Many of the crawlspaces I've inspected have elevated humidity problems. To control the humidity, you open the crawl vents in the winter to allow the warm moist air to flow naturally to the cooler dryer outside environment.

In the summer, you close the vents to keep the warm moist air outside and away from the much cooler dryer air in the crawlspace.

This is exactly opposite from the instructions our builder gave me and the way the vents in our foundation work.  He said to open the vents in summer and close them in winter.  We have two large vents that came with both solid panels and heavy mesh screens.  We also have a number of smaller vents around the foundation that open automatically when the air temperature exceeds 50 degrees and close during the cold months when the ambient temperature is less than 50.  This supports the builder's instructions to us about installing the screens in the larger vents when the weather turns warm and taken them out and installing the solid panels in the fall.  I have been doing this for 11 years without a problem.  And it makes sense.  You are not going to keep humidity out of a crawl space by closing vents in the summer, but you will create a stagnant  environment conducive to the growth of mold.