Capillary Break at Crawlspace Floor - Polyethylene Sheeting under Concrete Slab
When installing a concrete slab on a crawlspace floor, install a capillary break of polyethylene sheeting over the ground before pouring the slab.
- Use ≥6-mil polyethylene sheeting.
- Lap any seams in the sheeting by 6-12 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 posts or pipes coming up from the ground and at walls to provide a continuous vapor barrier.
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.
When constructing a home with a crawlspace, some builders will pour a concrete slab on the crawlspace floor, which can facilitate use of the crawlspace for storage or ease in accessing HVAC equipment that may be installed in the crawlspace. This thin slab, typically 2 to 4 inches thick, is sometimes referred to as a "rat slab" in reference to its resistance to penetration from burrowing rodents. It is important to keep in mind that a concrete slab alone will not prevent moisture from coming up into the crawlspace from the surrounding ground. Porous materials like concrete, wood, or masonry block can wick moisture upward through tiny cracks and pores via capillary action for surprisingly long distances. Capillary action in concrete is theoretically capable of pulling water upwards up to 6 miles (Lsibturek, 2014).
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 (see Figure 1) (EPA 2015). (Note, best practice in most climates is to seal and insulate the crawlspace if installing HVAC equipment there. See the guide Unvented Crawlspaces and Conditioned Basements for more information.)
To stop the migration of water from the ground into the crawlspace via capillary action, the builder should install a capillary break consisting of a water-impermeable material such as polyethylene sheeting under the slab. This can be installed over a material like gravel that also breaks capillary action due to the large spaces between the rocks, as discussed in the guide Capillary Break beneath Slab - Polyethylene Sheeting or Rigid Insulation.
The polyethylene sheeting capillary break is shown in Figures 2 and 3, along with other good water management practices such as sloping the surface grade and impervious surfaces like driveways and patios away from the house, installing gutters and downspouts that drain away from the house, installing a capillary break between the footing wall and the sill plate, and installing drainage pipe around the footer.
The polyethylene should be sealed at all seams and around any piping to provide a complete vapor barrier as well. When a concrete slab is poured over this, it holds the sheeting in place and provides an added measure of durability, ensuring the polyethylene provides lasting protection from ground moisture. If no concrete slab is poured, the polyethylene can be secured along the perimeter of the crawlspace by fastening it to the walls or staking it. (See the guide Capillary Break at Crawlspace Floors – Polyethylene Lapped Up Walls and Piers or Secured in the Ground.)
Figure 2 - Comprehensive water management features for a crawlspace include installing a capillary break over the crawlspace floor, sloping the surface grade away from the house, installing a capillary break beneath the sill plate, installing gutters and downspouts that drain away from the house, and installing footing drain pipe (not shown).
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, so crawlspace slabs are typically 2 to 4 inches thick. Installation of the polyethylene sheeting should be done by the foundation crew prior to pouring the concrete slab. Following are procedures recommended by the U.S. Environmental Protection Agency in their Indoor airPLUS Construction Specifications (EPA 2015). Before laying the polyethylene, after the stem walls are poured, some building scientists recommend as a best practice backfilling inside the stem wall to ensure that the crawlspace floor is above the exterior grade. This will eliminate the possibility of run-off draining directly into the crawlspace (although it won't eliminate the possibility of soils becoming saturated and wicking moisture into the crawlspace area; hence, polyethylene sheeting is still required). In locations where there is evidence that the groundwater table can rise to within six inches of the finished floor or where there is evidence that surface water doesn’t readily drain from the site, it is required by code that the crawlspace ground level be as high as or higher than the outside finished grade, unless an approved drainage system is installed (IRC 2015).
How to Install a Polyethylene Sheeting Capillary Break beneath a Concrete Crawlspace Slab
- Install a radon mitigation vent pipe if desired or required as described below.
- Install a minimum thickness of 6-mil polyethylene across the entire ground surface.
- Overlap the edges between the pieces of polyethylene by 6 to 12 inches and tape or seal all of the seams as shown in Figure 4. Seal the vapor barrier around the piers and pipes as described below.
- 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 shows the edges of the polyethylene secured to the foundation walls with pressure-treated wood strapping, instead of covering the polyethylene with concrete. This is an acceptable alternative that is described in the guide Capillary Break at Crawlspace Floor - Polyethylene Sheeting Lapped up Walls and Piers. 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.
How to Install a Continuous Capillary Break at Pier Footings
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 (see Figure 5).
- Tape the polyethylene ground cover to the capillary break at each interior footing.
Check the EPA Radon 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 under the crawlspace slab and vapor barrier 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 building up inside the home. This is especially important in tightly built homes where the likelihood for soil gas build-up may be increased.
The following guidance is from Building America research partner Building Science Corporation (Lstiburek 2004). For additional information see the Indoor airPLUS guidance Approved radon-resistant features installed in Radon Zone 1 homes.
How to Install a Radon Control System
- Lay a piece of 3-inch-diameter perforated pipe that is at least 5 feet long in a gravel-filled trench running across the interior area of the crawlspace as shown in Figure 6. 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
- Connect this horizontal pipe to a 3-inch-diameter vent pipe that runs vertically through the roof. The vent outlet should be located at least 1 foot above the roofline and at least 10 feet from any openings to the building such as windows, skylights, or ventilation intakes.
