# Experimental Evaluation and Numerical Simulation of the Thermal Performance of a Green Roof
## 3 Results and Discussion
### Experimental Investigation
Therefore, comparing the green and the original roofs U-values, a percentage difference of about \(-55\%\) can be highlighted. Regarding the middle season, the time range chosen to be analyzed was from 10 th of June to 26 th of June. During this period, the outdoor temperatures range was between 5.53 and 26.23 \({}^{\circ}\)C. The heat fluxes, indoor temperatures and internal surface temperatures recorded during middle season are reported in Figure 7, where Figure 7a refers to the green roof and Figure 7b to the original roof. The graphical representation of heat fluxes and temperatures is the same as that used in Figure 5. In order to provide a straightforward comparison among the acquired heat fluxes, the upper and lower limits of the secondary axes are the same. Even if the average indoor temperatures and the average internal surface temperatures are not significantly different between the two roof configurations, during this period it is possible to notice a more stable thermal behavior of the green roof. As a matter of fact, it is possible to notice that the heat fluxes along time of the original roof show much higher fluctuations than those observed for the green roof (the heat flux of the original roof ranged between \(-32.56\) and \(19.04\) W/m\({}^{2}\)). Due to the thermophysical properties of the material used for the roof, in this case the external surface temperatures showed different values. The external surface temperatures of the original roof reached values above \(40\)\({}^{\circ}\)C, while the external temperatures of the roof-lawn system reached values below \(22\)\({}^{\circ}\)C.  
Regarding the summer season, the time range chosen to be analyzed was from 10 th of June to 26 th of June. This period was selected due to the high temperatures recorded. During this period, the outdoor temperatures range was between 16.0 and 33.4 \({}^{\circ}\)C.  
The heat fluxes, indoor temperatures and internal surface temperatures recorded during summer are reported in Figure 8, where Figure 8a refers to the green roof and Figure 8b to the original roof. In this case, the graphical representation of heat fluxes and temperatures is the same as that used in Figure 5. In order to provide a straightforward comparison among the registered heat fluxes, the upper and lower limits of the secondary axes are the same. It is possible to observe a more stable thermal behavior of the green roof also during summertime. Considering the green roof, the average indoor temperatures and the average internal surface temperatures are equal to 24.80 and 24.60 \({}^{\circ}\)C, respectively. Figure 6: Green roof and original roof U-values. Figure 8: _Cont._  
Figure 7: Heat fluxes, indoor temperatures and internal surface temperatures registered during middle season for the green roof (**a**) and the original one (**b**). On the contrary, for the original roof, an average indoor temperature of 28.93 \({}^{\circ}\)C and an average internal surface temperature of 29.87 \({}^{\circ}\)C can be noticed. Regarding the heat fluxes, in the green roof a mean value of 2.19 W/m\({}^{2}\) was registered. On the contrary, in the original roof, a negative mean value of \(-\)1.86 W/m\({}^{2}\) highlighted the incoming direction of the heat flows. Analyzing the external surfaces' temperatures, the original roof reached 50 \({}^{\circ}\)C, while the external temperatures of the roof-lawn system reached values below 30 \({}^{\circ}\)C. As mentioned before, the influence of the thermophysical characteristics of the materials used played a fundamental role. The solar radiation was absorbed by the original roof covering tiles due to their high absorptance coefficient. This did not happen for the green roof, characterized by evapotranspiration phenomena. As previously mentioned, the dynamic performance of the roofs can be evaluated by means of the heat waves' phase shift. PS and DF parameters were obtained analyzing the summer months, when the solar radiation provides the highest influence.