Wet Basements... The Saga Continues

It used to be that customers assumed that their home’s basement was doomed to get wet. While water spouting out of the wall during heavy rains was pushing the limits, many customers felt that a little dampness was just par for the course, and made sure to keep boxes of keepsakes off the floor. But as customers become more savvy and demand better performing homes, they require that basements be free of moisture and dampness—which can contribute to rot, mold, and fungal growth, can lead to indoor air problems, and can damage wood floor joists and beams. Additionally, builders are looking for solutions to reduce litigation and make sure their customer satisfaction ratings stay high.

In our April 2002 QualityHome® newsletter, we presented best practices for insulation and moisture control of a basement wall configuration. These best practices were used in the recently constructed IBACOS Demonstration Home built in Pittsburgh, Pennsylvania (see the August 2003 QualityHome® newsletter for more information). In this article, we look at a range of basement insulation and moisture control measures that provide good to excellent performance while offering a broader range of construction options.

Foundation moisture can come from three sources: bulk water from soil entering through cracks or pores in the foundation wall, capillary moisture drawn up through the footing and into the foundation wall, diffusion of water vapor through the wall, and condensation of interior moist air on cold foundation surfaces.

To combat the first type of moisture, wall leakage, IBACOS recommends using a full elastomeric coating of the foundation exterior from the bottom edge of the footing to grade and providing an exterior drainage plane to drain water down and away from the wall to a footing drain. These two materials create a waterproof foundation system. This eliminates hydrostatic pressure on the wall, the driving force of wall leakage. The exterior drainage plane can be provided by a coarse gravel layer, a specialty drainage blanket (like Enkadrain, Mirafi, etc.), or rigid fiberglass insulation board. This latter material provides both drainage and insulation. All drainage layers should be separated from the soil backfill by a filter fabric to prevent clogging by very fine silt or clay particles over time.

The second type of moisture flow, capillary draw, can be minimized by providing a capillary break between the footing and the foundation wall. This is a very important action that builders are hesitant to take for fear of the wall structure slipping. However, we successfully used a capillary break in the IBACOS Demonstration Home, and feel that builders should be confident a capillary break won’t cause slippage. The capillary break can be achieved by applying either a sheet of polyethylene or a layer of a polymer-enhanced asphalt membrane on top of the footing, underneath the foundation wall. Note that this capillary break is a virtual extension of the typical polyethylene sheet under the basement floor slab and serves the same function.
Diffusion of moisture through a foundation wall is prevented by providing a good waterproof system on the outside and a capillary break between footing and wall.

When warm moist indoor air comes in contact with a cold foundation wall, condensation can occur. This is typically seen in spring and early summer when the soil behind the foundation is still cold. The best control of this foundation interior surface temperature is offered by exterior foundation insulation, typically rigid fiberglass or polystyrene foam. IBACOS strongly recommends this practice. Two alternative, but less desirable, interior insulation methods are: 1) low permeance foam insulation, which keeps moisture from passing through the insulation or 2) highly permeable insulation that will allow drying to the interior. It is also important to provide conditioned (heated or cooled) air in the space to promote drying conditions year round. See best practice details for these configurations.