Numerical simulation of the dual effect of green roof thermal performance

•Nonlinear and coupled heat and mass transfer equations has been solved in green roof simultaneously.•Plant metabolism (including photosynthesis) has been considered for the first time.•Results indicate that presence of plants mitigate roof heat absorption significantly.•Green roof reduces indoor cooling loads and outdoor heat island effect simultaneously. Green roof is one of technologies applied in reducing energy consumption when cooling of a building is of concern. The heat and mass transfer in green roof is expressed by the complex system of coupled nonlinear differential equations which should be solved with respect to the four elements of air, plants, soil and structure, simultaneously. Numerical solution is applied through finite difference method. Over 40 models among 100 are adopted for the evaluation of thermal, physical and biological parameters in order to achieve best accuracy. Modeling of photosynthesis and plants’ response to environmental change is simulated for the first time in green roof modeling history. Grid independency has been checked for two most challenging regions; plants and soil. The average difference between numerical results and experimental measurements is below 8%, indicating a good agreement. The shading effect of plants and drought of soil layers due to solar radiation are shown. The results, obtained through comparison of green and concrete roofs indicate that the green roof represents 77% reduction in heat flux transmission and 13K reduction in air temperature at one meter above the roof compared to conventional roof, revealing a significant effect in reducing the energy consumption required for cooling the buildings and urban heat island effect simultaneously.