One of the most noticeable discomforts in the home is humidity. Too little causes the skin to burn and tingle, too much causes a damp, muggy sensation. Controlling excessive humidity through proper crawlspace ventilation is essential.
Excessive crawlspace ventilation can cause many problems, not the least of which is decay of wood under the house and buckling floors. Water vapor from the ground is also a key contributor to indoor humidity.
Houses with a crawlspace are all made with ventilation grilles. These do very little to eliminate the humidity. Forced air ventilation is the most efficient solution. Many companies will now install automated systems which sense the crawlspace and outdoor humidity levels. When the outdoor humidity is less than the crawlspace they automatically start the forced air fans to force dryer air into the space and evacuate the humid crawlspace air.
Minimizing water vapor from entering the house also has a positive impact on the efficiency of your air conditioning unit. The more water vapor in the air, the harder the air conditioner has to work. See the other post on improving air conditioning efficiency here.