This International Standard specifies two laboratory test methods, based on slicing and scaling techniques, to determine the long-term changes in the thermal resistance of closed-cell (normally 90 %) cellular plastic materials that contain gases which, through diffusion processes, affect the properties of a foam with time. Using standard methods for the measurement of thermal resistance, method A consists of periodic measurements performed over a short time interval on thin specimens conditioned in a controlled ambient temperature environment. The results of relative change with time are used in conjunction with a mathematical technique to derive the thermal resistance of greater thicknesses of the material as a function of time. Method В describes a simple test to determine a conservative design life-time value (25 years and longer) for an unfaced, closed-cell, cellular plastic product. This method is limited currently to unfaced homogeneous materials. For this method, multiple specimens of the core and surfaces of materials with variations in the slope of the primary stage thermal resistivity and a time relationship of less than 10 % within a sample are considered to be homogeneous. Generally, products with natural skins or with density deviations normally found with such products may be considered acceptable for test by this technique.