Polyurethane (PU) and phenolic (PF) foams used for building isolation were analyzed by thermal gravity/differential thermal analysis to determine their pyrolysis behavior, including the decomposition point and the maximum reaction rate point. Besides, the shape deformations of PU and PF foams were observed, and their oxygen index and the calorific value in combustion were also studied. The results showed that the pyrolysis of both PU and PF can be divided into three stages from room temperature to 1 000 ℃ in the atmospheric air, with total mass loss of 94.345% for PF and 88.191% for PU, respectively. The oxygen index of PU and PF decreased with increasing the temperature and the duration of the heat treatment. With the temperature increasing, the calorific values of both materials were reduced remarkably. These results of the PU and PF could provide basic data of the thermal stability and fire safety design in the application of thermosetting insulation materials. In addition, the shape deformations of PU and PF foams were observed, and their oxygen index and the calorific value in combustion were also studied. The results showed that the pyrolysis of both PU and PF could be divided into three stages from room temperature to 1000 ° C in the atmospheric air, with total mass loss of 94.345% for PF and 88.191% for PU, respectively. The oxygen index of PU and PF decreased with increasing the temperature and the duration of the heat treatment. With the temperature increasing, the calorific values ​​of both materials were reduced remarkably. These results of the PU and PF could provide basic data of the thermal stability and fire safety design in the application of thermosetting insulation materials.