## 2 Materials and Methods
### Measurement Setup
#### 2.1.1 HFM Average Method  
The _HFM_ average method is a non-destructive method for measuring the thermal transmittance of opaque, plane building elements perpendicular to the heat flow with no significant lateral heat flow. It consists of monitoring the heat flux rate passing through the element (\(q\)) and the indoor (\(T_{IN}\)) and outdoor (\(T_{OUT}\)) environmental temperatures [76]. The _HFM_ standardized average method is widely used. It is considered to estimate steady-state conditions well, by monitoring the heat-flow rate and temperatures over an adequately extended duration. According to this method, defined in  
Figure 1: Flowchart of the research methodology.  
ISO 9869-1:2014 [76] and described in [PERSON] et al. [75], the thermal transmittance and its combined standard uncertainty (\(u_{c}\) (\(U\))) can be obtained by Equations (1) and (2):  
\[U_{\text{{mes HFM}}}\bigg{[}\frac{W}{m^{2}\cdot K}\bigg{]}=\ \frac{\sum_{j=1}^{n}q_{j}}{ \sum_{j=1}^{n}\left(T_{INj}-T_{OUT}\right)} \tag{1}\]  
\[u_{c}\,{}_{\text{{HFM}}}^{2}(U)=\Big{(}\frac{\delta U}{\delta q} \Big{)}^{2}\cdot u_{c}{}^{2}(q)+\Big{(}\frac{\delta U}{\delta T_{IN}}\Big{)}^{2 }\cdot u_{c}{}^{2}(T_{IN})+\Big{(}\frac{\delta U}{\delta T_{OUT}}\Big{)}^{2} \cdot u_{c}{}^{2}(T_{OUT})=\] \[=\Big{(}\frac{1}{T_{IN}-T_{OUT}}\Big{)}^{2}\cdot u_{c}{}^{2}(q)+ \Big{(}\frac{-q}{(T_{IN}-T_{OUT})^{2}}\Big{)}^{2}\cdot u_{c}{}^{2}(T_{IN})+ \Big{(}\frac{q}{(T_{IN}-T_{OUT})^{2}}\Big{)}^{2}\] \[\cdot u_{c}{}^{2}(T_{OUT})\]  
where \(q\) is the density of the heat-flow rate per unit area in [W/m\({}^{2}\)], \(T_{\text{IN}}\) is the environmental indoor temperature in [K], \(T_{OUT}\) is the environmental outdoor temperature in [K], the index j enumerates the individual measurements, \(u(q)\) is the uncertainty associated with the heat-flow-rate-measuring equipment, \(u(T_{IN})\) is the uncertainty associated with the interior environmental temperature measuring equipment, and \(u(T_{OUT})\) is the uncertainty associated with the exterior environmental temperature measuring equipment.  
The HFM average method is a standardized method described in ISO 9869-1:2014 [76]. This standard establishes that the duration of the test depends on the values obtained during the course of the test. In this sense, the standard defines three conditions that must be met simultaneously to end the monitoring process: (a) the test must have a minimum duration of 72 h or more, (b) the thermal transmittance obtained when the test finalized must not differ more than 5% from the value obtained 24 h before, and (c) the thermal transmittance obtained by analyzing data from the first time period of _INT(2-DT/3)_ days must not differ more than 5% from the value obtained from the data for the last period of the same duration, where \(DT\) is the lasting of the test in days.  
Moreover, the variability of results (random error \(e(\%)\)) was estimated with a 95.4% confidence level, according to [PERSON] et al. [77]. The coefficient of variation of the resulting U-values obtained following the _HFM_ average method was calculated with Equations (3) and (4) [77, 78]:  
\[CV_{\text{{HFM}}}[\%]=\sqrt{\frac{\sum_{i}^{n}\left(U\_{HFM\_mes_{i}}-\overline {U\_{HFM\_mes}}\right)^{2}}{n-1}}\times\frac{1}{\overline{U\_{HFM\_mes}}} \times 100 \tag{3}\]  
\[e(\%)=2\times CV_{\text{{HFM}}}(\%) \tag{4}\]  
where \(U\_{HFM\_mes_{i}}\) is the value of thermal transmittance of the facade in the cycle i, \(\overline{U\_{HFM\_mes}}\) is the average of _Um-values_ of the facade during \(n\) cycles, and n is the number of cycles (n = 3). According to the ASTM C1155-95 Standard [78], the coefficient of variation is expected to be less than 10% for the _HFM_ average method.