Building energy usage must be reduced because of the growing demand for energy worldwide. In order to reduce, heat gain through the roof, researchers are investigating insulating materials for roofing assembly. Using analytical techniques and Computational Fluid Dynamics (CFD) simulation, this study examines the thermal performance of coir-fiber reinforced natural rubber latex composites in a pitched roof assembly. Scaled-down test huts were used to perform experimental measurements outdoors in the first section of the investigation. CFD simulations were then used to confirm the findings. In the second section, heat flux transmission decrease and thermal conductivity (k-value) for various insulating schemes were assessed using the validated CFD model. The results demonstrated that coir fiber reinforced natural rubber composites are effective insulation materials appropriate for passive construction applications due to their low thermal conductivity (≈ 0.07–0.08 W/m·K).