Nowadays there is increasing activity in the development of materials from residual materials. Semi-structural materials made from waste materials are usually composites. Current use of these materials is minimal, but future economics can provide these materials with a competitive advantage. Polyurethane foam is an excellent material for various uses. It is made by foaming a liquid mixture of isocyanato -polyols in the presence of a blowing agent. This article contains an experimental study of improving the insulation of rigid polyurethane foam by the means of incorporating various amounts of each cotton stalks and corn stems. Four ratios (0.0, 10, 20, and 30 % by volume) of cotton stalks or corn stems were used with rigid urethane foam. The produced composite mad from rigid urethane foam and cotton stalks or corn stems were tested to measure bulk density, thermal conductivity and thermal resistivity. The results indicated that the thermal conductivity and thermal resistivity affected by the replacement ratios (%). The thermal conductivity was decreased with increasing replacement ratios. On the other hand, the thermal resistivity was increased with increasing replacement ratio (%). The results showed that the thermal properties of rigid urethane foam were improved with adding cotton stalks or corn stems. Corn stems showed the best, lowest, thermal conductivity values than cotton stalks. The lowest thermal conductivity was 0.01692 W/m.^oC at corn stems with replacement ratio of 30 (%) as well as, the highest thermal resistivity of 59.09 m.^oC/W at the same previous replacement ratio.