Capillary Break at Crawlspace Floors - Polyethylene Lapped Up Walls and Piers or Secured in the Ground
All crawlspaces should have a ground covering of polyethylene sheeting that serves as a capillary break and vapor barrier to keep ground moisture and soil gases from entering the crawlspace where they can be pulled into the home through cracks in or around the subfloor.
- Use ≥6-mil polyethylene sheeting.
- Lap any seams in the sheeting by 6 to 12 inches and seal the seams with a continuous bead of acoustical sealant, butyl rubber, or butyl acrylic caulk, or with tape manufactured to seal or patch polyethylene.
- Seal the sheeting around any pipes coming up from the ground.
- Lap sheeting up walls and piers at least 6 inches and attach to the walls or piers with mechanical fasteners. Or, secure sheeting to the ground by staking at the perimeters of the crawlspace.
See the Compliance Tab for related codes and standards requirements, and criteria to meet national programs such as DOE’s Zero Energy Ready Home program, ENERGY STAR Certified Homes, and Indoor airPLUS.
Moisture in crawlspaces can cause harm to the home by promoting mold and rot along floor joists and rim joists. Moisture can enter a crawlspace if it migrates into the soil beneath the crawlspace then enters the crawlspace as liquid water or water vapor from the soil, which can then condense on floor joists. To prevent water or water vapor from entering the crawlspace, the crawlspace floor should be covered with a heavy polyethylene plastic capillary break that is sealed to the walls and piers to provide a continuous vapor barrier. This plastic ground cover should be installed regardless of whether the crawlspace is vented or unvented. This plastic covering will also help keep soil gases from entering the home. This task should be included in the contract for the appropriate trade depending on the workflow at the specific job site.
Moisture can enter a crawlspace from sources other than vapor from the ground. In vented crawlspaces, the dominant source of 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 s