Crawl spaces are notorious for flooding, and being habitable places for mold and mildew. This means it is essential to waterproof your crawl space. Carefully study the terrain where you plan on building your home. Crawl space flooding will be a problem in areas where the water table is just below the crawl space. If the soil is always damp, or your property is near a body of water such as a lake, pond or river, you must provide exterior drainage and waterproofing around the exterior of your foundation. This includes a waterproofing agent applied to concrete, clean gravel the same depth as the foundation footing and two feet out from it, and finally a tile drainage system around the footing. Cover perforated plastic tile with more gravel, and pour in topsoil to about 5 inches deep. Inside your crawl space, you should install a sump pump that drains water to a water source. Also, install adequate gutters and drain pipes to drain away rainwater at a distance of at least eight feet from the house. Use a dehumidifier to keep the moisture content to a minimum and reduce air ventilation of the crawl space as much as possible. If moist, humid air enters the crawl space, it encourages mold growth. Measure a sufficient polyethylene vapor barrier to cover interior walls, and attach it to concrete with a duct sealer, this reduces moisture seepage through the concrete. Following these steps will help you ensure your crawl space stays dry.