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Foundations

The building envelope consists of the home's foundation, walls, windows, doors, and roof. It is the boundary between the outside climate and the home's internal environment. This envelope must be designed to control water, moisture, and air leakage, and to assist in maintaining the home's inside climate under different weather and temperature conditions.

Each component of the building envelope is designed to function as an integrated whole. Best practice guidelines for one key component of the building envelope—the foundation—are presented below.

Foundations

Most foundation water leakage or intrusion is due to either bulk moisture leaks or capillary action. Bulk moisture is the flow of water through holes and cracks. Capillary action occurs when water wicks into the cracks and open spaces of porous building materials, such as masonry block, concrete, or wood. These tiny cracks and pores can absorb water in any direction, even going upward. Moisture can also be carried by soil gas into homes.

Moisture may cause structural decay and can contribute to human health and comfort problems. Radon gas that enters a home is breathed by the occupants and may cause lung cancer.

The following practices apply to all foundation systems.

• Keep all untreated wood materials away from earth and concrete contact.

• Design the house structure with overhangs, gutters, drainage planes, and flashing to shed rainwater and conduct it away from the house.

• Slope the earth away from the house and ensure that no irrigation strikes near the foundation.

• Use a sill gasket for air sealing.

• Install a protective shield such as metal flashing, plastic L bracket, or a membrane to block capillary water wicking into the wall from the foundation. This material can serve as a termite shield as well.

• Damp proof all below grade portions of the exterior foundation wall to hinder the absorption of ground water.

• Place a continuous drainage plane over the damp proofing or exterior insulation on foundation walls to channel water to the foundation drain and relieve hydrostatic pressure. Drainage plane materials include special mats, high-density fiberglass insulation products, and washed gravel. All drainage planes should be protected with a filter fabric to prevent dirt from clogging the drainage channels.

• Basements require a foundation drain installed directly below the drainage plane and beside (not on top of) the footing. Foundation drains are needed for crawlspaces and slabs where the slab or the floor of the crawlspace is located below grade.

Slabs and Basement Floors

• Slab foundations and basement floors require six-millimeter polyethylene sheeting or rigid foam insulation acceptable for below grade use 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.

• A sand layer under the slab or basement floor should never be placed between a vapor retarder and a concrete slab. The concrete should be cast 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.

• Slab and basement floor drainage should include a gravel capillary break directly beneath the slab vapor retarder.

Crawlspace Foundation Systems

• In crawlspaces, install six millimeter polyethylene across the entire ground surface. Overlay and tape all seams by 12 inches. Seal the polyethylene at least 6 inches up the walls or to a height equal to ground level. Some Building America teams recommend installing a polyethylene groundcover at the beginning of construction, then install another one on top of the first one when the crawlspace is ready to be sealed up to cover all rips and holes. To improve durability, consider pouring a minimum 2-inch concrete slab over the polyethylene.

• Moisture problems are best avoided by building non-vented crawlspaces. One source of moisture problems in crawlspaces comes from the combination of moist air and cold temperatures. Air in crawlspaces may be moist due to the proximity of soil and air leaks from the house. In humid summers, moisture is carried into the crawlspace in the air drawn through traditional wall vents. When this warm moist air reaches cooler structural framing, the moisture condenses out and can cause mold and structural problems. In winter, cold air is drawn into the crawlspace and does little to dry out crawlspaces, but can lower temperatures, cause condensation and freeze exposed water pipes. A non-vented crawlspace is a more hospitable environment for the air distribution system to operate in.

Radon Control

In addition to other benefits, the gravel and vapor barriers under slabs, basement floors, and crawlspaces 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 used 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.

Other than by identifying areas that have had radon problems, there is no way 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 gases 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 the house has unacceptable radon levels, a fan can be added to the stack to actively draw soil gas away from the house. The EPA has a radon potential map, which provides potential radon levels by county.

The EPA provides local contacts, who can supply information about local variation in radon levels. 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.

Radon Control System