## 2 Materials and Methods
### Analysis of Variability of Results  
The introduction of a PVC film during the monitoring process changes the theoretical U-value of the wall due to its inherent thermal resistance plus the thermal contact resistance between the film and the wall surface. The theoretical maximum thermal resistance associated with the inclusion of a PVC film is estimated according to Equations (1) and (2), as follows:  
\[R_{film\_max}\bigg{(}\frac{m^{2\cdot K}}{W}\bigg{)}=0.0005+\ \frac{15\cdot 10^{-6}}{0.12}=6.25\cdot 10^{-4} \tag{6}\]  
where the thermal contact resistance is usually between 0.000005 to 0.00005 m\({}^{2}\cdot\)K/W according to [PERSON] and [PERSON] [57], the thickness of the film is usually between \(7\times 10^{-6}\) to \(15\times 10^{-6}\) m, the thermal conductivity of the material is between 0.12 to 0.25 W/m\(\cdot\)K and \(R_{film\_max}\) is the maximum thermal resistance associated to the inclusion of a PVC film, considering the most insulation case of the film inclusion.  
The theoretical effect of the inclusion of a PVC film on the thermal transmittance is calculated according to Equation (1) for the two case studies and shown in Table 4. The deviations between the theoretical thermal transmittance (NF) and the thermal transmittance when using a PVC film during the installation of the heat flow meter (F) are practically imperceptible values, 0.14% in Case study 1 and 0.02% in Case study 2.  
To assess the use of a protective film during the installation of heat flow meters, two values of the measured thermal transmittance were calculated for each case study. The first value was obtained using a heat flow meter plate installed with a layer of thermal interface material directly on the wall surface (\(U_{mNF}\)) and the second value was obtained using a heat flow meter plate installed with a PVC film between the layer of thermal interface material and the wall (\(U_{mF}\)). To check the adjustment between both measured U-values, the absolute value of the relative difference between measured U-values were calculated using the following expression:  
\[Absolute\ value\ of\ the\ relative\ difference\ U_{m\ NF}-U_{m\ F}(\%)=\left|\frac{(U_{m\ NF}-U_{m\ F})}{U_{m\ NF}}\times 100\right| \tag{7}\]  
where \(U_{mNF}\) is the measured U-value of the wall using the dynamic method using an HFM installed without film and \(U_{mF}\) is the measured U-value of the wall using the dynamic method using an HFM installed with a PVC film between the layer of thermal interface material and the wall.  
The variability of results was analysed by comparing the relative difference between the measured thermal transmittances obtained according to Equation (7), with the theoretical effect of including a PVC during the monitoring process shown in Table 4.  
\begin{table}
\begin{tabular}{c c c c} \hline \multirow{2}{*}{**Case Study**} & **Theoretical U-Value** & **Theoretical U-Value F** & **Deviation between** \\  & **NF (W/m\({}^{2}\cdot\)K)** & **(W/m\({}^{2}\cdot\)K)** & **U-Values(\%)** \\ \hline Case study 1 & 2.20 & 2.19 & 0.14 \\ Case study 2 & 0.36 & 0.36 & 0.02 \\ \hline \end{tabular}
\end{table}
Table 4: Theoretical impact of the inclusion of a PVC film on the thermal transmittance in the two case studies.