I did a lot of research on this a few years ago... There's two options when it comes to crawl spaces.
1. You want airflow. lack of air flow (like closing all of the vents that should be built into your foundation) can actually cause condensation and moisture, especially in the summer. It's OK to close them for a few months during the winter dry air time, but they need to be re-opened every spring, which is a total PIA, and something I would forget, plus I live in a mild winter climate... so I leave mine open year round.
2. Fully close off the crawl space. This means all of the vents closed and a really good vapor barrier wall to wall on the GROUND, and you have to run a good dehumidifier 24/7 that will pull all of the moisture out of the air. The PIA problem with this is emptying the system or running a drain line for it.
Also through my research, with both options, NEVER install a vapor barrier against the floor joist, this is almost guaranteed to trap humidity/moisture, and create mold.
Something else to look at. We have a termite bond on our house, these things cost a couple grand to have done now a days, so since it came with the house purchase, we pay the company like $100/yr to do a new inspection and check out the house/foundation. A couple years ago, our moisture level went from like a normal 12% (range should be like 10-16%) to like 25%. Huh. That's weird. I did a bunch of crawling around but couldn't find anything... my wife just so happened to turn the dryer on and poof. Huge plume of moisture where it had come loose at the floor seam. I fixed that, replaced all of the 15+ year old hose and when they came back the next year we were back to like 13%.