Spray Polyurethane Foam Insulation has excellent long term stability when used as a perimeter insulation.  When subjected to prolonged exposure to soil polyurethane foams hole up very well.  In a study done by Du Pont Company over a ten year period spray polyurethane foam was buried along with pine wood boards, sections of aluminum-faced panels and other building insulation (Du Pont Company Elastomer Chemicals Department Wilmington Delaware 19898).  The panels were buried to 1/2 of their lengths and polyurethane samples had 10" placed in the soil.  The samples were exposure for ten years being tested after one, three and ten years.  The results obtained of the polyurethane foam were excellent.  There was a small reduction in tensile strength, an increase in compressive strength, and a slight increase in weight loss in one plan exposed to 121 deg Celsius.  The pine boards were attacked by termites in three years and rotted in ten.  Polyurethane foams out performed all of the other building products.  The Polyurethane insulation after ten years in the ground show a few small surface pockets where rodents or insects had made some attempt to investigate the samples.