Nanodispersions of polyurethane components with a three dimensional gelled network of filler is formed by the addition of a very small quantity of vapor grown carbon nanofiber (CNF). Reactive foaming of these nanodispersions produced polyurethane foams with superior properties. The kinetic profiles of polymerization and foaming reactions are not affected by the addition of filler. The cellular structure of nanocomposite foam becomes more uniform. Thermal conductivity and fire retarding tendency of the nanocomposite foams are superior at a very low loading of filler (1% by weight in components which corresponds to <0.5% by weight in foam). The filler did not open cells or induce structural defects. (C) 2010 Elsevier Ltd. All rights reserved.