Your crawl space is likely not insulated because it is what we would call a "dirt crawl space" - meaning it is positioned directly above the earth with no form of protection. These types of crawl spaces can allow moisture, critters, and other unpleasant things into your crawl space. It is best to get your crawl space insulated and fitted with a vapor barrier to uncouple the effects of the ground from your crawl space. An insulated crawl space can also lead to warmer floors, lower heating costs, and healthier home air.