Maintaining thermal comfort in buildings has become a big challenge in developing countries. Cool roof or high reflective/emissive roof reduces absorbed building solar energy, roof surface temperature to reduce energy consumption and maintain thermal comfort. However, the impact on buildings thermal performance located in hot-dry climate and dusty conditions is not well-known. The study presents an experimental and numerical simulation of two identical non-air-conditioned rooms to estimate the impact of cool roof on building performance in dusty conditions of Makkah, Saudi Arabia in the summer season. The results show that the indoor thermal comfort was improved by decreasing the average indoor temperature, underneath roof surface and outside roof surface temperatures by 3.7 °C, 6 °C and 8.9 ° C, respectively. The use of cool roof reduces the energy consumption required for building cooling in clear conditions by about 50 kWh/m2/year. However, dust accumulation decreases the impact of cool roof impact and increases the energy consumption in cooling the building. The study concludes that with the proper roof cleaning process, cool roof is an effective method to improve the indoor thermal comfort and reduce building energy consumption in hot-dry and dusty climates.