Foundation System Moisture and Soil Gas Control for Marine Climates

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labs, crawlspaces, and basements are all found in the marine climate. 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 contact with earth and concrete.

·         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.

·         Exterior foundation wall insulation requires a protective coating at above-grade applications. Examples of protective coverings for exterior, above-grade insulation include: flashing, fiber-cement board, parging (stucco type material), treated plywood, or membrane material (EPDM* flexible roofing).

·         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. In combination, these systems should provide a continuous high quality water resistant layer capable of resisting both capillary and hydrostatic water penetration.

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

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.

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. Pressure treated wood strapping could be used to fasten the polyethylene to the wall.

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-inch concrete slab over the polyethylene.

In areas with 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 can condense out and cause mold and structural problems. In areas with freezing temperatures, cold air may be drawn into the crawlspace and does little to dry out crawlspaces, but can lower temperatures, cause condensation and freeze exposed waterpipes. Although warm-humid or frigid areas are limited in the marine climate, conditioned crawspaces should be considered for these areas. Conditioned crawlspaces limit condensation by controlling temperatures and moisture entry. A non-vented crawlspace is a more hospitable environment for the air distribution system to operate in. Conditioned crawspaces are described more fully in the Structure Thermal Performance section. Building America is currently conducting research on conditioned crawlspaces in the marine climate. Building America teams recommend using conditioned crawlspaces.

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.

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, basement floor, or crawlspace ground cover. 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.

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