Hybrid foams based on polyurethane and polysiloxane-domains were designed in terms of chemical structure and hierarchical morphology to improve the thermal insulation capability of conventional polyurethane foams. Polysiloxane-domains by sol-gel approach reacted with polyols to produce a polysiloxane-reactive-adduct, which completed its formation into polysiloxane aerogel-like structure during the polyurethane foaming process. Hybrids with 5wt% of the polysiloxane-domains exhibited a 22% reduction in thermal conductivity in comparison with pristine PUR. The chemical structure and the hierarchical morphology were studied by means of SAXS/WAXS and Si-29 and C-13 NMR. In particular the hybrid structure consisted of coarse polysiloxane-fractal-aggregates composed of silica nanoparticles embedded in the polyurethane matrix. Morphological analysis revealed a decrease of the cell size with the increase of the siloxane content ascribed to the nucleation effect of polysiloxane-domains during the foaming. The cell size reduction and the formation of hybrid-aerogel-like structures within the cell-walls are considered the key factors to reduce the thermal conductivity of hybrid foams. (C) 2014 Elsevier Ltd. All rights reserved.