Green and cool roofs' urban heat island mitigation potential in tropical climate

Urban heat island (UHI) can significantly affect building's thermal-energy performance. Urban materials absorb solar and infrared radiation and the accumulated heat is dissipated in the atmosphere increasing further the air temperature. Roofs are envelope components which with advanced solutions such as cool roofs or green roofs can provide significant energy savings in air-conditioned buildings and improved indoor thermal conditions. By means of dynamic simulations in EnergyPlus software a numerical comparative analysis between these two solutions was done in a tropical climate like Singapore's, taking into account climatological, thermal, optical and hydrological variables. Simulations of a typical summer day in Singapore were assessed to determine (i) UHI reductions for different green/cool roof scenarios; (ii) the diurnal heat fluxes dynamics and (iii) the buildings' thermal energy reduction for the investigated cases. The results show that during peak periods (9 am to 5 pm) cool roofs reduce heat gain by about 0.14 KWh/m(2) (8%) and green roofs mitigate considerably less to about 0.008 KWh/m(2) (0.4%). And for the whole of a summer design day, cool and green roof reduces heat gain by 15.53 (37%) and 13.14 (31%) KWh/m(2) respectively. The numerical simulation results confirm that an appropriate selection of roof materials contribute to the reduction of the negative effects of UHI but experimental data for air-conditioned buildings are yet to be carried out.