Vents are counterproductive
There are two types of crawl spaces: vented and sealed. Vented crawl spaces are associated with moisture problems and mold. Although older building codes required crawl spaces to be vented, newer versions of the code allow sealed crawl spaces. In most cases, a home with a sealed crawl space will use less energy than a home with a vented crawl space.
It may seem like a good idea to allow fresh air to circulate through the crawl space, at least during the summer. Locating vents on opposite sides of the crawl space should let air waft through, removing moisture and keeping things dry, right? Not really. What actually happens is that water vapor in humid summer air comes into contact with cool surfaces under the house and condenses into water. The dampness can foster the growth of mold, leading to decay (and the need for repairs). This is an especially likely scenario in the southeastern United States, where crawl spaces are common and summer air quite humid.
When crawl space walls are sealed and insulated and the air in the crawl space is heated and cooled, the potential for condensation problems is greatly reduced. Perimeter rigid foam insulation can be applied to either the inside or outside of the walls.
All crawl spaces require a layer of 6-mil (or heavier) polyethylene plastic spread over the floor of the crawl space to help keep moisture and soil gases from getting in. The plastic should be continuous, taped at any seams, and mechanically attached and sealed at the perimeter. For a superior crawlspace, consider covering the polyethylene with a thin slab.
For more information on best-practice details for sealed crawl spaces, see Building an Unvented Crawl Space.
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