Although homeowners rarely think about that small space under their home, builders should give crawlspaces careful consideration. Some of the most common—and damaging—building problems stem from moisture in crawlspaces. Poorly constructed crawlspaces can contribute to mold problems, indoor air problems, and inefficient operation of the home’s mechanical equipment. Carefully considering the design and construction of crawlspaces, as part of an overall quality construction strategy, can reduce problems for both builders and homeowners.
 
Preventing moist crawlspaces
Moist crawlspaces can contribute to rot, mold, and fungus growth, which can damage wood floor joists and beams. Furthermore, moisture in crawlspaces can lead to health problems. A carefully engineered moisture management strategy should be implemented to control moisture. These key preventative steps are part of such a strategy:

• Place a continuous, sealed 10 mil polyethylene vapor barrier on a gravel layer in the crawlspace and protect it with an unfinished slab. For more information about placing a vapor barrier in a crawlspace, download this free step-by-step document.
• Minimize moisture during the construction process.
• Slope the external grade so water flows away from the crawlspace.
• Install rain gutters, extensions, and leaders to carry excess water away from the home.
• Provide a capillary break on top of the footer.
• Install a footing drainage system to prevent rising groundwater from flooding the crawlspace.
• Bring conditioned air into the crawlspace.
• Insulate the exterior surface of the crawlspace wall.
• Seal the crawlspace against air leakage.

Benefits of unvented crawlspaces
Crawlspace vents effectively become the “determined hole” in a home. In winter, warm air exiting the attic or ceiling creates a negative pressure at the lowest openings in the home. This negative pressure causes cold air to enter through the crawlspace vents and rise through holes in the subfloor to the first floor. This phenomenon creates the “cold floor syndrome” prevalent in cold climate homes with vented crawlspaces. The cold air can also carry unwanted moisture and gases from the soil and odors from the crawlspace into the home.

Typically, the floor above a vented crawlspace is insulated. In unvented crawlspaces, the walls, instead of the floor, are insulated to limit heat transfer from outside to inside the crawlspace, and vice versa. When properly built, an unvented, insulated crawlspace helps to eliminate problems with comfort, odor, and health.

However, unvented crawlspaces aren’t recommended in certain areas. For example, a warm crawlspace in an extremely cold climate may eventually melt the permafrost beneath the home, causing the home to sink into the ground. Also, when homes are built in high water table areas (for instance, near a river at the bottom of a steep valley), it can be useful to seasonally vent the crawlspace to allow for drying after flooding.

Conditioning the crawlspace
Once the crawlspace is insulated and unvented, it should be treated the same way as any other space in the home—supplied with a certain amount of heated or cooled air from the mechanical system. Conditioning the crawlspace can help the mechanical components that are housed in the crawlspace (for example, the air handler, ductwork for the first floor zone, and plumbing line) operate more efficiently and last longer because they aren’t subject to temperature and humidity extremes. Ideally, the crawlspace temperature should be about 55°F to 65°F year-round. All ducts should be well sealed with mastic to prevent unwanted air leakage. This air leakage increases energy consumption because the outside air has to be heated in the winter and cooled in the summer.

More information about crawlspaces can be found in these online courses from BuildIQ University:

• Foundations: Basements & Crawlspaces
• Water Management: Basements & Crawlspaces