Concrete Slab over Polyethylene

Please Register or Login to Provide Feedback.

Climate

Except in U.S. Environmental Protection Area (EPA) Zone 1 Radon areas, polyethylene sheeting is not required in Dry (B) climates because the ground generally contains less moisture, so there is less potential for capillary movement of water from the earth into the crawlspace.

In EPA Zone 1 Radon areas, the polyethylene sheeting combined with a passive radon mitigation system will limit the amount of radioactive soil gas in the building, and avoid a dangerous build-up of the gas that can affect occupant health. It is also recommended that radon-resistant features be included in homes built in EPA Radon Zones 2 and 3.

Click here for the EPA Radon Map

Energy Star Version 3, (Rev 07)

Water Management Checklist, Water-Managed Site and Foundation. Capillary break at all crawlspace floors using >= 6 mil polyethylene sheeting, lapped 6-12 in., and installed using one of the following three options:  placed beneath a concrete slab; OR lapped up each wall or pier and fastened with furring strips or equivalent, OR, secured in the ground at the perimeter using stakes.  Polyethylene sheeting is not required in Dry (B) climates as shown in 2009 IECC Figure 301.1 and Table 301.1.

climate zone map

International Energy Conservation Code (IECC) Climate Regions

Please Register or Login to Provide Feedback.

Description

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).

A Damp Crawlspace

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.  Reference

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.

Comprehensive Water Management Features include a Capillary Break (>= 6-mil Polyethylene Sheeting) at all Crawlspace Floors

Figure 2 - Comprehensive Water Management Features include a Capillary Break (>= 6-mil Polyethylene Sheeting) at all Crawlspace Floors.  Reference

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.

Concrete Slab over Polyethylene Sheeting as a Capillary Break

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.  Reference

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):

  1. Install a minimum of 6-mil polyethylene across the entire ground surface.
  2. Overlap the edges between pieces of polyethylene by 6 to 12 inches, and tape all the seams.
  3. 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).
  4. 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.

Lapped and Taped Polyethylene Sheeting

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.  Reference

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):

  1. Install a capillary break over interior footings before masonry piers or steel columns are installed.
  2. Tape the polyethylene ground cover to the capillary break at each interior footing.

Taping Polyethylene Sheeting at Interior Pier Footings. It is important to provide continuous moisture control at pier footings

Figure 5 -  Taping Polyethylene Sheeting at Interior Pier Footings. It is important to provide continuous moisture control at pier footings.  Reference

Radon Control

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):

  1. 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.
  2. Lay polyethylene sheeting over the drain pipe and gravel bed, and seal (using a compatible caulk) all edges and around the vent stack.
  3. 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.
  4. 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.

A Radon Control System as Part of a Water-Managed and Air-Sealed Crawlspace for EPA Radon Zones 1, 2, and 3

Figure 6 - A Radon Control System as Part of a Water-Managed and Air-Sealed Crawlspace for EPA Radon Zones 1, 2, and 3.  Reference

Ensuring Success

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.

Scope

Capillary break at all crawlspace floors using ≥ 6 mil polyethylene sheeting, lapped 6-12 in., and placed beneath a concrete slab

Water Managed Site and Foundation

Capillary break at all crawlspace floors using >= 6 mil-polyethylene sheeting, lapped 6-12 inches, and placed beneath a concrete slab.

  1. Cover entire area with at least a 6-mil polyethylene sheeting.
  2. Overlap the polyethylene sheeting at least 6-12 inches.

ENERGY STAR Notes:

Only one item of detail 1.4 on the Water Management Checklist must be met to comply with ENERGY STAR.

Not required in Dry (B) climates as shown in 2009 IECC Figure 301.1 and Table 301.1.

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 Indoor airPLUS.

ENERGY STAR Notes for Existing Homes:

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 8) 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.

Training

Right and Wrong Images

Presentations

None Available

Videos

None Available

CAD Images

None Available

Compliance

Energy Star Version 3, (Rev 07)

Water Management Checklist, Water-Managed Site and Foundation.  Capillary break at all crawlspace floors using >= 6 mil polyethylene sheeting, lapped 6-12 in., and installed using one of the following three options:  placed beneath a concrete slab; OR lapped up each wall or pier and fastened with furring strips or equivalent, OR, secured in the ground at the perimeter using stakes.  Polyethylene sheeting is not required in Dry (B) climates as shown in 2009 IECC Figure 301.1 and Table 301.1. Polyethylene sheeting is also 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 and 3.

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 8) 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.

DOE Challenge Home

Exhibit 1: Mandatory Requirements. Certified under ENERGY STAR Qualified Homes Version 3.

2009 IRC

Section R408.3 Unvented crawl space. Exposed earth covered with a continuous Class I vapor retarder, with joints overlapping by 6 inches and sealed or taped.  Vapor retarder edges to extend at least 6 inches up the stem wall and attached and sealed to the stem wall.*

2012 IRC

Section R408.3 Unvented crawl space. Exposed earth covered with a continuous Class I vapor retarder, with joints overlapping by 6 inches and sealed or taped.  Vapor retarder edges to extend at least 6 inches up the stem wall and attached and sealed to the stem wall.*

*Due to copyright restrictions, exact code text is not provided.  For specific code text, refer to the applicable code

More Info.

Case Studies

None Available

References and Resources*

  1. Author(s): Lstiburek
    Organization(s): BSC
    Publication Date: November 2004

    Report outlining how conditioned crawlspaces perform better than vented crawlspaces in terms of safety, health, comfort, durability and energy consumption.

  2. Author(s): DOE
    Organization(s): DOE
    Publication Date: June 2013

    Standard requirements for DOE's Challenge Home national program certification.

  3. Author(s): BSC
    Organization(s): BSC
    Publication Date: May 2009

    Information sheet about groundwater control.

  4. Author(s): EPA
    Organization(s): EPA
    Publication Date: February 2013

    Website providing technical guidance to help home builders and their subcontractors, architects, and other housing professionals understand the intent and implementation of the specification requirements of the IAQ labeling program.

  5. Author(s): EPA
    Organization(s): EPA
    Publication Date: February 2011

    Guide describing details that serve as a visual reference for each of the line items in the Water Management System Builder Checklist.

Last Updated: 08/15/2013

Mobile Field Kit

The Building America Field Kit allows you to save items to your profile for review or use on-site.