Hygrothermal Performance of Cool Roofs Subjected to Hot and Humid Climates

Cool reflective (white) roofs use bright surfaces to reflect a significant portion of the incident short-wave solar radiation, which lowers the surfacetemperature compared to conventional (black) roofs. As such, white roofs help reduce the urban heat island effect during the summer. However, thequestions are: (a) do white roofing system lead to moisture-related problems in hot and humid climates? (b) what is the amount of energy savings as resultof using white roofing systems instead of black roofing systems of same insulation amount, and (c) what is the reduction in the amount of insulation in awhite roofing system having the same energy performance as the black roofing systems? To answer these questions, numerical simulations were conductedfor a roofing system that is commonly used in Saudi low-rise buildings in order to assess and compare the energy and hygrothermal performance of whiteand black roofing systems. The white and black roofing systems are subjected to hot and humid climates of the Saudi’s Eastern Province. The indoorconditions are taken based on ASHRAE recommendations for conditioned space. The results showed that black roof always performed with lowermoisture than white roof, and no risk of condensation and mould growth occurred in these roofing systems. Also, the results showed that white roofexperienced an increase in the heating energy load compared to that with a black roof. However, the decrease in the cooling energy load for a white roofwas typically much greater than the increase in the heating energy load. The main outcome of this study has shown the capabilities of using reflectivematerials with different short-wave solar absorption coefficients for enhancing the energy performance of roofing systems and/or reducing the amount ofinsulation that resulted in same energy performance as black roofing systems. The results of this study can be used in future for upgrading the SaudiBuilding Code so as to allow less roof insulation if reflective roofing system is installed.Keywords: White (cool) roof; black roof; green roof; moisture accumulation; hygrothermal performance; short-wave solar absorption coefficient.