Today's building professionals are coming to consensus that the best way to insulate a crawl space is to insulate the walls of the crawl space and put a vapor barrier on the ground to keep out moisture. This essentially brings your crawl space into the "conditioned space" of your home. Visit ENERGY STAR’s Basement and Crawlspace Air Sealing and Insulating website for more information. For detailed technical guidance, go to the Advanced Energy web site on crawl spaces.