space is often left for the new home owner to deal with and like I learned early in the Army: "out of sight, out of mind." It's one of those places like a water heater closet or the attic, we don't go in there unless we have to. But when left for long periods of time without proper care, damage can happen quickly when conditions such as mold growth or standing water occur, leaving you with a massive repair bill. So what is the best way to care for your crawl space? Ideally, you want to treat the crawl space like a basement and seal it up.
First you need to address any mold, mildew, or standing water you may have under the house. Then at an absolute minimum, a 6mil plastic vapor barrier should be installed over the dirt floor of the crawl space. Read here for more on vapor barriers:
So a vapor barrier is your minimum, BUT if you're truly interested in protecting your home from future moisture problems, you want to seal off the area from the dirt within the space and the humidity from the outside- a crawl space encapsulation. Meaning, a thick 10-20mil plastic needs to be installed not only on the crawl space floor but also up the walls at least 12 inches. I recommend installing the barrier up the foundation walls and stopping just 6 inches from the floor joists. This seals the whole area and if water seeps through the foundation walls it will stay on the back side of the barrier. It also leaves a "termite gap", which gives pest control a clear view of all exposed wood within the crawl space and is even required by building codes in many states. Finally all the seams should be sealed with a good, strong seam tape.
Keeping the outdoors where it belongs -outside-, you also need to close those vents up. You might make an exception in leaving one vent open to install a small exhaust fan, especially if you are in an area in which radon is prevalent. The best way to seal the vents is by cutting a piece of foam board insulation to the size of the vent and spraying that expanding foam all around it to fully seal it up. While you are at it, you can look for signs of light leaking into the crawlspace, like around the HVAC system, and spray foaming those areas too. Now don't forget the crawl space entry door! You may want to have someone close it while you are inside so you can see if there are any gaps. You want a good seal in this area, just like in your home.
Ultimately, after you get everything sealed properly, you want to check your moisture levels. Installing a dehumidifier is highly recommended to ensure that the moisture levels are and will continue to be where they need to be. When choosing a dehumidifier, be sure to determine the correct product for the size of the crawl space.
This can be a very daunting task with many factors and things that can go wrong, so if you are not fully confident in tackling this project yourself, go ahead and consult with a local professional.