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Capillary Break Beneath Slab - Polyethylene Sheeting or Rigid Insulation over Aggregate or Sand and Geotextile Mat

    Scope
    Scope Images
    Image
    Right - Rigid foam insulation is taped at seams and installed over a drainage pad of aggregate to serve as a capillary break under the basement slab.
    Scope

    Install a capillary break beneath slab foundations (for example, a slab-on-grade or basement slab foundation) consisting of 4 inches of aggregate stone or 4 inches of sand covered by geotextile matting. Install a vapor barrier over this consisting of 

    •  ≥ 6-mil polyethylene sheeting lapped 6-12 inches with seams sealed, or 
    • ≥ 1-inch extruded polystyrene rigid foam insulation with joints taped.

    Seal the sheeting or foam at the joints with foundation walls and around posts or pipes coming up from the ground to provide a continuous vapor barrier.

    See the Compliance Tab for links to related codes and standards and voluntary federal energy-efficiency program requirements.

    Description
    Description

    Water gets through the foundation of most houses, either through bulk moisture leaks or through a process called capillary action. Once inside, the water can create significant problems for the home, including structural damage, mold, and poor indoor air quality.

    Bulk moisture is the flow of water through holes and cracks and is addressed in the guide Exterior Surface of Below-Grade Walls. Bulk water usually moves with gravity down and through foundation walls if large openings or cracks allow it to flow freely into the building. Capillary action occurs when liquid water wicks into the tiny cracks and open spaces of porous building materials such as masonry block, concrete, or wood (see Figure 1 and Figure 2). Capillary suction enables porous materials like concrete to wick water in any direction, including upwards against gravity, for surprisingly long distances. Capillary action in concrete is theoretically capable of pulling water upward as far as 6 miles (Lsibturek, 2014). 

    Water intrusion. This image shows how water can wick through and up a foundation by way of capillary action
    Figure 1 - Capillary action pulls water up through a basement foundation wall that is not protected by a capillary break.

     

    This image shows how water can wick up from under the foundation and can easily seep into the crawlspace or basement
    Figure 2 - A capillary break of aggregate and polyethylene is installed under the slab but no capillary break separates the footing from the stem wall so water can wick up into the house.

     

    To establish a capillary break that will prevent water from wicking up into the basement or slab-on-grade foundation, the U.S. Environmental Protection Agency’s Indoor airPLUS Construction Specifications recommend builders install a drainage pad of either aggregate or sand and geotextile matting. The large spaces between the individual stones prevent capillary action from occurring. The geotextile matting material, which consists of waffle-like or dimpled high-density plastic sheet or a matrix of plastic wire, also has numerous air gaps that prevent capillary action from occurring. Both options are shown in Figure 3. 

    Basement slab. This image show both approaches to creating a successful capillary break using aggregate and also sand with a geotextile mat
    Figure 3 - A successful capillary break under a basement slab can be created from either aggregate or sand and geotextile matting covered by a vapor barrier of polyethylene or rigid foam.

     

    This base is then covered with a vapor barrier of either polyethylene sheeting or extruded polystyrene rigid insulation (EPA 2018).

    Polyethylene sheeting is used here primarily as a vapor retarder but it also provides a capillary break. Polyethylene is sometimes used as the vapor barrier between the footing and stem wall to prevent “rising damp,” capillary action that can pull moisture up into the footing walls, as shown in Figure 2.  

    Rigid foam can also be installed, either over the polyethylene sheeting or directly on the aggregate to serve as a vapor barrier. As with the polyethylene sheeting, if the insulation serves as the vapor barrier, the joints between the insulation panels must be taped and sealed, and the concrete slab should be in direct contact with (poured directly on) the insulation or poly sheeting.

    Some builders spray closed-cell spray foam directly onto the gravel base to serve as the vapor barrier, while providing an insulation layer under the slab.

    How to Install a Capillary Break with Aggregate or Sand and Geotextile Matting

    1. Level the soil beneath the slab area.
    2. Install a bed of ½-inch diameter aggregate gravel to a consistent depth of 4 inches. 
      Or, distribute a 4-inch layer of clean sand evenly across the entire pad area. Lay the geotextile matting in strips across the entire area surface, making sure it is in contact with the foundation edges. 
    3. Install radon vent pipe in gravel or sand if desired or required.
    4. Install a vapor barrier consisting of polyethylene sheeting or polystyrene rigid insulation.

