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 Single-Family New 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, Insulated Crawlspaces).
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.
- 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.
- For added durability, consider one of the following:
- 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.
- Lay a vinyl runner or extra plastic over any areas that will get traffic, such as a service path to the furnace.
- 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.
The map in Figure 1 shows the climate zones for states that have adopted energy codes equivalent to the International Energy Conservation Code (IECC) 2009, 12, 15, and 18. The map in Figure 2 shows the climate zones for states that have adopted energy codes equivalent to the IECC 2021. Climate zone-specific requirements specified in the IECC are shown in the Compliance Tab of this guide.
The Compliance tab contains both program and code information. Code language is excerpted and summarized below. For exact code language, refer to the applicable code, which may require purchase from the publisher. While we continually update our database, links may have changed since posting. Please contact our webmaster if you find broken links.
National Water Management System Builder Requirements
1. Water-Managed Site and Foundation.
1.4 Capillary break at all crawlspace floors using ≥ 6 mil polyethylene sheeting, lapped 6-12 in., & installed using one of the following:4, 5, 6
1.4.1 Placed beneath a concrete slab; OR,
1.4.2 Lapped up each wall or pier and fastened with furring strips or equivalent; OR,
1.4.3 Secured in the ground at the perimeter using stakes.
1.6 Class 1 vapor retarder not installed on interior side of air permeable insulation in exterior below-grade walls.8
Footnote 4) Not required in Dry (B) climates as shown in 2009 IECC Figure 301.1 and Table 301.1.
Footnote 5) Not required for raised pier foundations with no walls. To earn the ENERGY STAR, EPA recommends, but does not require, that radon-resistant features be included in homes built in EPA Radon Zones 1, 2 & 3. For more information, see www.epa.gov/indoorairplus.
Footnote 6) For an existing slab (e.g., in a home undergoing a gut rehabilitation), in lieu of a capillary break beneath the slab, a continuous and sealed Class I or Class II Vapor Retarder (per Footnote 7) is permitted to be installed on top of the entire slab. In such cases, up to 10% of the slab surface is permitted to be exempted from this requirement (e.g., for sill plates). In addition, for existing slabs in occupiable space, the Vapor Retarder shall be, or shall be protected by, a durable floor surface. If Class I Vapor Retarders are installed, they shall not be installed on the interior side of air permeable insulation or materials prone to moisture damage.
Footnote 7) Interior surface of an existing below-grade wall (e.g., in a home undergoing a gut rehab.) listed in Item 1.5a is permitted to be finished by:
- Installing a continuous and sealed drainage plane, capillary break, Class I Vapor Retarder (per Footnote 8) and air barrier that terminates into a foundation drainage system as specified in Item 1.8; OR
- If a drain tile is not required as specified in Footnote 9, adhering a capillary break and Class I Vapor Retarder (per Footnote 8) directly to the wall with the edges taped/sealed to make it continuous.
Note that no alternative compliance option is provided for existing below-grade wood-framed walls in Item 1.5b.
Footnote 8) The 2009 IRC defines Class I vapor retarders as a material or assembly with a rating of ≤ 0.1 perm, using the desiccant method with Proc. A of ASTM E 96. The following materials are typically ≤ 0.1 perm and shall not be used on the interior side of air permeable insulation in above-grade exterior walls in warm-humid climates or below-grade exterior walls in any climate: rubber membranes, polyethylene film, glass, aluminum foil, sheet metal, and foil-faced insulating / non-insulating sheathings. These materials can be used on the interior side of walls if air permeable insulation is not present (e.g., foil-faced rigid foam board adjacent to a below-grade concrete foundation wall is permitted). Note that this list is not comprehensive and other materials with a perm rating ≤ 0.1 also shall not be used. Also, if mfr. spec.’s for a product indicate a perm rating ≥ 0.1, then it may be used, even if it is in this list. Also note that open-cell and closed-cell foam generally have ratings above this limit and may be used unless mfr. spec.’s indicate a perm rating ≤ 0.1. Several exemptions to these requirements apply:
- Class I vapor retarders, such as ceramic tile, may be used at shower and tub walls;
- Class I vapor retarders, such as mirrors, may be used if mounted with clips or other spacers that allow air to circulate behind them.
Please see the ENERGY STAR Single-Family New Homes Implementation Timeline for the program version and revision currently applicable in your state.
Exhibit 1 Mandatory Requirements.
Exhibit 1, Item 1) Certified under the ENERGY STAR Qualified Homes Program or the ENERGY STAR Multifamily New Construction Program.
