Vented dirt crawlspaces attract mold, mildew, dust mites, and pests of all kinds. This is due to the Stack Effect. The Stack Effect occurs because hot air leaves the home through your attic and upper levels, as it does, a new air is pulled upwards from the crawlspace. If the crawlspace is vented, then that air is being drawn in from outside. That outside air carries moisture that condenses when it reaches the cooler crawlspace and then promotes mold and mildew growth. These mold spores are then drawn up into the rest of the home when warm air leaves the attic.
The best way to prevent this vicious cycle is to seal off the crawlspace from the outside air, allowing conditioned air from inside the home to cycle into a sealed crawlspace. This encapsulation increases your energy efficiency, prolongs the life of your air conditioning system, and saves your home from expensive repairs due to pest or mold damage.