## 3 Numerical Simulations
### Model Accuracy Verifications and Validation
Making use of two load-bearing LSF wall prototypes ((1) a partition [20] and (2) a facade, as illustrated in Figure 4), their surface-to-surface thermal resistances were measured under controlled lab conditions. The LSF wall prototypes were placed between a hot box (heated by a thermal resistance up to around 40 \({}^{\circ}\)C) and a cold box (cooled by a fridge down to around 5 \({}^{\circ}\)C). These kinds of \"climatic chambers\" allowed us to implement a nearly steady-state temperature difference between the tested LSF walls of around 35 \({}^{\circ}\)C, for a mean average temperature of the tested LSF walls equal to 22.5 \({}^{\circ}\)C. To mitigate the eventual flanking lateral heat losses, the perimeter for the tested LSF wall was covered with polyurethane foam insulation (80 mm thick), while it was supported by a layer of more rigid thermal insulation (100 mm thick XPS). The heat transfer across the tested LSF wall samples was monitored by making use of thermocouples (TC) to measure temperatures, and Heat Flow Meters (HFMs) to measure heat flux. These measurements were performed making use of the Heat Flow Meter method described in standard ISO 9869-1 [7]. However, to improve the measurements' accuracy and reduce the test duration, two HFMs were used simultaneously, for each sensor location, at both cold and hot wall surfaces, as suggested by [PERSON] and [PERSON] [50]. Figure 4: Tested load-bearing LSF facade prototype: (**a**) steel frame; (**b**) sensors’ position (cold surface) [35]. \begin{table}
\begin{tabular}{l l c c} \hline \hline  & & \multicolumn{2}{c}{_U-Value (W/(m\({}^{2}\).K))_} \\ \cline{3-4}
**Approach** & **Tool** & **Partition** & **Facade** \\ \hline
**Analytical** & ISO 6946 & 0.3182 & 0.2246 \\ \hline \multirow{4}{*}{**Numerical**} & THERM & 0.3182 & 0.2246 \\  & FLIXO & 0.3182 & 0.2246 \\  & PSIPLAN & 0.3182 & 0.2246 \\ \cline{1-1}  & ANSYS & 0.3182 & 0.2246 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Thermal transmittance calculated for the simplified partition and facade LSF walls, assuming homogeneous layers (i.e., without a steel stud). For each wall configuration, three tests were performed; for each test, the sensors were placed at different height locations, as illustrated in Figure 4b. Moreover, these measurements were performed along two vertical lines: (1) in the middle of the LSF wall prototype, i.e., near the vertical steel stud, and (2) between the steel studs, i.e., in the middle of the insulation cavity. This test setup allowed us to monitor the two typical thermal behaviour zones on an LSF wall: (1) a region with higher heat flux (HFM1), originated by the steel stud's thermal bridge, and (2) the remaining part of the wall with smaller heat flux (HFM2), due to the cavity thermal insulation. More details about these measurements could be found in previous research works published by the authors [20; 35]. Besides the measured conductive thermal resistances, displayed in Table 6 as \(R_{cond}\), it was possible to also measure the average surface thermal resistances for each side of the tested LSF walls. The measured values were, for the partition LSF wall, \(R_{\text{si}}\) = 0.10 m\({}^{2}\cdot\)K/W and \(R_{\text{se}}\) = 0.11 m\({}^{2}\cdot\)K/W, and for the facade, \(R_{\text{si}}\) = 0.10 m\({}^{2}\cdot\)K/W and \(R_{\text{se}}\) = 0.13 m\({}^{2}\cdot\)K/W. Notice that the internal surface resistance (\(R_{\text{si}}\)) was measured on the hot side of the tested wall, while the external one (\(R_{\text{se}}\)) was measured on the cold side of the same wall. Figure 5 displays the measured heat flux values and surface temperatures over time (24 h) for the facade LSF wall, when the sensors were located at the highest height, as illustrated in Figure 4b. The measured values recorded at two different sensor locations are displayed in these plots: (1) near the steel stud (Figure 5a), and (2) in the wall cavity (Figure 5b). It is clearly visible that after a couple of hours, the values stabilize, confirming the intended steady-state regime.