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The effect of several nanofillers on thermal insulating properties of polyurethane rigid foams (PUR) was analyzed. The nanofillers used differ for chemical nature and aspect ratio. We used both pristine and organically-modified layered silicates (OMLS) and inorganic spherical nanopowders. The fillers were first dispersed in the polyol component through use of sonication; then doped polyol was mixed with isocyanate. The effect of polyol viscosity and nature (polyether or polyester) on the degree of the dispersion of the filler was also taken into account. The results obtained showed that the chemical nature and viscosity of polyols as well as the kind of compatibilization of the filler play a very important role in favoring the filler dispersion on a nanometer scale; the aspect ratio of the filler is instead not so important. When a good degree of dispersion is achieved, the thermal insulating properties of the foam can be slightly enhanced, as the fillers may act as nucleating agents. Some drawbacks were also found in using nanofillers: the fillers may bring in some cases an increase of the open cell content making the diffusion rate of the blowing agents faster thus leading to a worsening of thermal insulating properties during time.