The insulation efficiency of rigid polyurethane foams can be improved by increasing the cell isotropy of the polymer matrix and decreasing the heat radiation effect. The influence of the foam growth direction on the thermal properties of foams may be considerable, as shown by measurement of the thermal diffusivity. The modified Angstrom method was used for the measurement of the thermal diffusivity D of rigid polyurethane foams. The influence of sample preparation and the measurement conditions on the D-values were studied. It was found that to compare the insulation properties of polyurethane foams, the measurement conditions must be carefully chosen. The ability to use small-dimension samples makes this method very useful for the local measurement of the anisotropy of thermal properties of insulation foams.