A complete water management strategy is critical in order to keep the home from excessive moisture issues. Mold, rot, ruined insulation, poor indoor air quality (IAQ), and the potential for structural damage are all a result of excessive water entering the building envelope. While a home’s overall moisture protection strategy may include sump pumps and other technologies to help remove water, the primary goal should be to keep water from entering the home in the first place. Designs that include roof overhangs, gutters and downspouts that carry water away from the foundation, site grading away from the house, and other preventative steps will keep water from pooling around the home’s foundation (Ueno and Lstiburek 2011; BSC 2009).
See Final Grade for guidance on managing water drainage around the home’s foundation.
Figure 1 - Moisture Infiltration in Below-Grade Wall. Water saturated concrete, mineral stains on the wall, and standing water on the floor are obvious signs of moisture infiltration in below-grade walls.
Finishing the Exterior Surface of Below-Grade Walls that are Concrete
Most foundation and below-grade walls are constructed with concrete, concrete masonry, or insulated concrete forms (ICFs). Concrete products are porous, and unless treated, are not waterproof, allowing water to migrate into the building (BSC 2006). Telltale signs that concrete foundations have water issues include mineral stains, mold, wet (saturated) areas, and even puddles on the floor.
To ensure that concrete-based foundations and below-grade walls remain dry and water does not seep into the home, you must do the following (with each bullet described in more detail below):
Customize the below-grade poured concrete mixture to yield concrete that is more impermeable to water migration.
Cover the surface of all below-grade walls with damp-proof coating.
If installing below-grade insulated concrete forms, use manufacturer-approved materials for the damp-proof coating.
Customize Below-Grade Concrete Mixture for More Water Impermeability
Not all concrete is mixed to the same specifications. The overall strength of concrete and the ability to repel water can be adjusted depending on the ratio of water, cement, sand, and aggregate used. Concrete is compression rated using a pounds per square inch (PSI) measurement. Most mixed concrete will range between 3,000 PSI and 4,000 PSI, but greater strengths are available. The higher the PSI the more water resistant the concrete will be once it is cured.
Also, additives called admixtures can be incorporated into the concrete before it is poured that can alter curing time, improve freeze protection, and improve water impermeability. When determining the PSI and admixtures for concrete, it is important to make decisions based on climate and local building codes.
Because increasing the PSI and water impermeability of the concrete will also increase the cost of the material, you may consider increasing the PSI for only the foundation or below-grade walls and select a lesser PSI concrete mix for other parts of the house like sidewalks, parking areas, and patios.
Apply a Damp-Proof Coating to Concrete Below-Grade Walls
In addition to customizing the concrete mixture, you can apply a waterproof treatment, known as a damp-proof coating (as shown in Figure 2). Damp proofing is usually an asphalt emulsion that can be brush- or roller-applied, or may take the form of a spray-on coating, or closed-cell polyurethane foam.
Figure 2 - A Below-Grade Foundation Wall with a Damp-Proof Coating. Customized concrete mixtures (more water impermeable) and damp-proof coatings help protect below-grade concrete walls from water absorption. Here a damp-proof coating is painted onto the concrete, which is covered with the secured ends of the plastic sheeting that is located under the concrete slab.
For poured concrete walls, a damp-proof coating can be applied directly to the surface; however, for masonry block walls and below-grade insulated concrete forms the surfaces must first be prepared.
Preparing Masonry Block Walls for Exterior Damp-Proof Coating
The surfaces of concrete masonry block walls must be coated with a layer of parging before damp-proofing can be applied. Parging is a mortar applied to the surface of a masonry wall to create a smooth, continuous surface free of holes. It will give the damp-proof coating something even to adhere to and form a flat plane.
The steps for preparing masonry block walls for exterior damp proofing are as follows:
Mix parging material per manufacturer’s directions.
Using a trowel, apply parging directly to the exterior of the masonry or rough surface wall, creating a smooth, even plane.
Allow to dry, per manufacturer’s recommendations, prior to applying damp-proof coating.
Preparing Below-Grade Insulated Concrete Forms for Damp-Proof Coating
Below-grade insulated concrete forms (ICFs) are blocks or panels consisting of two layers of rigid plastic foam that sandwich a 4- or 6-inch layer of concrete (see Insulated Concrete Forms). ICFs require slightly more attention due to the possibility of trapping moisture between the concrete and the insulation. Refer to the manufacturer’s recommendations when applying any coating as some coatings may dissolve the foam form.
The steps for damp-proofing ICFs are as follows:
Contact the manufacturer or refer to the manufacturer’s documentation to determine the appropriate damp-proofing material.
Apply per the manufacturer’s directions.
Finishing the Exterior Surface of Below-Grade Walls that are Wood
The 2012 International Residential Code (IRC) allows wood foundation walls, although this is not considered a Building America best practice. If using wood as a below-grade wall, be sure to do the following for the exterior finish:
First, refer to local and national codes and Authority Having Jurisdiction (AHJ) instructions regarding the required preservative-treated lumber and moisture barrier for your climate zone.
Select the materials that meet all regulations and are recommended for below-grade construction.
Install per specifications.
The following overall steps are to be considered within the specific code and AHJ instructions.
1. Install preservative-treated lumber for all below-grade walls. This lumber is immersed in a liquid preservative and placed in a pressure chamber to force the chemical into the wood. It is important to select wood for below-grade applications that has been pressure-treated with an appropriate and code- or jurisdiction-approved preservative for the specific climate zone and application.
Ensure the lumber panel joints are sealed the full length with a caulking compound that produces a moisture-proof seal.
Cover all below-grade lumber with a moisture barrier:
a. Apply 6-mil-thick polyethylene sheeting or a self-adhesive waterproof membrane to the entire exterior side of the below-grade walls before backfilling.
b. Lap the joints by 6 inches and seal with manufacturer-recommended adhesive.
Note: Do not nail or otherwise puncture the sheeting as this allows moisture to contact the concrete.
The Exterior Finish as Part of an Overall Water Management Strategy
Finally, as shown in Figure 3 below, applying a damp-proof finish to a below-grade wall is just one important water management strategy in designing the below-grade wall (Ueno and Lstiburek 2011; BSC 2009). The overall strategy needs to consider many factors including national and local codes, climate zone, and insulation options (Aldrich et al. 2012; BSC 2002).
Figure 3 - A Complete Water Management Strategy for a Below-Grade Wall