## 2 Materials and Methods  
This paper presents an experimental study of a high thermal mass building, which was thermally insulated with two different materials to reach the thermal transmittance limit provided by the Italian national regulation. In order to investigate the behavior of different external insulating walls in a temperate climate, the measurement campaigns were carried out in summer 2018 and 2019. The monitored mockup is an experimental building made in lightweight concrete (LWC) with lightweight recycled glass aggregates: This concrete mix design allows reaching a mechanical resistance class of LC 8/9. LWC is an innovative material with good resistance properties and thermal characteristics, i.e., thermal conductivity \(\lambda\) of 0.27 W m\({}^{-1}\) K\({}^{-1}\), specific heat capacity \(c\) of 1000 J kg\({}^{-1}\) K\({}^{-1}\), and density \(\rho\) of 1200 kg m\({}^{-3}\). Thus, LWC represents an optimal solution to construct a monolithic building, as the one analyzed in this research, which is characterized by a high thickness \(t\) of 70 cm. The aim of this study is to compare the behavior of the building envelope with different external insulating layers. Therefore, the study focused on the performance of the Southeast-oriented facade, the most exposed to daylight, which were insulated with two different external insulating linings. Specifically, the initial wall composed of a single layer of LWC, named \"as build\" state (Wall 0, material Type 0), was divided in half: In the first half, a cork based lightweight plaster (Wall 1, material Type 1) was applied, in the second half, an XPS panel (Wall 2, material Type 2). The cork based lightweight plaster (Type 1) is characterized by a thickness \(t\) of 4 cm, a thermal conductivity \(\lambda\) of 0.037 W m\({}^{-1}\) K\({}^{-1}\), a specific heat capacity \(c\) of 1100 J kg\({}^{-1}\) K\({}^{-1}\), and a density \(\rho\) of 280 kg m\({}^{-3}\). An extruded polystyrene insulation, XPS board (Type 2), is featured by a thickness \(t\) of 8 cm, a thermal conductivity \(\lambda\) of 0.036 W m\({}^{-1}\) K\({}^{-1}\), a specific heat capacity \(c\) of 1450 J kg\({}^{-1}\) K\({}^{-1}\), a density \(\rho\) of 35 kg m\({}^{-3}\). In order to protect the external insulating layers, a smooth finishing plaster (Type 3) is applied, which is composed of natural lime binders, mineral expanded fillers, and pure limestone powder, grain size 0-0.6 mm. The lightweight smooth finishing plaster is characterized by a thickness \(t\) of 5 mm, a thermal conductivity \(\lambda\) of 0.128 W m\({}^{-1}\) K\({}^{-1}\), a specific heat capacity \(c\) of 1000 J kg\({}^{-1}\) K\({}^{-1}\), and a density \(\rho\) of 1165 kg m\({}^{-3}\). The external smooth finishing plaster has the same color and surface roughness for both analyzed surfaces. The two thermal insulation materials, Type 1 and Type 2, were chosen for similar thermal conductivity \(\lambda\), but different specific weight \(\rho\) and water vapor permeability \(\mu\). The thermal lightweight plaster, composed of natural hydraulic lime, has a higher porosity and breathability than the XPS panel, which is extruded polystyrene with a closed cell, characterized by good moisture resistance but less water vapor permeability. The thermo-physical properties of the selected materials are presented in Table 2. \begin{table}
\begin{tabular}{c c c c c}  & **Type 0** & **Type 1** & **Type 2** & **Type 3** \\ Thickness \(t\) & (cm) & 70 & 4 & 8 & 0.5 \\ Thermal conductivity \(\lambda\) & (W m\({}^{-1}\) K\({}^{-1}\)) & 0.27 & 0.037 & 0.036 & 0.128 \\ Specific heat \(c\) & (J kg\({}^{-1}\) K\({}^{-1}\)) & 1000 & 1100 & 1450 & 1000 \\ Density \(\rho\) & (kg m\({}^{-3}\)) & 1200 & 280 & 35 & 1165 \\ Thermal diffusivity \(a\) & (m\({}^{2}\) Ms\({}^{-1}\)) & 0.225 & 0.120 & 0.709 & 0.111 \\ Water vapor permeability \(\mu\) & - & 22.6 & 3 & 150 & 15 \\ Surface absorption coefficient \(\alpha\) & - & 0.6 & - & - & 0.3 \\ \end{tabular}
\end{table} 
Table 2: Thermal characteristics of the insulating materials.