- Lay polyethylene sheeting over the drain pipe and gravel bed and seal all edges and around the vent stack (using a compatible caulk) as described above.
- Mechanically fasten the polyethylene to the foundation wall or pour a two-inch-thick 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 reduce radon levels below 4 pico-curies per liter, 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 rate at which the soil gas is removed.
Visually inspect the polyethylene prior to pouring the slab. Seams should overlap at least 6 inches and be taped with an acrylic- or butyl-based adhesive tape. Ordinary cloth-backed duct tape is not appropriate, as the bond will not last.
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:
1.4.1 Capillary break at all crawlspace floors using ≥ 6 mil polyethylene sheeting, lapped 6-12 in., & placed beneath a concrete slab; 3, 4, 5
(3) Not required in Dry (B) climates as shown in 2009 IECC Figure 301.1 and Table 301.1.
(4) Not required for raised pier foundations with no walls. To earn the ENERGY STAR, EPA recommends, but does not require, that radonresistant features be included in homes built in EPA Radon Zones 1, 2 & 3. or more information, see www.epa.gov/indoorairplus.
(5) 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.
(7) 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 manufacturer’s specifications 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 manufacturer’s specifications 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.
Builders Responsibilities: It is the exclusive responsibility of builders to ensure that each certified home is constructed to meet these requirements. While builders are not required to maintain documentation demonstrating compliance for each individual certified home, builders are required to develop a process to ensure compliance for each certified home (e.g., incorporate these requirements into the Scope of Work for relevant sub-contractors, require the site supervisor to inspect each home for these requirements, and / or sub-contract the verification of these requirements to a Rater). In the event that the EPA determines that a certified home was constructed without meeting these requirements, the home may be decertified.
ENERGY STAR Revision 08 requirements are required for homes permitted starting 07/01/2016.
The U.S. Department of Energy (DOE) Zero Energy Ready Home National Program Requirements requires (Exhibit 1, Items 1 and 6) that all homes meet ENERGY STAR Certified Homes Version 3 or 3.1 and the U.S. Environmental Protection Agency Indoor airPLUS Construction Specifications.
EPA Indoor airPLUS
The U.S. Environmental Protection Agency (EPA) Indoor airPLUS Construction Specifications requires homes to meet the ENERGY STAR Certified Homes requirements, which fulfills Indoor airPLUS requirements to install polyethylene sheeting or XPS insulation under concrete basement slabs or polyethylene sheeting at crawlspace floors. Additional requirements include the following.
1.2 Capillary Break Installation
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 2009 IECC Figure 301.1.
- Areas with free-draining soils – identified as Group 1 (Table R405.1, 2009 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
NOTE: Completion of the ENERGY STAR requirements now satisfies the following Indoor airPLUS requirement:
- Air seal all sump covers (Builder-W 1.7).
Additional Indoor airPLUS Requirements:
- Construct homes in EPA Radon Zone 1 (see www.epa.gov/radon/zonemap.html) with radon-resistant features to conform to ASTM E1465; or IRC, Appendix F; or NFPA 5000, Chapter 49. Consult EPA's "Building Radon Out" (EPA 402-K-01-002) for general guidance on installing radon-resistant features.
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 to conform with the radon-resistant standard used, e.g., “Radon Reduction System” or “Radon Pipe” or “Radon 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. For crawlspaces, install at least 5 ft. of horizontal perforated drain tile on either side of the T-fitting, attached to the vertical radon vent pipe beneath the sheeting and running parallel to the long dimension of the house.
- Radon fan installed in the attic (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.
- Foundation air sealing with polyurethane caulk or the equivalent at all slab openings, penetrations and control or expansion joints.
Note: Consult local building codes to determine whether additional radon requirements apply. In January 2013 ANSI-AARST published a standard of practice for “Reducing Radon in New Construction of 1&2 Family Dwellings and Townhouses (CCAH-2013)”, available at www.aarst.org/bookstore.shtml.
- 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 www.epa.gov/radon/whereyoulive.html 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 pre-emptively include a radon vent fan. EPA also recommends, but does not require, radon-resistant features for homes built in EPA Radon Zones 2 and 3. EPA further recommends that all homes built in EPA Radon Zones 2 and 3 with 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.
- The U.S. Surgeon General and EPA recommend that all homes built in Radon Zones 1, 2 and 3 be tested for radon. Provide buyers with EPA’s Citizen’s Guide to Radon, encourage them to test for radon and refer them to www2.epa.gov/radon 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.
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 R506.2.3 Vapor retarder. A 6 mil polyethylene (or other approved) vapor retarder to have joints lapped not less than 6 inches must be placed between the concrete slab and the base course or prepare subgrade if no base course exists. Exceptions: Detached garages, utility buildings, other unheated accessory structures, unheated storage rooms less than 70 square feet, carports, driveways, walks, patios, and other flatwork not likely to be later enclosed or heated, and where approved by the building official.*
*Due to copyright restrictions, exact code text is not provided. For specific code text, refer to the applicable code.
Foundation Wall Water/Damp-Proofing = Foundation Wall Water Barrier