In moist warm climates like the U.S. Southeast, venting the crawlspace can actually increase the moisture problems and sometimes lead to mold and rot.
This free design guide explains the option of a sealed crawlspace and how to build it.
(http://www.jlconline.com/isroot/jlconline/ImagesOnline/images/htmlarticles/html/2005/0512/1205news1crawl.jpg)
http://www.advancedenergy.org/buildings/knowledge_library/crawl_spaces/