*3.1. 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 di fferent climatic conditions.

Regarding the winter season, the time range chosen to be analyzed was from 1st of December to 31st 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 ◦C.

The data registered during this period are reported in Figure 5, where Figure 5a refers to the green roof and Figure 5b to the traditional roof (distinguished in the label as "*ORIGINAL*"). In Figure 5a, 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 5.** Heat fluxes, indoor temperatures and internal surface temperatures registered during winter for the green roof (**a**) and the original one (**b**).

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 ◦C, respectively. On the contrary, for the original roof, an average indoor temperature of 8.95 ◦C and an average internal surface temperature of 8.58 ◦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 <sup>W</sup>/m2, 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.

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/(m2K) was obtained for the green roof. On the other hand, a value of 3.021 W/(m2K) was found for the original roof. Therefore, comparing the green and the original roofs U-values, a percentage difference of about −55% can be highlighted.

**Figure 6.** Green roof and original roof U-values.

Regarding the middle season, the time range chosen to be analyzed was from 10th of June to 26th of June. During this period, the outdoor temperatures range was between 5.53 and 26.23 ◦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 <sup>W</sup>/m2). 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 ◦C, while the external temperatures of the roof-lawn system reached values below 22 ◦C.

Regarding the summer season, the time range chosen to be analyzed was from 10th of June to 26th 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 ◦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 ◦C, respectively.

**Figure 7.** Heat fluxes, indoor temperatures and internal surface temperatures registered during middle season for the green roof (**a**) and the original one (**b**).

**Figure 8.** *Cont*.

**Figure 8.** Heat fluxes, indoor temperatures and internal surface temperatures registered during summer for the green roof (**a**) and the original one (**b**).

On the contrary, for the original roof, an average indoor temperature of 28.93 ◦C and an average internal surface temperature of 29.87 ◦C can be noticed. Regarding the heat fluxes, in the green roof a mean value of 2.19 <sup>W</sup>/m<sup>2</sup> was registered. On the contrary, in the original roof, a negative mean value of −1.86 <sup>W</sup>/m<sup>2</sup> highlighted the incoming direction of the heat flows. Analyzing the external surfaces' temperatures, the original roof reached 50 ◦C, while the external temperatures of the roof-lawn system reached values below 30 ◦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. The average PS of the green roof was equal to 6 h and 50 min, much higher than the original roof, with a PS of 3 h and 30 min. In terms of decrement factor, the green roof showed a DF equal to 0.19, while the original roof was characterized by a DF equal to 0.37. It is essential to have a thermal wave phase shift of at least 8 h, or of no less than 10 h in areas characterized by a hot summer. The phase shift value, often neglected during the design phase, is surely critical for determining summer thermal comfort, with effects in terms of energy savings. In summer, the heat stored by the envelope is gradually released inside the rooms with a time delay that attenuates and postpones the heat peak, thus reducing the cooling energy needs. Here, the roof-lawn system is simply placed on a reinforced concrete slab which was not optimized to obtain the best performance of the roof. For this reason, future developments will concern the optimization of the structural part of the roof, designing a stratigraphy able to work with the overlying roof-lawn.

## *3.2. Equivalent Thermophysical Properties*

On-site measurement data were used for generating a model through Comsol software. Thus, different equivalent thermophysical properties were iteratively tested and the internal surface temperatures were simulated. The search for equivalent thermophysical properties aimed at finding the best reproduction of the green roof behavior, trying to ge<sup>t</sup> the best match between internal measured and simulated surface temperatures. The best matching is shown in Figure 9a, where the comparison between measured (green line) and simulated (black dotted line) internal surface temperatures is reported.

**Figure 9.** Green roof equivalent model. Comparison between measured and simulated internal surface temperature: summer (**a**); middle season (**b**); winter (**c**).

During summer, the overlap between experimental and simulated data was found, setting the following equivalent parameters: thermal conductivity equal to 0.4 W/(mK), specific heat capacity equal to 840 J/kgK and mass density equal to 1100 kg/m3. The equivalent thermophysical properties mentioned before were also tested during middle season (April) and winter (December), when the external climatic conditions are different (see Figure 9b,c). The EF coefficients were calculated in all the mentioned seasons: during summer, EF = 0.96 was obtained; during the middle season, EF = 0.96 was computed; and, finally, during winter, EF = 0.93 was found.

The EF values are all higher than 0.9, satisfying the desired condition reported in the methodology section (see the flow-chart in Figure 4). The equivalent thermophysical properties can therefore be used in building energy simulation tools.

## *3.3. Building Energy Simulations*

The equivalent thermophysical properties defined in the previous section were used as inputs in the building energy software, in order to simulate the influence of the green roof on the annual energy needs. As mentioned in the methodology section, a simple detached building was created by means of TRNSYS software.

Comparing the effects related to the installation of the green roof respect to the original one, Figure 10 shows the difference between the annual heating and cooling energy needs. It is possible to observe a percentage difference equal to −21.14% for heating and −34.70% for cooling.

**Figure 10.** Comparison between annual heating and cooling energy needs.

In addition, the energy simulations were performed, taking into account different climatic conditions. Following the climatic classification reported in [28], Table 2 lists the heating and cooling energy needs obtained using four different weather data: Rome, Manaus, Abu Dhabi and Moscow were considered as references for mild temperate, tropical, dry and snowy conditions, respectively. Comparing the green and the original roofs, the values reported in Table 2 always obtain negative percentage variations, highlighting that the green roof could be applied under different climatic conditions, showing positive effects during winter and summer.


**Table 2.** Comparison among heating and cooling energy needs in different climatic conditions.

Starting from the obtained results, it is possible to affirm that the green roof has a good insulating effect, reducing the energy needs of the building in cold seasons and keeping it cooler under warm climatic conditions (due to its inertial behavior). It is worth mentioning that a green roof can also protect the roof's materials from temperature fluctuations, ensuring the transpiration of the layers.
