This International Standard specifies a method for the determination of dimensional stability of rigid cellular plastics when subjected to specific conditions of temperature and relative humidity. The method suggests a range of conditions from which one or more of the desired test conditions can be selected. Additional conditions may be used as agreed upon by the purchaser and the supplier. The term "dimensional stability of a rigid cellular plastic" implies the absence of irreversible change in dimensions in each of three directions perpendicular to each other when a test specimen of specified size is exposed to stated conditions for a specified period. In practice, such irreversible changes do occur and are measured by the method described in this International Standard.