# Experimental Evaluation and Numerical Simulation of the Thermal Performance of a Green Roof
## 3 Results and Discussion  
### Experimental Investigation  
The results obtained during the measurement campaign are reported in this section. The monitoring was conducted from October 2018 to September 2019. The results of one year of measurements are a wealth of collected data. For the sake of brevity, partial data referring to winter and summer are presented here. This approach can be useful for readers to understand the thermal behavior of the green roof under different climatic conditions. Regarding the winter season, the time range chosen to be analyzed was from 1 st of December to 31 st of December. This period was selected due to the low temperatures recorded. During this month the outdoor temperature range was between 3.83 and 14.16 \({}^{\circ}\)C.  
The data registered during this period are reported in Figure 5, where Figure 5 refers to the green roof and Figure 5 to the traditional roof (distinguished in the label as \"_ORIGNAL_\"). In Figure 5, the heat flux (called _q_GREEN_) is represented by the continuous red line, the indoor air temperature (called _Ti_GREEN_) is represented by the dashed green line and the internal surface temperature (called _Tsi_GREEN_) is represented by the continuous green line. On the other side of the roof, the surface temperature registered under the green roof was called _Textrados_GREEN_ (orange dotted line) and the temperature of the upper layer of the green roof (measured inserting the sensor below a first small layer of soil) was called _Ts_roof-lawn_ (continuous line of light green color). Figure 4: Methodological approach flow-chart. On the other hand, in Figure 5b, the heat flux (called _q_ORIGINAL_) is depicted by the continuous red line, the indoor air temperature (called _Ti_ORIGINAL_) is represented by the dashed black line and the internal surface temperature (called _Tsi_ORIGINAL_) is depicted by the continuous black line. On the other side of the roof, the external surface temperature was called _Tse_ORIGINAL_ and it is represented by the orange dotted line. Observing both heat fluxes and temperatures, it is possible to notice a more stable thermal behavior of the green roof if compared with the original one. Referring to the green roof, analyzing the indoor temperatures and the internal surface temperatures, it is possible to obtain average values equal to 14.14 and 13.39 \({}^{\circ}\)C, respectively. On the contrary, for the original roof, an average indoor temperature of 8.95 \({}^{\circ}\)C and an average internal surface temperature of 8.58 \({}^{\circ}\)C can be observed. In order to provide a straightforward comparison among the acquired heat fluxes and temperatures, the upper limit of the ordinate axes was limited to the same value. Considering the green roof, the highest value for the heat flux was equal to about 14 W/m\({}^{2}\), a much lower value than those registered for the original roof. The different inertial behavior of the original roof allowed the observation of a strong fluctuation of heat flows, also showing negative values. Taking into account the external surface temperatures, it is possible to notice that the original roof is characterized by higher values. The main reason to explain these results is strictly related to the thermophysical characteristics of the employed materials. The roof covering tiles have a high solar radiation absorption coefficient, unlike green roofs, which are characterized by evapotranspiration phenomena. Figure 5: Heat fluxes, indoor temperatures and internal surface temperatures registered during winter for the green roof (**a**) and the original one (**b**). The registered data allowed us to calculate the steady-state thermal transmittance of the roofs. Applying Equation (2), the U-values shown in Figure 6 were computed; it can be noticed that, after a few days, both thermal transmittances tend to reach a stationary value. A thermal transmittance of 1.361 W/(m\({}^{2}\)K) was obtained for the green roof. On the other hand, a value of 3.021 W/(m\({}^{2}\)K) was found for the original roof.