Some positive aspects of ventilating home crawlspaces are prevention of the formation of high moisture that results in mold, mildew, fungus or rot on wooden structures in the crawlspace and the avoidance of pest problems. However, negative aspects of ventilating crawlspaces include increased energy costs and the fact that ventilation does not always solve moisture problems. Improperly constructed crawlspaces can negatively affect the comfort and energy efficiency of homes.
Building codes have traditionally mandated ventilating crawlspaces with outside air as the primary means of controlling moisture within crawlspaces. This works well in areas that have low humidity. However, in areas that have ambient humidity of more than 80 percent, such as in the Southeastern United States, ventilation tends to exacerbate the formation of moisture in crawlspaces. Humid air enters the crawlspace and condenses inside, making the wooden structures in it such as the floor above it vulnerable to mold and rot. It also increases cooling costs in the summer and heating costs in the winter.
Some builders recommend a totally sealed crawlspace that does not vent to the outside. This involves installing vapor barriers all around the crawlspace. Such an encapsulated crawlspace essentially makes the crawlspace part of the house, making it immune to outdoor humidity and temperature fluctuations, thereby resulting in lower energy costs. The lower heating and cooling costs, and better air quality offset the additional cost of installing the vapor barrier.