A crawl space is an unfinished, accessible area below the first floor of a building. Remember: The main purpose of insulation is to enclose the living space in a thermal envelope. Therefore, if the space below a floor is unconditioned, either the floor or the walls below it must be insulated.
There are two types of crawl spaces: vented and unvented. Vented crawl spaces open to the outside, while unvented crawl spaces are part of the basement. How and what is insulated depends on whether the space is vented or unvented.