## 2 Materials and Methods
### Description of the Analysed Green Roof Installed in the Experimental Site  
Going from the indoor to the outdoor sides of the building, as shown in Figure 5, the green roof compound is composed of the following layers: a root barrier (with a waterproofing membrane), a drainage layer (made of a polyethylene geo-net, hot-coupled with a non-woven geotextile with filtering functions), a water storage layer (constituted by cushions filled with expanded perlite), a filter fabric (composed by a geotextile felt, 100% polypropylene calendered), a growing medium (which is a mixture of peat, lapillus, pumice, zeolite and slow releasing fertilizers and is infesting weeds free) and the vegetation layer.  
Figure 4: Trend of the solar radiation during the monitoring period.  
Figure 5: Sketch of the green-roof layers.  
Figure 3: Trend of the outdoor air relative humidity during the monitoring period.  
In order to also analyse the effects provided by different plant species and different thicknesses of the water storage layer on the green roof thermal behaviour, the roof was divided into six sectors, where three different Mediterranean autochthonous species (_Halimione Portalucoides_, _Rosmarinus Officinalis Prostratus_ and _Crithmun Maritiumum_) and two different thicknesses of the water storage layer (10 cm for plots 1, 2, 3 - P1, P2, P3 - and 15 cm for plots 4, 5, 6 - P4, P5, P6 -) were used according to the scheme reported in Figure 6.  
Table 2 reports, instead, the main physical parameters characterizing each of the green roofs' six plots. The data listed in Table 2 are the same as those used in [69].  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Layer** & **Element Type** & **Thickness [cm]** & **Plant Species** \\ \hline
1 & Structural support & 20 & _Hollow brick_ \\
2 & Waterproofing membrane & - & _Bituminous paint_ \\  & and root barrier & - & _Polyethylene geo-net, hot-coupled_ \\
3 & Drainage layer & 0.5 & _with a no woven geotextile_ \\  & & 10 (P1, P2, P3) & _Pillows filled with expanded perlite_ \\
4 & Water storage layer & 15 (P4, P5, P6) & _Geotextile felt, 100\%polypropylene_ \\
5 & Filter fabric & - & _Calendered_ \\
6 & Growing medium & 15 & _Pumice, lapillus and peat_ \\  & & & _Halimione Portalucoides_ \\  & & & (P1 and P6) \\
7 & Vegetation layer & - & _Rosmarinus Officinalis Prostratus_ \\  & & & (P2 and P5) \\  & & & _Crithmun Maritium_ (P3 and P4) \\ \hline \hline \end{tabular}
\end{table}
Table 1: Description of the layers constituting the green roof plot.  
Figure 6: Scheme of the plant species planted in the different sectors.  
In addition to thermal conductivity, density and specific heat, which characterize the thermo-physical behaviour of the soil, some other properties typical of the specific plants, which have an important impact on the heat exchanges through the green-roof, were also considered.  
Particularly, the \"leaf reflectance\" (dimensionless), that is, the ratio of the incoming light which is reflected by a leaf, and the \"leaf area index\"--LAI (m\({}^{2}\)/m\({}^{2}\))--defined as the one-sided green leaf area per unit of ground surface area. The latter, in particular, which has a great influence on the shading and transpiration effects, has a positive effect, especially during summer seasons; in fact, the higher the LAI, the higher the cooling reduction [50; 69; 70].  
As for the vegetation characteristics, Figure 7 shows how the _Halimione Portalacoides_ (both P1 and P6) and the _Rosmarinus Officinals Prostratus_ (only P5) reached full coverage (100%) in less than 12 months while the _Rosmarinus Officinals Prostratus_ in P2 achieved a maxim coverage of about 85% in the same period and the _Crithmum Maritium_ (both P3 and P4) did not accomplish more than 40%-60%, showing the difficulty of establishing it in the considered environment [52].