Rigid polyurethane foam is a widely used insulation material in various industrial fi elds. Enhancing its insulating performance

often involves controlling pore size to reduce radiative conductivity and, in some cases, decrease gas thermal conductivity

through the Knudsen eff ect. While numerous studies have addressed reducing the pore size of thermoplastic polymer foam

to the nano-scale, limited research has focused on rigid polyurethane foam. Although some studies have shown that pore

size changes with the addition of nucleating agents, a comprehensive understanding of the mechanisms governing pore size

remains elusive. Therefore, this study investigates the factors determining the pore size of rigid polyurethane foam. Our

fi ndings confi rm that the size of air bubbles formed during the blending of prepolymers has the most signifi cant impact on

pore size. Furthermore, we demonstrate that regulating the size of air bubbles before the urethane reaction allows for control

over the fi nal pore size. Experimental details substantiating these fi ndings are presented.