# Experimental Evaluation and Numerical Simulation of the Thermal Performance of a Green Roof
## 2 Materials and Methods  
### On-Site Measurements  
The experimental survey was done by analyzing a roof-lawn system installed on the roof of a single-story building (see Figure 1a) situated in the countryside near the city of Latina (about 70 km south of Rome). This research aimed at evaluating the thermal characteristics of a green roof by means of an extended measurement campaign. The roof-lawn system is an innovative patent, characterized by the species of the Zoysia genus (distinguished by a slow growth and a typical wave effect). The realization of the water system only reintegrates the losses due to evapotranspiration. The level of maintenance required for the green roof is extremely low, since the routine maintenance is not required. Once it reached the maximum growth, the vegetation has a characteristic wave effect with average foliage heights that do not exceed 25 cm. The whole system components are reported in Figure 1b. The stratigraphy of the roof is shown in Figure 2. The green roof is characterized by different layers: on the upper part of the roof, a waterproofing sheath was installed to avoid water infiltration; over that, the green roof was built on a draining mat and an inorganic substrate. The structural part of the roof is composed of a reinforced concrete layer, with a thickness of about 8 cm. The overall thickness of the green roof is equal to 20 cm. Only a half of the building roof is characterized by the installation of the roof-lawn system, while the other half remained in the previous condition. The original roof is made of a reinforced concrete slab with a thickness of about 8 cm, covered with tiles. A waterproof membrane is installed on the concrete slab. The two rooms under the roofs are characterized by the same orientation and the same occupation rate. To assess the behavior of the green roof and define the characteristics of the system in terms of stationary and dynamic thermal performance, heat flow sensors, air temperatures probes and surface temperature sensors were installed, as shown in Figure 3. The measuring instruments' technical data are reported in Table 1.  
Figure 1: (**a**) Single-story building on which the green roof is installed and (**b**) roof-lawn whole system (an elaboration from [20]). Figure 2: Green roof stratigraphy. It is worth mentioning that the external surface temperature probe installed on the upper layer of the green roof was inserted below a first small layer of soil, in order to guarantee the best thermal contact. The schematic representation reported in Figure 3 could be misleading, as it represents the external surface temperature probe (blue circle) on the outermost part of the green layer. Of course, the upper part of a green roof is made of grass and surface temperatures cannot be measured. Therefore, the surface temperature probe was installed, placing the sensor in the upper part of the soil, where the grass grows. All sensors were connected to the data-loggers, recording heat fluxes, air and surface temperatures with a timestep equal to 10 min, for 24 h per day. The measurement campaign started in October 2018 and finished after one year, in September 2019. All the acquired data were used to calculate the thermal transmittances (also known as U-value or merely U) of the green roof and the original roof. Heat fluxes and air temperatures can be used for calculating the U-value by applying the following formula  
\[q=U(T_{i}-T_{e}) \tag{1}\]  
where \(q\) is the heat flux density, and \(T_{i}\) and \(T_{e}\) are the air temperature in the internal and external environment, respectively. According to the standard ISO 9869-1 [21], heat fluxes and air temperature values were used for calculating the stationary U-value of the roof by applying the average progressive method, following the formula  
\[U=\frac{\sum_{j=1}^{N}q_{j}}{\sum_{j=1}^{N}\Bigl{(}T_{ij}-T_{ej}\Bigr{)}} \tag{2}\]  
where \(N\) is the total registered samples.