Painting Roofs White: An Affordable Strategy to Improve Indoor Comfort in African Buildings

Corrugated metal sheets are the most widely used roofing solution in Africa. This low cost solution prevents rain from pouring inside the building, but lacks any thermal insulation and reaches very high temperatures by absorption of solar radiation. A solution to prevent indoor overheating is to paint the outer covering of the roof with white coatings having a high solar reflectance to prevent heating, and a high emissivity to dissipate accumulated heat. The lower temperature of the roof further improves the indoor comfort through the reduction of the thermal radiation from the ceiling. The present study estimates the effectiveness of this solution, through the study case of a factory building located in Rwanda. A simulation model was generated including the geometrical, constructive and operational characteristics of the building. Different roof coatings in use for different periods were characterized in Rwanda, using portable equipment for measuring reflectance in the 380–1700 nm wavelength range and directional emittance in the 1.5–21 μm range. For conventional paints, the thermo-optical parameters range from those of a red roof with solar reflectance (SR) of 0.290 and hemispherical total emittance (HTE) of 0.740 and those of a grey one with SR of 0.045 and HTE of 0.877. Among the new white paints, including both local and European products, the highest SR is 0.887 for a coating with HTE of 0.872. The lowest reflectance (0.647) corresponds to the paint with the highest emittance (0.906) related to a content of siliceous aggregate to improve acoustic performance. The results of energy simulations considering the thermo-optical properties measured in situ show that white paints achieve significant energy savings with respect to conventional ones, allowing, in some cases, to avoid the use of air conditioning altogether to keep the temperature inside the factory below 25 °C in the hottest month.