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CABO Limits Rigid Foam in Ground Contact

In the move to control damage by termites in the South and West, the ICC (International Code Council, formerly CABO) will include a provision in the 1998 International One-and Two-family Dwelling Code, due out in late 1998, that restricts the use of rigid foam insulation below grade in areas with a high probability of termite infestation.The provision prohibits the use of "foam plastics", such as polyisocyanurate and extruded or expanded polystyrene, under slabs, and under or on the exterior of foundation walls, whether the walls are at the perimeter or inside the building. Foam plastics installed above grade cannot be installed within 6 inches of grade. An exception will allow insulated concrete forms (IFC's) to be used foundations when the framing is of noncombustible materials (steel) or pressure-treated wood. It is not clear how the measure will affect buildings in which ICF's extend from the footings above-grade to the rafters.

While there is no definitive explanation for the high incidence of infestation in homes with below-grade foam insulation, termite inspectors have reported that the cover provided by the foam prohibits them from observing the termite migration from the earth to the wood structure. In addition, recent experiments at Colorado State University demonstrate that termites, when faced with a choice of routes, follow paths in which the air flow contains higher concentrations of carbon dioxide, a product found in rigid foam insulation. Carbon dioxide is also a by-product of rotting wood, and the concentration of this gas is higher in termite colonies than ambient air.

According to Dr. Neil Ogg, an entomologist and assistant department head at Clemson University's Department of Pesticide Regulation,"When placed below grade, foam insulation provides a superhighway from the soil, the termites' home, to the wood of the structure, the termites' food source." Ogg pointed to a Celotex Corporation study of 880 Florida homes with solid exterior insulation extending below grade. In that study, 92% of the homes were found to be significantly infested with termites after 10 years. No studies of similar homes without foam insulation over the same time period were available.

The effect of the measure on energy efficiency is difficult to determine, although the areas targeted by the new restriction, the South and California, are least likely to benefit from belowgrade insulation. Check with your local code officials to determine how this restriction will affect building in your area.