The high cost of production, transmission, and distribution of energy for household and industrial consumption has lead to development of efficient methods and solutions for energy loss prevention. Rigid polyurethane foams as thermal insulation materials have a better performance than other thermal insulators. This is the main reason for the widespread use of rigid polyurethane as an insulator. This type of insulator is used in ducts of cooling and ventilation systems in buildings and insulation of transmission pipe lines in humid environments. Generally, over time, consuming products and commodities lose their properties and must be replaced with new materials. Natural and accelerated aging methods are commonly used to find the lifetime of products. To accelerate the aging phenomenon, severe experimental conditions are applied in order to reduce the aging time, such that changes of properties can be observed. The extrapolation of these changes leads to eventuation and estimation of the product behavior in long run. In this work, the hydrothermal effects on the changes of mechanical properties and aging of rigid polyurethane insulation foam have been investigated. Here, by hydrothermal we mean simultaneous presence of water and heat. For this purpose, different samples were prepared according to the pertinent standards, and placed in distillated water at room temperature for 6 days. After assurance of full penetration of water into samples, the samples were kept at -20, 70 and 95 °C. These conditions were maintained for 16, 26, and 33 days. Then, the samples were taken out of the experimental conditions, and were subject to various characterizations test methods. To ensure the lack of presence of water in samples, they were kept in an oven at 70°C for 48 hours. The characterization test methods involved compression test, scanning electron microscopy (SEM), and attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR). Diagram of stress-strain displayed reduction of compression modulus of foam with increasing temperature and aging time. This reduction was observed at -20 and 95 °C for 16, 26, and 33 days. Deviation of curves from that of the control sample increased with increased temperature and aging time. This deviation reached up to 10% of strain at constant stress at 95 °C and 33 days. Effect of temperature on polyurethane foam degradation was more than that of the aging time. As a result, the temperature increased the kinetics of degradation. The images of SEM test showed degradation of the foam’s apparent structure at different temperatures and aging times. However, this degradation was more remarkable for images at 95 °C and 33 days. ATR-FTIR test was carried out as a method for analysis of chemical degradation of polymeric structure. This test was performed on samples aged for 33 days at -20 and 95 °C. At -20 °C, because of the presence of water and absence of heat, hydrolysis was the only effective factor of chemical degradation. However, more damage was observed at 95 °C. In fact, this thermal degradation is added to the degradation caused by hydrolysis. The comparison of these spectrums demonstrated that the main mechanism of degradation is hydrolysis, and the degradation by hydrolysis is much more than the thermal degradation. The water absorption test was carried out to determine the diffusion coefficient into the polyurethane foam at 25, 70 and 95 °C. The Calculation of the diffusion coefficient at these temperatures showed its strong dependence on temperature. The Arrhenius equation was used to determine this dependency and express it in a mathematical expression. This expression is the most precise empirical formula used for diffusion in solids. Thus, the diffusion coefficient at infinite temperature and activation energy of diffusion of water into the closed cell polyurethane foam was calculated. The extrapolation of aging for 16, 26, and 33 days at 95 o C was equal to 348, 566 and 730, respectively.