Foundation System Moisture and Soil Gas Control for Hot-Dry and Mixed-Dry Climates

Document:

Slabs are a common foundation system in the hot-dry and mixed-dry climates. Building foundations should be designed and constructed to prevent the entry of moisture and other soil gases. Moisture may cause structural decay and can contribute to human health and comfort problems. Radon that enters a home exposes occupants and may cause lung cancer. 

Best Practice: Slabs require 6-ml polyethylene sheeting directly beneath the concrete that accomplishes vapor control and capillary control for the slab. The vapor retarder should continuously wrap the slab as well as the grade beam. Other solutions may work for the grade beam, such as applying damp proofing. A moisture retarder is needed between the stem wall and framing.

Best Practice: A sand layer under the slab should never be placed between a vapor retarder and a  concrete slab. Cast the concrete directly on top of the vapor barrier. Differential drying and cracking is better handled with a low water-to-concrete ratio and wetted burlap covering during initial curing.

Best Practice: Sub-slab drainage should consist of a gravel capillary break directly beneath the slab vapor retarder. In addition to other benefits, the gravel and vapor barrier are important first steps to radon control. The gravel provides a path for radon and other soil gas to escape to the atmosphere rather than being drawn into the house. And the vapor retarder helps to block soil gas entry into the house. Where gravel is scarce, builders often pour slabs onto sand. When sand or other native fill is used, a 3- or 4-inch perforated and corrugated pipe loop can be use for both drainage and radon control. Or drainage matting may be installed over sand. Both approaches are described in a U.S. Environmental Protection Agency (EPA) document described in the following list, Building Radon Out (2001).

Best Practice: Other than identifying areas that have had radon problems, it is not possible to predict radon levels in houses prior to construction, so it is important to include inexpensive radon control measures. One measure recommended by the EPA to control potentially high radon levels and other soil gasses, is a passive soil gas stack connected to a perforated drain pipe embedded in the gravel under the slab. The stack may also be attached to a perforated pipe loop or mat. If it turns out the house has unacceptable radon levels, a fan can be added to the stack to actively draw soil gas away from the house. To determine potential radon levels in the county in which you are building, visit the EPA’s radon potential map at www.epa.gov/radon/zonemap.html.

For information about local variation in radon levels you can find local contacts at the following EPA Web site: www.epa.gov/iaq/whereyoulive.html. The EPA divides counties into one of three zones based on radon level potential. The EPA recommends that all homes built in Zone 1 (high radon potential) areas have radon reduction systems.

Foundation Energy Performance

Slabs are a common foundation system in the Southwest.

Best Practice: In the hot-dry portion of these climate zones, slabs are generally not insulated, even at the perimeter, because of the low overall heating load. Slabs in the mixed-dry climate should be insulated at the perimeter with one inch of borate-treated foam board insulation or rigid glass fiber insulation.

Crawlspace Foundation Systems

Although not prevalent, some houses in the hot-dry and mixed-dry climates are built with crawlspaces.

Best Practice: Crawlspaces should be built as a conditioned space. Insulation should be applied to exterior walls and vents to the exterior should not be installed. The crawlspace should receive conditioned air from at least one supply duct, and transfer grills should allow for the transfer of air back to the living space. Other features include a ground cover that is continuous and sealed to the perimeter walls and piers, air sealing of the exterior walls, and sealed air distribution ducts. If a heating system is installed in the crawlspace, use only a sealed combustion appliance. For more information see:

·         The addendum to the Site Supervisors chapter, look for the section entitled: Details for Mechanically Vented Crawlspaces.

·         Yost, Nathan. May 2003. “The Case for Conditioned, Unvented Crawl Spaces.” Building Safety Journal. Available on the BSC Web site at: www.buildingscience.com/resources/articles/24-27_Yost_for_author.pdf

For more information, please visit Building America or download the full reports below:

Cold and Very Cold Climates

Hot-Dry and Mixed-Dry Climates

Hot and Humid Climates

Marine Climates

Mixed-Humid Climates