Dirt floors
Dirt crawl spaces have the high humidity levels necessary for mold growth. Humidity levels from 50-90% are easily found in dirt crawl spaces that have never flooded.
Condensation
Because of high humidity level in a dirt crawl space, cool surfaces such as the earth at 55 degrees, concrete and metal surfaces will sweat with water vapor condensing out of the air. This causes rot, smells, and is a water sources for critters.
Mold Spores
Mold can grow on dirt, insulation, framing and even under your carpet on the first floor.
Outside Air
Most dirt crawl spaces are vented to get the moisture out. The problem is that vents let warm humid air in during the summer. This adds moisture to the crawlspace, and makes the house harder to air condition as all this moisture needs to be wrung out of the air. In the winter, the vents let in cold air, which causes very cold, uncomfortable floors upstairs and higher heating costs.
