Without a capillary break, moisture can wick-up through the pores of a concrete slab and enter the crawlspace, which can lead to mold and the moisture-related failure of building materials. Also, the air exchanged between the moisture-laden crawlspace and the house can carry mold and contaminants into the home; this exchange is intensified when leaky forced-air heating and air-conditioning ducts are located in the crawlspace (EPA 2013).
Figure 1 - A Damp Crawlspace. This crawlspace lacks the polyethylene sheeting at the floor, and as result, water vapor migrates up from the earth into the crawlspace, into the insulation (which is poorly installed and wet in this photo), and then into the house.
An essential part of a comprehensive approach to creating a water-managed home and foundation is to provide a capillary break (i.e., a water impermeable material) at the crawlspace floor installed in one of three ways:
Figure 2 below illustrates this capillary break at the crawlspace floor as part of a comprehensive water-manage strategy.
Figure 2 - Comprehensive Water Management Features include a Capillary Break (≥ 6-mil Polyethylene Sheeting) at all Crawlspace Floors.
Installing Polyethylene Sheeting (a Capillary Break) beneath a Concrete Slab
One best practice for moisture control in a crawlspace includes polyethylene sheeting installed beneath a minimum 2-inch-thick slab. This thin slab is often referred to as a "rat slab," in reference to its resistance to penetration from burrowing rodents.
Figure 3 - Concrete Slab over Polyethylene Sheeting as a Capillary Break. Best practice for controlling moisture in a crawlspace includes pouring a thin slab over a continuous 6-mil polyethylene groundcover. The concrete provides an added measure of durability, ensuring the polyethylene provides lasting protection from ground moisture.
Since a crawlspace is not living space, it does not need a full-depth 4-inch slab, as is typical for basements that may be subject to structural live loading. For basement slabs and slab-on-grade foundations, a more elaborated treatment of the sub-slab surface area is required, as provided in the following Solution Center Guides:
Installation of the polyethylene sheeting should be done by the foundation crew prior to pouring the concrete slab, following these procedures (EPA 2013):
Install a minimum of 6-mil polyethylene across the entire ground surface.
Overlap the edges between pieces of polyethylene by 6 to 12 inches, and tape all the seams.
Seal the polyethylene at least 6 inches up the walls or to a height equal to ground level. Best practice water management for new home construction should include backfilling the stem-wall foundation prior to framing the walls to ensure that the crawlspace floor is above the exterior grade. This will eliminate the possibility of run-off draining directly into the crawlspace (though it won't eliminate the possibility of soils becoming saturated and wicking moisture into the crawlspace area; hence, polyethylene sheeting is still required).
Pour the slab over the polyethylene, taking care not to damage the polyethylene. Do not allow the foundation crew to place a sand layer between the polyethylene and the concrete slab, as sometimes occurs. The belief is that this added layer of sand will allow the slab to dry to the bottom and help prevent the slab from curling. However, this practice is not recommended. The sand gets saturated from the concrete and wet-method curing, which only adds to the long-term moisture loading as the sand slowly dries to the crawlspace. Differential drying is better handled with a low water-to-cement ratio in the concrete mix, and covering the slab with wetted burlap or employing another effective curing method.
Figure 4 - Lapped and Taped Polyethylene Sheeting. When installing the polyethylene, lap the pieces 6 to 12 inches and tape the seams to create a continuous moisture-control layer.
Note in the illustration above that the edges of the polyethylene around the perimeter may be secured to the foundation walls with pressure-treated wood strapping, instead of covering the polyethylene with concrete. This is acceptable. The important thing is to provide a lasting moisture barrier to help keep the crawlspace dry. Pouring concrete over the polyethylene, however, provides an added measure of durability that will ensure the groundcover lasts as long as the rest of the building.
A Note about Taping Polyethylene Sheeting at Each Interior Pier Footing
In addition to securing the edges of the polyethylene to the foundation perimeter, care must be taken to provide continuous moisture control at pier footings as follows (Lstiburek 2004):
Install a capillary break over interior footings before masonry piers or steel columns are installed.
Tape the polyethylene ground cover to the capillary break at each interior footing.
Figure 5 - Taping Polyethylene Sheeting at Interior Pier Footings. It is important to provide continuous moisture control at pier footings.
Check the EPA zone map to verify the Radon Zone of your building project. In areas where the risk of radon is high, a passive radon-control system should be installed to safely ventilate soil gases to the exterior of the home. This is highly recommended in Radon Zone 1, where the risk is highest. Even in Radon Zones 2 and 3, however, a radon control system will reduce the chance of soil gas concentrations from building up inside the home. This is especially important in tightly built homes where the likelihood for soil gas build-up may be increased.
An effective radon control system should include a 4-inch perforated drain pipe installed along the interior perimeter of the foundation footing. This perimeter piping tees into perforated pipe running across a gravel-filled trench in the interior area of the crawlspace, and it connects to a vent stack running out through the roof. The vent outlet should be located safely above the roofline, and not be located near any windows, skylights, or ventilation intakes. When installing this system, use the following guidance (Lstiburek 2004):
Bury the perforated pipe in 4 to 6 inches of course gravel (no fines). The larger the spaces between the gravel, the easier it will be for the vent stack to depressurize the area beneath the polyethylene cover.
Lay polyethylene sheeting over the drain pipe and gravel bed, and seal (using a compatible caulk) all edges and around the vent stack.
In addition to caulk at the edges along the foundation, mechanically fasten the polyethylene to the foundation wall, or pour a minimum two-inch slab over the top to improve the durability of the system.
Monitor the radon concentration levels after the home is enclosed. If passive flow through the vent stack is insufficient to adequately reduce radon levels, a fan rated for continuous duty can be installed on the vent stack in the attic. This will provide active depressurization of the gravel bed beneath the polyethylene, and increase the ventilation rate of the soil gas.
Figure 6 - A Radon Control System as Part of a Water-Managed and Air-Sealed Crawlspace for EPA Radon Zones 1, 2, and 3.