Exhibit 1, Item 6) Certified under EPA Indoor airPLUS.
1.2 Capillary Break Installation.
- Install polyethylene sheeting or extruded polystyrene (XPS) insulation beneath concrete slabs, including basement floors. Ensure sheeting is in direct contact with the concrete slab above (ENERGY STAR requirement).
- Install a capillary break at all crawlspace floors using ≥ 6 mil polyethylene sheeting, lapped 6 to 12 in. (ENERGY STAR requirement).
- Under the polyethylene sheeting or extruded polystyrene (XPS), insulation installed to meet ENERGY STAR Water Management System Builder Checklist Item 1.3:
- Install a 4 in. layer of 1/2 in. diameter or greater clean aggregate; OR
- Install a 4 in. uniform layer of sand, overlain with either a layer of geotextile drainage matting throughout or strips of geotextile drainage matting along the perimeter installed according to the manufacturer’s instructions.
Exceptions to the aggregate or sand requirement (Not applicable in EPA Radon Zone 1):
- Dry climates, as defined by 2015 IECC Figure 301.1.
- Areas with free-draining soils – identified as Group 1 (Table R405.1, 2015 IRC) by a certified hydrologist, soil scientist, or engineer through a site visit.
- Slab-on-grade foundations.
Alternative path for gut-rehabs: For an existing slab in a home undergoing a gut rehabilitation in Radon Zones 2 and 3, the alternate slab treatment in the ENERGY STAR Water Management System Builder Checklist, footnote 5, shall apply as an alternative to polyethylene and aggregate or sand under the slab. Homes undergoing gut rehabilitation in Radon Zone 1 must also install an active radon system utilizing sub-slab depressurization, and radon levels shall be verified upon final inspection to be below the EPA action level (4pCi/l) to receive qualification.
Note: In EPA Radon Zone 1 (see Specification 2.1):
- Polyethylene sheeting must be installed and overlapped by 6 to 12 in. at the seams.
- ENERGY STAR staking method for poly sheeting may not be used in crawlspaces with no slab.
- ENERGY STAR exceptions for capillary break (polyethylene) under slabs do not apply. Poly is required in Radon Zone 1.
Advisory: 10 mil polyethylene is recommended if crawlspace floors are not covered with a concrete slab.
2.1 Radon-Resistant Construction. Construct homes in EPA Radon Zone 1 with radon-resistant features (a passive system at minimum). EPA recommends that radon-resistant features are installed according to ANSI/AARST CCAH for 1-2 family dwellings and townhouses (max. total foundation area of 2500 sq. ft.) OR ANSI/AARST CC-1000 for larger foundations.
Visually verify the following requirements:
- Capillary break installed according to Specification 1.2, irrespective of climate zone.
- A 3 or 4 in. diameter gas-tight vertical vent pipe, clearly labeled as a component of a radon reduction system. The vent pipe shall be connected to an open T-fitting in the aggregate layer (or connected to geotextile drainage matting according to the manufacturer’s instructions) beneath the polyethylene sheeting, extending up through the conditioned spaces and terminating a minimum of 12 in. above the roof opening. At least 10 ft. of horizontal perforated drain tile is to be attached to the T-fitting beneath the polyethylene sheeting placed over earthen crawlspaces and below concrete slabs. Note: suction points are not permitted on sump lids.
- Radon fan (i.e., an active system) OR an electrical receptacle installed in an accessible attic location near the radon vent pipe (i.e., a passive system) to facilitate future fan installation if needed. A space surrounding the radon pipe, having a vertical height of not less than 48 inches and a diameter of not less than 21 inches, shall be provided in the attic area where the radon fan can be installed, if required.
- Homes with no accessible attic location for a fan must utilize another exterior location or a garage that is not below conditioned space per ANSI/AARST CCAH. The branch circuit supply shall be labeled at the electrical panel indicating its intended use.
- Foundation air sealing with polyurethane caulk or the equivalent at all slab openings, penetrations and control or expansion joints.
Note: Larger buildings and multifamily properties may share mitigation systems across multiple units or may require multiple soil gas vent systems to accommodate large building footprints. See ANSI/AARST CC-1000 for electric metering requirements in shared (collateral) mitigation systems, as well as for maximum nominal sizes of soil gas collection plenums and corresponding pipe sizes.
Note: Consult local building codes to determine whether additional radon requirements apply. Also consult EPA's "Building Radon Out" (EPA 402-K-01-002) for general guidance on installing radon-resistant features.
