Many theoretical and empirical models exist to predict the effective thermal conductivity of polymer foams. However, most of the models only consider the effect of porosity, while the pore size effect is ignored. The objective of this study is to understand the effect of pore size on the thermal conductivity of polymer foams, especially when it reduces to the micro and nanometer scales. A wide range of pore sizes from 1 nm to 1 mm were studied in conjunction with the porosity effect using finite element analysis and molecular dynamics simulation methods. Experimental data was used to validate the modeling result. It is shown that the pore size has significant effect on thermal conductivity, even for microcellular and conventional foams. The contribution of heat conduction through air is negligible when pore size is reduced to the micrometer scale. The extremely low thermal conductivity of nanofoams is attributed to extensive diffusive scattering of heat carriers in the solid phase of polymer matrix, instead of air. This study provides quantitative understanding of the pore size effect on thermal conductivity of polymer foams. It is also shown that polyetherimide (PEI) nanofoams could have a thermal conductivity as low as 0.015 W/m-K. POLYM. ENG. SCI., 2013. © 2013 Society of Plastics Engineers