Foundation System Moisture and Soil Gas Control for Cold Climates

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Slabs, crawlspaces, and basements are all found in the cold and very cold climates. All building foundations should be designed and constructed to prevent the entry of moisture and other soil gases.

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 that enters a home exposes 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 as described in the Drainage section.

·         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.

·         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 requires 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.

Based on local code and Termite Infestation Probability (TIP) maps, use environmentally appropriate termite treatments, bait systems, and treated building materials for assemblies that are near soil or have ground contact (check with County Cooperative Extension Programs in the area you are building).

Slabs and Basement Floors

Best Practice: Slab foundations and basement floor require 6-ml 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.

Best Practice: A sand layer under the slab or basement floor 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: Slab and basement floor drainage should include a gravel capillary break directly beneath the slab vapor retarder.

Crawlspace Foundation Systems

Best Practice: In crawlspaces, install 6-mil 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 make the following recommendations: install 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, some Building America teams recommend pouring a minimum 2” concrete slab over the polyethylene.

Best Practice: Moisture problems are best avoided by building non-vented crawlspaces. Non-vented systems are described more fully in the Structure Thermal Performance section. 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 waterpipes.

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 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.

Radon-resistant construction practices are described in the following documents for a variety of foundation systems:

·         ASTM WK2469 New Standard (Formerly E1465-92)(draft) Guide for Radon Control Options for the Design and Construction of New Low-Rise Residential Buildings

·         Model Standards and Techniques for Control of Radon in New Residential Buildings (U.S. EPA 1994)

·         Building Radon Out: A Step-by-Step Guide on How to Build Radon-Resistant Homes (U.S. EPA 2001) available on the Web at www.epa.gov/199/iaq/radon/images/buildradonout.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