- Elevated levels of radon have been found in homes built in all three zones on EPA’s Map of Radon Zones. Consult your state radon program for current information about radon in your area. Go to EPA's radon website and click on your state for contact information.
- EPA recommends, but does not require, that all homes built with radon-resistant features in EPA Radon Zone 1 include a radon vent fan. EPA also recommends radon-resistant features for homes built in EPA Radon Zones 2 and 3, and that all homes with or without radon-resistant features be tested for radon prior to occupancy. A radon vent fan should be installed when the test result is 4 pCi/L (the EPA action level) or more.
- Provide buyers with EPA’s Citizen’s Guide to Radon, encourage them to test for radon and refer them to EPA's radon website for more information.
- If soil or groundwater contamination is suspected on or near the building site (e.g., former industrial sites), volatile chemical contaminants from soil gas or vapor intrusion into a building may pose an IAQ risk. In such cases, EPA recommends radon-resistant features consistent with Specification 2.1, which can minimize or prevent the vapor intrusion into a house. See the EPA Vapor Intrusion Primer or ASTM E2600 for more information. You should also consult your state, tribal, or local environmental regulatory agency for information on the location of contaminated sites, including those subject to Superfund (CERCLA), Resource Conservation and Recovery Act (RCRA) cleanup requirements, or the Brownfields program. Visit EPA’s “Where You Live” for more information.
See Indoor airPLUS Specifications for exceptions and for an alternative path for gut rehabs.
Section R408.1 Ventilation. Ventilated crawlspaces should have at least 1 ft2 of vent opening for each 150 ft2 of floor area, unless the ground is covered with a Class 1 vapor retarder, then 1 ft2 of vent area is required for each 1,500 ft2 of floor area.
Section R408.2 Openings for under-floor ventilation. Ventilated crawlspaces should have at least 1 ft2 of vent opening for each 150 ft2 of crawlspace floor area, unless the ground is covered with a Class 1 vapor retarder, then the total area of ventilation openings should be equal to 1/1,500 of crawlspace floor area.
Section R408.3 Unvented crawl space. Ventilation openings are not required in the crawlspace if the exposed earth is covered with a continuous Class I vapor retarder with seams that are overlapped by 6 inches and sealed or taped and with the edges fastened and sealed to the foundation walls at least 6 inches above the ground level.
Section R102.7.1 Additions, alterations, or repairs. Additions, alterations, renovations, or repairs shall conform to the provisions of this code, without requiring the unaltered portions of the existing building to comply with the requirements of this code, unless otherwise stated. (See code for additional requirements and exceptions.)
Appendix J regulates the repair, renovation, alteration, and reconstruction of existing buildings and is intended to encourage their continued safe use.
Investigate the crawlspace of an existing home to determine whether the home has a vapor barrier covering the crawlspace floor and what state it is in. If no vapor barrier is present, install one as outlined in the Description tab. If a 6-mil vapor barrier is present in the crawlspace but it is unsealed at the seams, or not attached to walls or piers, it may be possible to repair it by taping any tears, overlapping and taping any seams, and attaching pieces with house wrap tape or another compatible sealing tape to extend the vapor barrier up the walls. The barrier should be fastened to the walls and piers with furring strips or other mechanical fasteners as described in the Description tab. Another, more durable, option is to cover the existing vapor barrier with a new layer of ≥ 6-mil polyethylene then seal that layer at all seams and secure it to the walls and piers as described in the Description tab.
If any standing water, mold, rotten wood, or hazardous materials are present in the crawlspace, these conditions should be addressed before replacing the vapor barrier.
See the following Building America Solution Center guides for more information:
Radon guidance from EPA, HUD, NIH, OSHA, and FEMA can be found in Homeowner's and Renter's Guide to Reducing Radon After Disasters and Radon: Worker and Employer Guide to Hazards and Recommended Controls.
The U.S. Department of Energy’s Standard Work Specifications has additional information on moisture and vapor barriers in crawlspaces and worker safety in crawl spaces.
Access to some references may require purchase from the publisher. While we continually update our database, links may have changed since posting. Please contact our webmaster if you find broken links.
The following authors and organizations contributed to the content in this Guide.
Without an effective moisture barrier, water can penetrate the pores of a concrete foundation slab, which can potentially lead to moisture-related issues. To prevent moisture from wicking up into the concrete slab and foundation walls, a water-proof layer is installed under the concrete consisting of 6-mil plastic sheeting overlapped and taped at the seams or soil-contact-rated rigid foam taped at all seams. Under this barrier is a 4-inch layer of gravel to help moisture drain away. In crawlspaces, plastic sheeting should installed over the dirt floor.