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:
- Install aggregate to a depth of 4 inches and then cover with either polyethylene sheeting or extruded polystyrene rigid insulation, or
- 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:
- Install at least 6-mil polyethylene sheeting and overlap the sheeting at least 6-12 inches, or
- 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.
Figure 1 - Water intrusion. This image shows how water can wick through and up a foundation by way of capillary action.
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.
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.
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:
Level the soil beneath the slab area.
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.
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. 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:
Select at least 6-mil polyethylene sheeting as a minimum thickness.
Place the polyethylene sheeting over the entire surface of the aggregate material and make sure it touches each perimeter wall.
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).
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. 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.
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:
Lay the polystyrene rigid insulation over the entire foundation area making sure the insulation is in direct contact with aggregate material and all edges.
Tape and seal the joints between insulation pieces.
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.
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