Crawlspace Ventilation Facts

• In a 1000 Sq. Ft. crawlspace, as much as 12 gallons of moisture vapor can be released from the soil in a single day.
• Sources of moisture include evaporation from the soil, diffusion through the basement walls, and from humid air entering the crawlspace.
• In a crawlspace, the soil temperature, living space temperature, and ductwork temperature all contribute to lowering the temperature in the crawlspace. Outside air entering will then be cooled, and condensation will occur.
• Some homes have experienced warped or bucked flooring as a result of moisture vapor from a crawl space.
• Venting of crawlspaces will only work if the outside air is dryer than the crawlspace air, or when enough warm outside air enters and warms the crawlspace.
• Outside air in the summer may actually make the crawlspace wetter, not dryer.
• Venting a crawlspace can both increase the heating and cooling loads.
• An alternative to venting is to completely cover the soil in the crawlspace with a vapor
• retarder, such as a 6 mil poly or equivalent cover. Capillary breaks can be installed, or by covering walls and piers with a vapor retarder. Wet crawlspaces need a mechanical dehumidifier system.
• Exterior soil should be sloped away from the house, at least 6” for the first 10’.
• Where there is a probability of standing water, a french drain, sump pump and sloped soil can be used.
• According to several engineering studies, crawlspaces perform better with a sealed system, rather than standard vented crawlspaces. Air conditioners run more efficiently, since they have to remove less moisture from the air. Duct insulation stays dryer. Humidity levels within the crawlspace and house are more uniform from season to season. This results in less movement of hardwood flooring, trim, and cabinetry.

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