This paper is focused on demonstrating that it is possible to reduce the thermal conductivity of polymeric foams by reducing the average cell size below the micron. For this purpose, a wide set of samples with cell sizes from 90 nm to 100 gm and relative densities from 0.12 to 0.6 has been produced and analyzed. In addition, a characterization procedure that allows identifying independently some of the heat transfer mechanisms in polymeric foams has been developed. As a result, it has been demonstrated that Knudsen effect takes place in nanocellular polymeric foams, being this effect very positive to reduce the overall heat transfer. Through the understanding of the underlying mechanisms it has possible to model the thermal conductivity behavior of these materials in the entire range of cell sizes, and overall porosities. It has been proved that the reduction of the cell size has an effect on the thermal conductivity through the solid phase, this contribution is reduced due to an increment of the tortuosity of the cellular structure and/or a confinement effect in the polymeric matrix. (C) 2014 Elsevier Ltd. All rights reserved.