Building codes vary from region to region, but there are some basic requirements when considering ventilation for a crawl space. The primary goal is to eliminate or minimize moisture in a crawl space. Additionally, take steps to ensure that any moisture that does occur in a crawl space can be eliminated quickly so that it does not affect the living space above.
Particularly in areas with high water tables, such as the southern United States, homes were regularly constructed on posts or pillars. Over time, builders recognized that crawl space construction had thermal benefits over this type of construction. Crawl spaces tend to stay cooler in the summer and warmer in the winter because the ground temperature has a more direct effect. This is a benefit to energy concerns.
Crawl space construction, while thermally beneficial, does not have the same air exchange rate as post or pillar construction. For this reason, crawl spaces tend to become damp.
Building codes began to mandate crawl space ventilation around the 1950s. The theory is that ventilation will allow for moisture to evaporate in the crawl space area and then to dissipate to the outside through the ventilation treatments.
It has been found that crawl space ventilation is a double-edged sword. While outside air can help dissipate moisture, it appears most effective in cooler climates. In hot, humid regions, outside air can actually have a negative effect and create moisture in a crawl space.
While codes can vary, the general rule is that crawl spaces must have ventilation through the walls that meet 1 square foot of free space per 150 square feet of floor space. These vents are to ensure cross-ventilation and are required to be located with 3 feet of the corner of the structure. This requirement can be reduced dramatically (as much as 1 square foot of free space per 1,500 square feet of floor space) with an approved vapor barrier installed over the ground surface.
While a vapor barrier is not necessarily a code requirement, it is highly recommended. A vapor barrier retards moisture from the ground surface from entering the crawl space. Additionally, take care to reduce potential moisture from entering the crawl space in other ways. This may include grading around the crawl space to direct surface water away from the area.