The authors studied the thermal conductivity, water absorption and linear burn rate of natural fibre reinforced polymers (NFRP). High density polyethylene (HOPE) and degradable starch-based polymers were used as matrices. Discrete sisal (Agave Sisalacea) fibres were used as reinforcement. The effect of fibre content, moisture and void fractions on thermal conductivity was studied. From water absorption tests, it was determined that water diffusion in NFRPs corresponds to a Fickian process only in the early stages of diffusion. Diffusivities were obtained for that case. Linear burn rate experiments showed that sisal reinforced polymers burn twice as fast as unreinforced HDPE specimens, however the reinforced specimens show better structural integrity during burning than the unreinforced ones, probably due to an increase in viscosity and melt strength due to the presence of reinforcing fibres.