The commercial development and growth rate of rigid urethane foams has not met the forecasts presented by the chemical industry. Difficulty in meeting building code require ments and specifications established by regu latoiy groups coupled with relatively poor comparative economics of applied insulation has retarded the growth and acceptance of many foam products. A review of extended foams systems based on the incorporation of low cost commodity chemicals into foam sys tems will be presented. These modified sys tems, while significantly lower in cost, have gained only limited acceptance due to the loss of dimensional stability, lower heat dis tortion properties, and increased permeability.