Below Grade

Energy codes require that below-grade walls must be insulated in most climate zones. In certain climate zones, insulation around and below footings, as well as frost-protected shallow foundations, may also be required. An effective means to insulate below-grade surfaces or insulate below concrete slabs, especially during new construction, is the application of closed-cell foam to the exterior side of the foundation or underside of the slab. This application provides a continuous, air-impermeable water-resistant layer of insulation around the entire perimeter of the building.

Building codes may not approve the installation of foam plastics on building exteriors below grade in areas where termite infestation is considered “very heavy.” Check with local building code authorities for the applicability of applying SPF below grade in your area. Additionally, building codes may have physical property requirements for below-grade insulations, including maximum water absorption limits and minimum compressive strength values.  


Closed-cell SPF provides several functions which are important for below-grade applications. These include:
• Insulation
• Air Sealing
• Moisture Control
• Water Resistance
• Compressive Strength

The building science industry has researched and written a large body of information on heat, air, and moisture control in below-grade applications.  Closed-cell rigid foam board and SPF products are generally recommended for all below-grade installations where bulk water may contact the installation. Closed-cell SPF can generally be used in any location indicated as appropriate for rigid foam board, even if ccSPF is not specifically included as an option. Insulation materials that either easily absorb water or are air-permeable are typically not recommended for below-slab installation.

There are several advantages for using ccSPF in lieu of foam board insulation in below-slab applications. ccSPF provides a continuous layer of insulation without any cracks and gaps found at the edges of rigid foam boards. This helps to provide a better seal to prevent the infiltration of soil gases, as well as water vapor and liquid water. Secondly, ccSPF can help level the gravel base, minimizing concrete use and reducing uneven edges of the foam board. This prevents breakage of the foam and trip hazards after installation of the foam boards prior to installation of the concrete floor. The monolithic SPF layer stays in place without the need for pins to prevent the rigid foam insulation boards from becoming displaced.