    How to Install Polyethylene Sheeting as a Vapor Barrier

    1. Select at least 6-mil polyethylene sheeting as a minimum thickness.
    2. Place the polyethylene sheeting over the entire foundation area making sure it touches each perimeter wall (see Figure 3 and Figure 4). The polyethylene can be extended to serve as a capillary break under the footing of a slab-on-grade foundation (as shown in Figure 5) or between the footing and the stem wall (as shown in Figure 6). 
    3. Lay the lengths of sheeting side-by-side and overlap the edges by at least 6 inches. 
      Tip: Overlap the polyethylene sheets by 12 inches to compensate for uneven cut lines.
    4. Seal the sheets together at the overlap using either a continuous bead of acoustical sealant, butyl rubber, or butyl acrylic caulk, or with tape manufactured to seal or patch polyethylene such as some builder's tapes and tapes used to repair polyethylene greenhouses (EPA 2018).
    5. Seal around any penetrations through the sheeting such as plumbing or radon mitigation piping.
    6. Pour the concrete slab directly over the polyethylene (see Figures 5, 6, and 7) or install rigid foam and then pour the concrete slab (see Figure 4). 
    Image
    Good water management practices like sloping grade away from house, and installing gutters, perimeter drain pipe, a capillary break, and free-draining soils or drainage mat protect the foundation from water saturation.
    Good water management practices like sloping grade away from house, and installing gutters, perimeter drain pipe, a capillary break, and free-draining soils or drainage mat protect the foundation from water saturation.
    Good water management practices like sloping grade away from house, and installing gutters, perimeter drain pipe, a capillary break, and free-draining soils or drainage mat protect the foundation from water saturation.
    Figure 4 - Polyethylene sheeting works with the gravel as a capillary break under the slab and serves as a capillary break between the footing and the stem wall to prevent wicking of water from underneath the slab. (Source: Upgrading Below Grade Spaces).
    House detail for hot-humid climate showing a slab-on-grade foundation with a capillary break of aggregate and polyethylene which also serves protects the footing.
    Figure 5. House detail for hot-humid climate showing a slab-on-grade foundation with a capillary break of aggregate and polyethylene which also serves protects the footing. (Source: Building Science Corporation).
    House detail showing a slab-on-grade foundation with a capillary break of aggregate and polyethylene, which also separates the footing from the foundation wall.
    Figure 6. House detail showing a slab-on-grade foundation with a capillary break of aggregate and polyethylene, which also separates the footing from the foundation wall. (Source: Steve Easley & Associates).
    Image
    Pouring the slab
    Pouring the slab
    Pouring the slab
    Figure 7 - Polyethylene sheeting can be seen as cement is poured to create the foundation. Care must be taken when walking on the sheeting to ensure it does not tear or pull apart at the seams. (Source: Steve Easley & Associates).

    How to Install Polystyrene Rigid Insulation as a Vapor Barrier

    1. Lay the polystyrene rigid insulation over the entire foundation area, making sure it touches all perimeter walls. See Figure 8.
    2. Tape and seal the joints between insulation pieces.
    3. Tape around all piping that comes through the rigid foam insulation (see Figure 9).
    4. Pour the concrete slab over the sealed polystyrene insulation.
       
    Rigid foam insulation is installed directly over the aggregate capillary break to serve as a vapor barrier beneath the basement slab in this house detail for the mixed humid climate.
    Figure 8. Rigid foam insulation is installed directly over the aggregate capillary break to serve as a vapor barrier beneath the basement slab in this house detail for the mixed humid climate. (Source: Building Science Corporation).
    Image
    Right - Rigid foam insulation is taped at seams and installed over a drainage pad of aggregate to serve as a capillary break under the basement slab.
    Right - Rigid foam insulation is taped at seams and installed over a drainage pad of aggregate to serve as a capillary break under the basement slab.
    Right - Rigid foam insulation is taped at seams and installed over a drainage pad of aggregate to serve as a capillary break under the basement slab.
    Figure 9. Rigid foam insulation is installed over a drainage pad of aggregate to serve as a capillary break under the basement slab. Seams and pipes are sealed with tape and gaskets so foam can serve as a continuous vapor barrier as well. (Source: IBACOS).
    Success
    Ensuring Success

    It is critical that care be taken when installing and working around the polyethylene and/or rigid foam prior to pouring the foundation slab to maintain a complete, well-secured, and unbroken vapor barrier over the aggregate or sand and geotextile matting capillary break.  

    Climate
    Climate

    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. 

