Many building professionals now recognize that building an unventilated crawl space (or closing vents after the crawlspace dries out following construction) is the best option in homes using proper moisture control and exterior drainage techniques.
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 crawlspace 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 build and maintain an airtight, insulated access door in the perimeter wall.
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.
However, vented crawl spaces experience serious moisture and mold problems that cost builders and the homeowner significant resources to repair. Also, wintertime ventilation makes crawl spaces colder and increases the heat loss from the home – venting crawl spaces wastes energy, and can lead to freezing pipes and uncomfortable floors.
In short, crawlspace vents should be sealed, crawlspace 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.
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