## 3 Results
### Results at Room Level  
The influence of low emissivity coatings that were applied to the interior side of one external wall turned out a slight, but not marginal contribution for the reduction of the heat losses through the external walls (Figure 11) that can be summarised, as follows:  
* the application of a100 paint (\(\varepsilon\) = 0.41) show a reduction of \(\sim\)35% of the surface heat transfer coefficient h\({}_{\rm i}\) both in measurements and in the results that were obtained by applying the ISO 6946:2007 (Table 5); and,
* the application of a50 paint (\(\varepsilon\) = 0.60) show a reduction of \(\sim\)19% of the surface heat transfer coefficient hi, similar results with a reduction of \(\sim\)21% was obtained calculating h\({}_{\rm i}\) (predicted) according to ISO 6946:2007 (Table 5);  
As expected, the reduction of the surface heat transfer coefficient affects the resulting \(U\)-value. In this specific case, it has to be underlined that, for research purposes (reduce the heat flux measurement uncertainty), an unrealistic wall component (6 mm of MDF) was used for the experiments. For building components that are characterised by poor insulating properties, the surface heat transfer represents the main driver of the \(U\)-value and this explains the high reduction of \(U\)-value up to 20% (Figure 11b). However, this reduction is not realistic, since uninsulated wall components very rarely present \(U>1.5\)-2 W/m\({}^{2}\) K, which means that, in real situations, a reduction of the \(U\)-value higher than 10% cannot be achievable. For moderately insulated building components, i.e., characterised by \(U=0.5\) W/m\({}^{2}\) K, the achievable reduction decrease below 4%, as can be observed in Figure 12. These results are in line with those that were reported by [16]: in fact, considering that heat losses through opaque building envelope account only for a fraction of the heating energy consumption, solutions that claim energy savings above 5% (even with minor emissivity) should be considered with some skepticism. Furthermore, important shortcomings should be highlighted: an undesirable effect of the reduction of the surface heat transfer coefficient determines a lowering of the indoor surface temperature (up \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Sample Name** & **Al Paint Concentration** & \(\varphi_{\rm rad}\) & \(\varepsilon\) \\ \hline  & **[\%]** & **[W/m\({}^{2}\)]** & **[-]** \\ \hline a100 & 100\% & 11.54 & 0.410 \\ a80 & 80\% & 13.86 & 0.500 \\ a60 & 60\% & 14.42 & 0.522 \\ a50 & 50\% & 16.32 & 0.597 \\ a40 & 40\% & 19.60 & 0.732 \\ a20 & 20\% & 19.73 & 0.737 \\ a0 & 0\% & 23.04 & 0.881 \\ c (reference aluminium foil) & - & & 0.040 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Summary of the results obtained with the HFM procedure (5 mm thick air gap). Figure 10: Sample (a) mixtures: emissivity as a function of the aluminium paint concentration. to 1.5 \({}^{\circ}\)C of reduction was observed by applying paint with \(\varepsilon\) = 0.41); this effect, which is clearly represented in Figure 11a, can determine surface condensation and mould growth. For this reason, the feasibility of the intervention should be carefully verified before applying low emissivity paints in both uninsulated walls and in walls that still present moisture-related pathologies. As discussed, the application of low emissivity paints benefits of the reduction of heat losses through the opaque components. Nevertheless, another not negligible benefit can be the improvement  
Figure 11: (**a**) Internal measured surface heat transfer coefficient \(\mathbf{h}_{\mathrm{i}}\) and surface temperature \(\mathrm{T}_{\mathrm{si}}\) as a function of the wall surface emissivity \(\varepsilon\); and, (**b**) Measured _U_-values obtained with the different paints mixtures. Figure 12: Reduction of the _U_-value that can be achieved with a100 paint (\(\varepsilon\sim\) 0.41) and a50 paint (\(\varepsilon\sim\) 0.60) compared to the a0 paint (\(\varepsilon\sim\) 0.88) for different level of insulation.