The aim of this study was to investigate the effect of three different nanofillers on the properties of rigid polyurethane foams, which were prepared by a one-step, laboratory-scale method from a two-component system at the ratio of NCO groups to OH groups equaled to 2. The reaction mixture consisted of the proper amounts of the commercial oligoether polyol, catalysts, water, nanoclays, and polymeric diphenylmethane diisocyanate. Three types of montmorillonite were used as clay component, i.e., montmorillonite modified by methyl tallow bis-2-hydroxyethyl ammonium (Cloisite((R))30B), a synthetic layered silicate (Laponite((R))RD), and an aluminium phyllosilicate absorbent, essentially impure clay consisting mostly of montmorillonite (Bentonite). Thermal properties of rigid polyurethane foams and nanocomposite foams were investigated by dynamical thermal analysis, thermogravimetry, oxygen index, and thermal conductivity measurements. It has been shown that the foams modified by selected nanofillers are characterized by high mechanical strength, as well as improved fire barrier properties. (c) 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 2272-2281, 2013