This study investigates the potential of naturally occurring biodegradable fibers for use as building thermal insulation. The use of biodegradable building thermal insulation would alleviate the environmental problems presently associated with the disposal of currently used man-made non-biodegradable thermal insulations. The apparent thermal conductivity(lambda) for biodegradable coconut and sugarcane fiber were investigated in accordance with ASTM C 518 over the density ranges 40 kg/m3 to 90 kg/m3 and 70 kg/m3 to 120 kg/m3 for the test temperature ranges 13.2oC to 21.8oC and 18oC to 32oC, respectively. The experimental data were used to determine empirical equations for lambda variation with density and temperature for both coconut and sugarcane fiber. Comparison of l at 24oC for coconut and sugarcane fiber were made with seven different conventional insulation from published data. The results indicated that the minimum lambda for both coconut and sugarcane fiber are within the range normally associated with building thermal insulation. The lambda variation with density and mean temperature for both coconut and sugarcane fiber were consistent with the behavior of loose-fill thermal insulation.