    Image
    IECC climate zone map
    IECC climate zone map
    IECC climate zone map
    Figure 1. Climate Zone Map from IECC 2009, 12, 15, and 18. (Source: 2012 IECC)
    Image
    Climate Zone Map from IECC 2021
    Climate Zone Map from IECC 2021
    Climate Zone Map from IECC 2021
    Figure 2. Climate Zone Map from IECC 2021. (Source: 2021 IECC)
    Training
    Right and Wrong Images
    Image
    Right-Polyethylene sheeting is correctly installed over aggregate and taped to pillars and foundation wall
    Right-Polyethylene sheeting is correctly installed over aggregate and taped to pillars and foundation wall
    Image
    Wrong - Polyethylene sheeting should be lapped up sides of walls and pillars and taped
    Wrong - Polyethylene sheeting should be lapped up sides of walls and pillars and taped
    Image
    Right - This rigid insulation is correctly installed in a level layer over aggregate; the seams will be taped
    Right - This rigid insulation is correctly installed in a level layer over aggregate; the seams will be taped
    Image
    Right – Polyethylene sheeting is laid over aggregate and over footing to provide a capillary break between the ground and the slab and between the footing and the stem wall
    Right – Polyethylene sheeting is laid over aggregate and over footing to provide a capillary break between the ground and the slab and between the footing and the stem wall
    Image
    Right – Polyethylene sheeting completely covers the aggregate and the footing with no tears or open seams
    Right – Polyethylene sheeting completely covers the aggregate and the footing with no tears or open seams
    Image
    Wrong – Polyethylene sheeting does not completely cover the aggregate and the footing
    Wrong – Polyethylene sheeting does not completely cover the aggregate and the footing
    Image
    Wrong – Polyethylene sheeting does not extend beyond edge of aggregate to separate the footer from the stem wall which will be poured next
    Wrong – Polyethylene sheeting does not extend beyond edge of aggregate to separate the footer from the stem wall which will be poured next
    Image
    Right – The vapor barrier is thoroughly sealed with tape at all seams.
    Right – The vapor barrier is thoroughly sealed with tape at all seams.
    Image
    Right - Workers installed rigid foam on the floor and the sill plates of these pre-cast, pre-insulated concrete wall panels to provide a continuous layer of insulation under and around the not-yet-poured basement floor slab.
    Right - Workers installed rigid foam on the floor and the sill plates of these pre-cast, pre-insulated concrete wall panels to provide a continuous layer of insulation under and around the not-yet-poured basement floor slab.
    Image
    Right – Two inches of rigid foam was installed on the ground before pouring the basement floor slab while precast, pre-insulated concrete panels comprise the basement walls.
    Right – Two inches of rigid foam was installed on the ground before pouring the basement floor slab while precast, pre-insulated concrete panels comprise the basement walls.
    Image
    Right - The slab-on-grade foundation is insulated with two layers (R-20) of XPS foam under the-slab and R-10 on the exterior of the stem walls.
    Right - The slab-on-grade foundation is insulated with two layers (R-20) of XPS foam under the-slab and R-10 on the exterior of the stem walls.
    Videos
    CAD
    CAD Files
    Rigid Insulation Over Aggregate
    Rigid Insulation Over Aggregate
    Download: DWG PDF
    Polyethylene Over Aggregate
    Polyethylene Over Aggregate
    Download: DWG PDF
    Polyethylene sheeting covers the aggregate and extends down and under the footing to provide a capillary break between the ground and the slab and monolithic footing
    Polyethylene sheeting covers the aggregate and extends down and under the footing to provide a capillary break between the ground and the slab and monolithic footing
    Download: DWG PDF
    Compliance
    More

    More Info.

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    Case Studies
    References and Resources*
    Author(s)
    U.S. Environmental Protection Agency,
    ENERGY STAR
    Organization(s)
    EPA,
    ENERGY STAR
    Publication Date
    Description
    Guide describing details that serve as a visual reference for each of the line items in the Water Management System Builder Checklist.
    Author(s)
    U.S. Department of Energy
    Organization(s)
    DOE
    Publication Date
    Description
    Website describing the U.S. Department of Energy's residential buildings program.
    Author(s)
    Huelman Patrick H,
    Breidenbach Sam,
    Schirber Steve
    Organization(s)
    NorthernSTAR,
    University of Minnesota,
    UMN,
    NSTAR,
    U.S. Department of Energy,
    DOE
    Publication Date
    Description
    Presentation describing the challenges of insulating an existing basement, presented to the Building America residential energy-efficiency stakeholder meeting.
    Author(s)
    Lstiburek Joseph W
    Organization(s)
    Building Science Corporation,
    BSC
    Publication Date
    Description
    Article describing correct ways to eliminate water damage by incorporating a sacrificial layer to foundation walls and footings.
    *For non-dated media, such as websites, the date listed is the date accessed.
    Contributors to this Guide

    The following authors and organizations contributed to the content in this Guide.

    Sales
    Building Science Measures
    Building Science-to-Sales Translator

    Foundation Capillary Break over Aggregate = Foundation Floor Water Barrier

    Image(s)
    Technical Description

    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.

    Foundation Floor Water Barrier
    Sales Message

    Foundation floor water barriers help drain water away from under the slab. What this means to you is peace-of-mind knowing your home has a comprehensive set of measures that minimize the risk of water damage in your basement. Wouldn’t you agree every home should have full water protection?

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