Basement Walls

Installing insulation on the exterior of a basement wall (exterior insulation) is usually good practice. Exterior insulation can mini­mize thermal bridging and subsequently reduce heat loss through the foundation, can protect waterproofing, and can help serve as a capillary break to moisture intrusion, and can help prevent any freeze-thaw cycle damage to the foundation. Some exterior insula­tion materials are susceptible to insect infestation, so material selection is paramount to good performance. Foam insulation impregnated with insecticidal boric acid has yielded some success in discouraging termite infestations. Although termites avoid it, boric acid slowly leaches out of most materials exposed to moisture. Installation of a good gravel or manufactured “rain screen” drainage element outside the insulation can reduce moisture prob­lems significantly and structurally protect the insulation.

More comprehensive analysis is needed to better identify appro­priate protective coatings, address insulation moisture absorption, and understand long-term insulation R-value degradation. One study conducted by the Minnesota Department of Public Service, Office of Energy Conservation, surveyed 59 houses in the Minneapolis-St. Paul area from April to June of 1988. The study sampled foundation insulation specimens and soil specimens to determine long-term performance.

The survey’s results showed that the durability and performance of exterior foundations are due to installation quality and above­grade protective coatings rather than the type of insulation mater­ial used. Most coatings help to minimize moisture absorption and foster R-value retention. However, almost 60 percent of the bitu­men coatings (commonly used to protect spray urethane insulation) sampled showed flaking, gouging, or other damage that could reduce effectiveness.1

The U. S. Department of Energy is working with regulatory groups to help establish appropriate guidelines that provide cost – effective thermal protection for buildings. Building scientists theo­rize that the best way to build a dry basement is to insulate the outside of exterior walls with a rigid, fibrous, insulating drainage layer, such as fiberglass or rock wool, and omit the common appli­cation of exterior damp-proof coating or interior vapor retarder.1 Conventional damp proofing should be applied from 6" below grade down to 3′ below grade. The fibrous insulation acts as a capillary break that keeps bulk water out even during floods. The concrete will always dry to the exterior because of the vapor pressure dif­ferentials. This construction resists summer wall condensation and potentially can act as a passive dehumidifier for the basement. In winter, water vapor will diffuse inward whenever the relative humidity of the basement air is below 33 percent.

Cavity foundation materials, such as concrete block, potentially lend themselves to both retrofit and new construction installations of foamed-in, blown-in, and poured-in insulations. The most com­monly used materials include foamed-in insulations or poured-in polystyrene beads and granular materials such as vermiculite.

Concrete block is also available with insulating inserts for new con­struction. These materials reduce convection within the cells (the hollow cavity), but significant levels of heat can still conduct through the webs of the masonry. Some concrete block manufac­turers attempt to increase the thermal resistance of their product by adding materials such as polystyrene or wood chips to the con­crete mix.

Insulation also may be applied to the interior of a foundation or basement wall. This method is especially suited for renovation or remodeling projects without excavation. Material analysis is essen­tial for proper placement, as well as for ensuring the safety and structural integrity of the home. For example, some types of insu­lation require separation from habitable spaces by a fire-rated material because they are extremely flammable and release toxic gases when ignited. Interior insulation applications fail to protect the waterproofing or structure as well as exterior insulation. Proper installation of sealants and vapor retarders is important for adequate performance of interior insulation.