In this study, we investigated the properties of polyurethane-polydimethylsiloxane (PU-PDMS) hybrid foams with different types and contents of physical blowing agents (PBA). Two different types of blowing agents including physical blowing agents and thermally expandable microspheres (TEM) were applied. The apparent density was measured using foam cut into the specified size, and the pore size was measured through image software. Additionally, the microstructure of foams was confirmed using scanning electron microscopy and transmission electron microscopy. Thermal conductivity related to the microstructure of the foams was compared. When hydrocarbon-based PBAs were added in the amount of 0.5 phr, the apparent density and pore size of the foam were minimal, but the pore size was larger than that of the neat foam. On the other hand, 3 phr of TEM most effectively reduced both the apparent density and pore size among the PBAs. The rise in resin viscosity due to TEM can enhance the stability of bubble production, leading to the formation of more uniform and smaller pores. These results indicate that TEM proves to be highly efficient among the PBAs employed to decrease the weight and pore size of PU-PDMS hybrid foams.