Capillary Break Beneath Slab - Polyethylene Sheeting or Rigid Insulation over Geotextile Matting

Scope

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.

Notes:

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, it is recommended, but not required, that radon-resistant features be included in homes built in EPA Radon Zones 1, 2 and 3. For more information, visit the EPA's radon information website.

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 geotextile matting or aggregate to control water intrusion from water vapor, in one of two ways:

1. Install at least 6-mil polyethylene sheeting and overlap the sheeting at least 6 to 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).

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

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.

Installing the Geotextile Matting Capillary Break
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. 

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

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. Geotextile matting provides the same capillary break and can be used as a substitute for gravel. The waffle-like or dimpled high-density plastic sheet or the matrix of plastic wire forms an air gap, and a synthetic filter fabric keeps poured concrete from clogging the matting materials.

To install the geotextile matting you must:

1. Level the soil beneath the slab area.
2. Distribute a 4-inch layer of clean sand evenly across the entire pad area.
3. Lay the geotextile matting, in strips, across the entire area surface, making sure it is in contact with foundation edges.

After the geotextile matting is in place, the polyethylene sheeting or polystyrene rigid insulation can be installed. It is important to note that the geotextile matting is the capillary break, and the polyethylene sheeting or polystyrene rigid insulation is used as a vapor barrier to help reduce water intrusion.

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

Polyethylene Sheeting as a Vapor Barrier
Polyethylene sheeting is primarily a vapor retarder, but also provides a capillary break. 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 geotextile matting 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).

Figure 4 - Complete capillary break. Here you can see that at every possible point of entry, either polyethylent sheets or caulk has been installed to prevent wicking of water from underneath the slab. 

Polystyrene Rigid Insulation as a Vapor Barrier
Another option is to install 1-inch or greater polystyrene rigid insulation over the geotextile matting. 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 the geotextile matting and all edges.
2. Tape and seal the joints between insulation pieces.
3. Pour the concrete slab over the sealed polystyrene insulation.

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. 

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 is not 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 polystryene insulation will prevent water from wicking up and through the foundation.

Climate

ENERGY STAR Version 3, (Rev. 6)

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.

International Energy Conservation Code (IECC) Climate Regions

Right and Wrong Images

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Compliance

ENERGY STAR Version 3, (Rev. 6)

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.

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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