Crawlspaces are common in homes. They are usually found connected to the main basement, below a room that was added to the main house, some years after the home was built.
They can be scary places that are difficult to access, have dirt floors, falling fiberglass insulation, debris, mice and spiders- YIKES!
The floor joists are usually insulated with fiberglass. Which still allows the cold outside air to enter the crawlspace from gaps in the sill-band. The same gaps that allow cold air to enter also allows mice and insects to enter as well. The exposed dirt floor allows water vapor and other natural vapors like Radon into your basement and into your home. The exposed earth gives off a musty smell and is potentially harmful.