## 4 Discussion and Conclusions  
This research assesses the influence of the heat flow meter plate non-perfect thermal contact on the accuracy of the in situ measurement of walls' thermal transmittance using the standardised heat flow meter method. The assessment considers usual practices in the installation of heat flow meters, consisting of the application of a PVC film to avoid damage to the wall in occupied buildings, in two case studies with different theoretical thermal transmittance values. The experimental campaign was designed to isolate measurement errors related to non-perfect thermal contact from those related to the measurement accuracy and the analysis of data. In situ measurements were conducted in the same conditions and equipment, with two heat flow meters: one was carefully installed with a layer of thermal interface material directly on the wall surface and the other was carefully installed by applying a film between the thermal interface material and the wall. Data were analysed in 24-h periods using the dynamic method. Finally, the variability of results was analysed by comparing the deviation between the measured thermal transmittances obtained from the two heat flow meters with the theoretical effect of including a PVC film during the monitoring process. Test completion results indicate that the minimum duration of experimental campaigns was influenced by the installation of the heat flow meter plate. Generally, when the heat flow meter was installed by applying a PVC film between the thermal interface material and the wall to protect its surface, the minimum duration of the test was longer than when the heat flow meter was directly installed on the wall surface without a PVC film. In Case study 1, the test lasted 24 h more and in Case study 2, the test lasted 48 h more. The findings show that the use of a PVC film hinders the installation of sensors, worsening the thermal contact between sensor and wall. Relative differences between the measured U-values were found to be greater than the expected by the theoretical calculation and also by the ISO standard quantification in both case studies. Deviations between the two measured thermal transmittance values, using an HFM installed with a layer of thermal interface material directly on the wall surface and an HFM installed  
Figure 5: Deviation between the measured thermal transmittances for the two case studies, using an HFM installed with a layer of thermal interface material directly on the wall surface and an HFM installed with a PVC film between the layer of thermal interface material and the wall surface. with a PVC film between the layer of thermal interface material and the wall surface, were found to be significantly different than those expected resulting from the calculation of the theoretical effect of including a PVC during the monitoring process summarised in Table 4. In the theoretical estimation, the use of a PVC film between the layer of thermal interface material and the wall surface had an expected deviation of 0.14% in Case study 1 and 0.02% in Case study 2. However, in the experimental tests results from the third cycle onwards showed that for Case study 1, the relative differences were around 24% to 27%, and in Case study 2 around 19% to 21%. On the other hand, random variations caused by a non-perfect thermal contact between the sensors and the surface are quantified by 5% according to ISO 9869-1:2014 [31]. The experimental results also differ significantly from those quantified by ISO standard [31]. These considerable differences between the expected differences and the measured ones might be due to collateral effects when installing an HFM with a PVC film between the layer of thermal interface material and the wall surface that hampers a good thermal contact, as [PERSON] et al. [27] suggested. Including a PVC film to protect the wall may complicate the sensor installation process and consequently, the quality of the thermal contact might be hampered.