Capillary Break Beneath Slab - Polyethylene Sheeting or Rigid Insulation Over Aggregate

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ENERGY STAR Version 3, (Rev. 07)

Water Management Checklist, Water-Managed Site and Foundation. Capillary break beneath all slabs (e.g., slab on grade, basement slab) except crawlspace slabs using either: >= 6 mil polyethylene sheeting, lapped 6-12 in., or >= 1” extruded polystyrene insulation with taped joints. 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 more information, see EPA's Indoor airPlus Website.

DOE Challenge Home

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

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International Energy Conservation Code (IECC) Climate Regions

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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 where large openings or cracks allow it to flow freely into the building. Capillary action occurs when liquid water wicks into the cracks and open spaces of porous building materials, such as masonry block, concrete, or wood. These tiny cracks and pores can absorb water in any direction, even going upward.

In order to protect the home from water intrusion through capillary action, a barrier or capillary break must be installed under the slab. There are two primary ways to install this capillary break and prevent water from wicking up into the basement or slab-on-grade construction:

  1. Install aggregate to a depth of 4 inches and then cover with either polyethylene sheeting or extruded polystyrene rigid insulation, or
  2. Install sand to a depth of 4 inches, cover with geotextile matting and then cover with either polyethylene sheeting or extruded polystyrene rigid insulation.

Once the primary capillary break has been installed, a vapor barrier should be installed directly in contact with the aggregate or geotextile matting, in one of two ways:

  1. Install at least 6-mil polyethylene sheeting and overlap the sheeting at least 6-12 inches, or
  2. Install at least 1-inch extruded polystyrene insulation to be in contact with the slab and tape all joints (DOE 2008).

The most common approach to creating the foundation slab is to use aggregate gravel under the slab. However, using a sand base and laying down geotextile matting is also an appropriate slab construction design as long as the correct steps are taken when creating the capillary break.

Water intrusion. This image shows how water can wick through and up a foundation by way of capillary action

Figure 1 - Water intrusion. This image shows how water can wick through and up a foundation by way of capillary action.  Reference

Installing the Capillary Break over Aggregate

The primary way to stop water from passing through the foundation wall is to address the water management issues during construction of the foundation.  Only a complete and well-sealed capillary break will prevent water under the slab from flowing up the walls of the foundation, as shown in the illustration below.

This image shows how water can wick up from under the foundation and can easily seep into the crawlspace or basement

Figure 2 - Easy Access. This image shows how water can wick up from under the foundation and can easily seep into the crawlspace or basement.  Reference

A gravel layer beneath a slab provides a capillary break; the large spaces between the individual stones prevent liquid water from moving up to make contact with the slab.

To install the aggregate material you must:

  1. Level the soil beneath the slab area.
  2. Install a consistent ½-inch diameter aggregate gravel to a consistent 4-inch depth.

After the aggregate material is in place, the next part of the capillary break system can be installed: the polyethylene sheeting or polystyrene rigid insulation. It is important to note that the aggregate is the capillary break, and the polyethylene sheeting or polystyrene rigid insulation is used as a vapor barrier to help reduce water intrusion.

Basement slab. This image show both approaches to creating a successful capillary break using aggregate and also sand with a geotextile mat

Figure 3 - Basement slab. This image show both approaches to creating a successful capillary break using aggregate and also sand with a geotextile mat.  Reference

Polyethylene Sheeting as a Vapor Barrier

Polyethylene sheeting is primarily a vapor retarder. A vapor retarder installed beneath a basement floor slab allows low-vapor-permeability floorings such as vinyl tile or sheet goods to be applied over the slab. Without a vapor retarder beneath the slab, water vapor that migrates through a concrete slab can accumulate under low-perm floorings until the flooring adhesive fails (EPA 2012).

Install the vapor barrier using polyethylene sheeting as follows:

  1. Select at least 6-mil polyethylene sheeting as a minimum thickness.
  2. Place the polyethylene sheeting over the entire surface of the aggregate material and make sure it touches each perimeter wall.
  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 any mistakes in cutting the sheets).
  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 2012).


Notice at every possible point of entry, either polyethylene sheets or caulk has been installed to prevent the wicking of water from under the slab

Figure 4 - Complete capillary break. Notice at every possible point of entry, either polyethylene sheets or caulk has been installed to prevent the wicking of water from under the slab.  Reference

Polystyrene Rigid Insulation as a Vapor Barrier

Another option is to install 1 inch or greater of polystyrene rigid insulation over the aggregate material. As with the polyethylene sheeting, the joints between the insulation panels must be taped and sealed, and the insulation must be in contact with the bottom of the slab.

Install the capillary break using polystyrene rigid insulation as follows:

  1. Lay the polystyrene rigid insulation over the entire foundation area making sure the insulation is in direct contact with aggregate material and all edges.
  2. Tape and seal the joints between insulation pieces.
  3. Pour the concrete slab over the sealed polystyrene insulation.

Here the polyethylene sheeting can still 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

Figure 5 - Pouring the slab. Here the polyethylene sheeting can still 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.  Reference

Exceptions

  • In areas of free-draining soils (identified as Group 1 by a certified hydrologist, soil scientist, or engineer through a site visit), a gravel layer or geotextile matting may not be required under concrete slabs.
  • Polyethylene sheeting is not required in Dry (B) climates, as defined by IECC Figure 301.1, unless the sheeting is required for radon resistance (EPA 2012).

Ensuring Success

It is critical that care be taken when installing and working around the water barrier prior to pouring the concrete slab and foundation. Only a complete, well-secured, and unbroken capillary break measure of 6-mil. polyethylene sheeting or 1-in. extruded polystyrene insulation will prevent water from wicking up and through the foundation.

Scope

Capillary break beneath all slabs (e.g., slab on grade, basement slab) except crawlspace slabs using either: ≥ 6 mil polyethylene sheeting lapped 6-12 in., or ≥ 1 in. extruded polystyrene insulation with taped joints

Water Managed Site and Foundation

Capillary break beneath all slabs (e.g., slab on grade, basement slab) except crawlspace slabs using either: >= 6-mil polyethylene sheeting lapped 6-12 inches, or >= 1-inch extruded polystyrene insulation with taped joints.

  1. Install at least a 6-mil polyethylene sheeting and overlap the sheeting at least 6-12 inches.
  2. If not using polyethylene sheeting, install at least 1-inch extruded polystyrene insulation to be in contact with the slab and tape all joints.

ENERGY STAR Notes:

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.

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Compliance

ENERGY STAR Version 3, (Rev. 07)

Water Management Checklist, Water-Managed Site and Foundation. Capillary break beneath all slabs (e.g., slab on grade, basement slab) except crawlspace slabs using either:  >= 6 mil polyethylene sheeting, lapped 6-12 in., or >= 1” extruded polystyrene insulation with taped joints.  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 more information, see EPA's Indoor airPlus Website.

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 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.*

2012 IRC

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:  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.*

2009 IECC

This is not specifically addressed in the 2009 IECC.

2012 IECC

This is not specifically addressed in the 2012 IECC.

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

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References and Resources*

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

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

  2. Author(s): Ueno, Lstiburek
    Organization(s): BSC
    Publication Date: May 2012

    Document providing information on basement insulation, air sealing and water management retrofits.

  3. Author(s): DOE
    Organization(s): DOE
  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): NSTAR
    Organization(s): NSTAR
  6. 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

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