## 4 Conclusions  
With the help of efficient insulation systems for building, energy efficiency can be increased. In particular, natural insulation materials can be favorably used in buildings in combination with standard panels in order to propose sustainable solutions and healthy indoor environment. Not only natural scraps, but also waste components from industrial and cultivation processes can be successfully used to these aims. In this context, four kinds of recycled waste materials were used in order to compose insulation panels for building opaque envelope and their properties were analyzed in order to find the best solutions by considering both thermal behavior and environmental impact. These panels were fabricated with cold-water-based polyurethane glue by means of cold pressing. A cross-comparative analysis is the core part of the paper: it was carried out in order to select the best compromise solutions in terms of thermal and environmental analysis. It was observed that cork has the best thermal performance (lower thermal conductivity) and the panels with higher weight ratio of cork are the best ones. Nevertheless, it has a not very low environmental impact with respect to rice husk and coffee chaff. Therefore, the best sustainable solutions are the panels composed of 37% cork and 63% rice husk, 51% cork and 49% coffee chaff, or 58% cork, 21% rice husk, and 21% coffee chaff, being about the same the thermal conductivities (0.063-0.068 W/mK) but showing the lower environmental impact. This kind of analysis has proven to be very effective for fixing of \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & \begin{tabular}{c} **GWP** \\ **(kg CO2 eq)** \\ \end{tabular} & \begin{tabular}{c} **\(\lambda\) (W/mK)** \\ **(kg CO2 eq)** \\ \end{tabular} & \begin{tabular}{c} **\(\lambda\) (W/mK)** \\ **(kg CO2 eq)** \\ \end{tabular} & \
\begin{tabular}{c} **GWP** \\ **(kg CO2 eq)** \\ \end{tabular} &
\begin{tabular}{c} **\(\lambda\) (W/mK)** \\ **(kg CO2 eq)** \\ \end{tabular} \\ \hline \multirow{2}{*}{1} & \multicolumn{2}{c}{100\% RH} & \multicolumn{2}{c}{37\% COR + 63\% RH} & \multicolumn{2}{c}{100\% COR} \\ \cline{2-6}  & 1.11 & 0.07 & 2.17 & 1.11 & 0.07 & 0.055 \\ \hline \multirow{2}{*}{2} & \multicolumn{2}{c}{100\% CC} & \multicolumn{2}{c}{51\% COR + 49\% CC} & \multicolumn{2}{c}{100\% COR} \\ \cline{2-6}  & 0.56 & 0.076 & 2.16 & 0.56 & 0.076 & 0.055 \\ \hline \multirow{3}{*}{3} & \multicolumn{2}{c}{100\% ELT} & \multicolumn{2}{c}{100\% COR} & \multicolumn{2}{c}{-} \\ \cline{2-6}  & 3.61 & 0.135 & 3.07 & 3.61 & 0.135 & \\ \hline \multirow{3}{*}{4} & \multicolumn{2}{c}{50\% RH + 50\% ELT} & \multicolumn{2}{c}{30\% COR + 35\% ELT + 35\% RH} & \multicolumn{2}{c}{100\% COR} \\ \cline{2-6}  & 2.36 & 0.102 & 2.65 & 2.36 & 0.102 & 0.055 \\ \hline \multirow{3}{*}{5} & \multicolumn{2}{c}{50\% RH + 50\% CC} & \multicolumn{2}{c}{58\% COR + 21\% RH + 21\% CC} & \multicolumn{2}{c}{100\% COR} \\ \cline{2-6}  & 0.83 & 0.073 & 2.18 & 0.83 & 0.073 & 0.055 \\ \hline \multirow{3}{*}{6} & \multicolumn{2}{c}{33\% RH + 33\% CC + 33\% ELT} & \multicolumn{2}{c}{30\% COR + 23\% RH + 23\% CC + 23\% ELT} & \multicolumn{2}{c}{100\% COR} \\ \cline{2-6}  & 1.76 & 0.094 & 2.46 & 1.76 & 0.094 & 0.055 \\ \hline \hline \end{tabular}
\end{table} 
Table 8: Global Warming Potential and Thermal Conductivity values obtained for the best identified mixture (in the grey column the best mixing ratios are reported).