This standard specifies a test method for evaluating the long-term deformation performance of rigid foam plastics (such as polystyrene, phenolic resin, and other materials) under constant pressure. The core principle involves applying a continuous compressive load to a standard-sized specimen under specified temperature and relative humidity conditions, and measuring its thickness change over 48 hours using precision instruments to calculate key indicators such as creep strain or creep modulus. To conduct tests according to this standard, equipment such as a constant-load compression testing machine with an accuracy of ±1%, a constant temperature and humidity chamber capable of maintaining specific conditions, and displacement measurement devices (e.g., dial indicators or sensors) for precise thickness measurement are required. This method provides a critical basis for predicting the long-term dimensional stability of foam plastics in applications such as building insulation.