Crawl Spaces are notorious for bad odors and high humidity. Frequently, those problems start making their way up into the living areas of a home. There are some good affordable solutions that can help.
If your crawl space has dirt floor, then a smart first step would be to install a vapor barrier. A vapor barrier is a plastic sheet that is resistant to water vapor.
Read the full article at: plasticrollsheet.wordpress.com
After installing a vapor barrier, a next good step would be a crawl space ventilation unit which expels musty damp air and replaces it with fresher dry air from floors above, such as, the WAVE Crawl Space Unit.
Learn how the unit works here: http://www.wavehomesolutions.com/ventilation/