In short, crawl space vents should be sealed, crawl space walls insulated, rim joists and spaces where air may leak should be sealed, and the ground should have a continuous sealed cover such as taped polyethylene.
This information is based on a report prepared for the US Department of Energy’s Building America
Program (see Building America cover page for more information). The report is freely available to the public at www.buildingamerica.gov.
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Crawl space encapsulation and sealed crawl spaces increase energy efficiency and air quality, while reducing moisture and critter issues. Many building professionals now recognize that building an unventilated crawl space (or closing vents after the crawl space dries out following construction) is the best option in homes using proper moisture control and exterior drainage techniques.
Sealed and insulated crawl spaces perform better in terms of safety and health (pest control), comfort (warm floors, uniform temperatures), durability (moisture) and energy consumption than passively vented crawl spaces. Crawl spaces should be dry so that they are less likely to have pests, termites and mold. A dry crawl space is therefore safer and healthier than a wet crawl space. Ground cover prevents evaporation of ground moisture into the crawl space.
If you have or will have an unventilated crawl space, the best approach is to seal and insulate the foundation walls rather than the floor between the crawl space and the house. This strategy has the advantage of keeping piping and ductwork within the conditioned volume of the house so these building components don't require insulation for energy efficiency or protection against freezing. It’s best to locate the access door to the crawlspace inside the home through the subfloor unless you build and maintain an airtight, insulated access door in the perimeter wall.