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April Showers: Water Control Tips: Tip 4

Tip 4: Method for Controlling Excessive Moisture Vapor

All foundation materials are porous and will allow some moisture vapor to pass through from the exterior. In addition, the cooler temperatures in crawlspaces and basements create a condition which tends to draw in the moisture through condensation. 

In homes where excessive moisture vapor is present, some control method should be used to prevent problems with wood destroying insects and fungus. In  basements, when moisture penetration is slow, the most common method for  controlling moisture vapor is with a dehumidifier. In crawlspaces, moisture vapor penetration usually comes through the soil floor and is usually controlled with a vapor barrier and crawlspace vents.

Vapor Barrier
Vapor barriers are intended to prevent the moisture vapor entering crawlspaces area. The vapor barrier is usually comprised of sheets of 6 millimeter polyethylene (plastic). The sheets are laid in rows over the soil. The edges of the sheets are overlapped and tapped to form a continuous barrier that will trap the rising moisture. In most locations, the vapor barrier is placed only over the soil and secured with weights. In some regions, the vapor barrier is continuous over the foundation wall and secured to the sill plate.

Crawlspace Vents
Crawlspace vents prevent moisture vapor from accumulating in the crawlspace by providing opening for the moisture to pass to the exterior. Vents are required on new homes. The general requirement is one square foot of vent opening for 150 square feet of under-floor area.