Some cellular plastics (foams) incorporate a chlorofluorocarbon (CFC) to improve their thermal insulating properties. During the service life of this type of insulation air diffuses into the foam cells and the CFC gas diffuses out, each at a rate that depends on type of polymer and temperature, eventually reducing the effectiveness of insulation. This effect is known as aging. Although literature on the aging process is extensive, there are no generally accepted procedures for evaluating long-term thermal resistance of new gas-filled foams. The mechanisms of the aging process are reviewed, and integrated approach to the evaluation of aging of gas-filled foams is proposed.