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Crawl Spaces and Ventilation


Many houses are built over a crawl space or an open space that is deep enough to allow a person to gain access to the underfloor area by crawling. The minimum depth of the crawl space should be 24 inches under the floor joists or 18 inches under the girder. An 18 inch clearance beneath any ductwork is also desirable. Moisture problems within a house that show up as condensation and/or frost on windows in the living area can result from dampness in the crawl space, or from inadequately vented living space. Moisture problems in crawl spaces may be due to the construction of a crawl space in an area of high water table, improper grading of the lot for drainage or the omission of moisture control devices such as vapor barriers, ground cover and ventilation openings. Obvious symptoms of excessive moisture passing upward through the floors may include any of the following: a musty odor - mold on the walls near the floor - in corners and in closets - moisture condensation on insulated windows or storm windows and moisture condensation in the walls with resulting paint peeling. Some of the more common ways moisture gets into crawl spaces are:

 -Ground water moving through the foundation walls or up through the earth floor

-Surface water moving through the foundation walls or flowing in through ventilators

-Capillary rise of ground moisture

-Green slabs, or concrete foundations not fully cured when the rest of the house was built above it

Moisture defense

A ground cover of 6-mil polyethylene sheet plastic must be placed over the entire area of the crawl space soil, lapped at least 6 inches and held in place by bricks or other weights. (Black) polyethylene works better than clear, because the black plastic slows the growth of plants, which can occur even in a relatively dark crawl space. In cases where concrete is poured to create a floor in the crawl space, the 6 or 8 mil polyethylene cover is still required.

Proper crawl space ventilation

Building codes in most areas requires cross-ventilation in a crawl space equal to at least 1.5 percent of the square foot area within the crawl space. Venting allows damp areas to dry-out, reducing the likelihood of mold growth, and to minimize moisture pressure or build-up under homes. Provide year-round air circulation with multiple vents, a minimum of 1 square foot for each 150 square feet of under-floor space area. One ventilating opening shall be within 3 feet of each corner. Ventilation fans can be used in the crawl space area to circulate the air, promote drying and reduce dead air spaces. Ventilation openings to the outdoors are not required if, ventilation/conditioned openings to the homes interior are provided. Per (IRC) International Residential Code, Section R408.1 and R408.2