Traditionally crawlspaces have been vented to the outside through code required air vents.

Now research is showing that sealed crawlspaces with a poly moisture barrier over the soil often have lower humidity and fewer moisture problems than vented crawlspaces. This is especially true in high humidity climates.

Here is a good resource on the subject. Check for your climate suggestions and perhaps ask your building inspector what the requirements are for your area. (Thanks, Ryan!) —http://www.advancedenergy.org/buildings/knowledge_library/crawl_spaces/