f crawlspace moisture is bulk water intrusion from improper grading of the site, lack of perimeter drainage, improper irrigation practices, etc. See these other Building America Solution Center guides for more information about good water management practices around the home site: Final Grade Slopes Away from Foundation, Patio Slabs, Porch Slabs, Walks, and Driveways Slope Away from House, Gutters and Downspouts, and Footing Drain Pipe.
In humid climates, water vapor in warm outdoor air can enter through crawlspace vents and condense on cooler framing. In humid climates, building scientists recommend building insulated, unvented crawlspaces (see Unvented Crawlspaces and Conditioned Basements).
Because radon accumulation in a home cannot be measured until after the home is built, as an added precaution, consider installing a passive radon venting system, which collects soil gasses under the plastic and vents them through the roof via a vent pipe (see Vertical Radon Ventilation Pipe).
How to Install a Polyethylene Ground Cover
- Spread a layer of 6-mil or thicker polyethylene across the entire ground surface. Edges should lap up side walls.
- Overlap all seams by 12 inches and tape them. Ensure that surfaces where tape will be applied are clean and dry.
- Attach the polyethylene to the walls at least 6 inches up the walls or to a height at least equal to the exterior ground level. Attach the polyethylene with pressure-treated wood furring strips or other mechanical fasteners. The edges can be sealed to the wall with fiberglass mesh tape and duct mastic. Or, secure the sheeting to the ground by staking at the perimeters of the crawlspace with landscape fabric stakes.
4. Lap the vapor retarder up the sides of any interior columns at least 4 inches above the crawl space floor; mechanically fasten the polyethylene to the pier and seal the edges.
5. For added durability, consider one of the following:
a. Lay one polyethylene groundcover at the beginning of construction. When construction is completed, install a second sheet over the first sheet to cover any rips in the first sheet (make sure the first sheet is dry and free of organic matter). Seal the top sheet to the walls.
b. Lay a vinyl runner or extra plastic over any areas that will get traffic, such as a service path to the furnace.
c. Cover the single sealed layer of polyethylene with a concrete slab 2 or more inches thick. (See the guide Capillary Break at Crawlspace Floor – Polyethylene Sheeting under Concrete Slab.)
Visually inspect the crawlspace to ensure that the earthen floor is completely covered with a layer of polyethylene sheeting at least 6 millimeters thick, that the sheeting extends up the sides of each wall or pier and is mechanically fastened with wood furring strips or other fasteners, and that any seams in the plastic are overlapped 6 to 12 inches and taped or sealed.
ENERGY STAR Version 3 (Rev 08) Water Management Checklist, 1. Water-Managed Site and Foundation. 1.4. Capillary break at all crawlspace floors using ≥6 mil polyethylene sheeting.
Polyethylene sheeting is not required in Dry (B) climates shown in the International Energy Conservation Code (IECC) climate map (2009 IECC, Figure 301.1and Table 301.1), except in U.S. EPA Zone 1 Radon areas (see the EPA Radon Map). Polyethylene sheeting is also not required for raised pier foundations with no walls. Click here for the 2009 IECC Interactive Climate Zone Map.
Although not required in dry climates designated as EPA Radon Zone 2 or 3, polyethylene sheeting is still recommended, as is a passive radon venting system, to help keep soil gasses out of the home and to mitigate radon should it be present at the house site.
International Energy Conservation Code (IECC) Climate Regions
ENERGY STAR Certified Homes (Version 3/3.1, Revision 08), Water Management System Builder Requirements
1. Water-Managed Site and Foundation:
1.4.2 Capillary break at all crawlspace floors using ≥ 6 mil polyethylene sheeting, lapped 6-12 in., & lapped up each wall or pier and fastened with furring strips or equivalent AND 1.4.3