Suggest installing a sump pump in a tub with holes in the lowest spot in the crawlspace piped to the outside. Cover the earth with 6 mil. plastic and close the vents as the best way to avoid mold growth on the joist and floor. If sump impossible, go Steve's way plus air tight vapor barrier on the bottom of the joist. May also insulate or use foam board with foil on the bottom as vapor retarder. The problem is to avoid condensation on the bottom of the floor deck during humid times of the year. The home A/C cools the floor below the dewpoint of the outside/crawlspace air. Condensation forms on the joist and floor causing mold and rot. More outside humid air is not the answer. Dehumidification may also be required to keep the crawlspave below 60% relative humidity.