## 4 Discussion  
The results of this research revealed that in situ measurement techniques can be useful to assess the build quality of a new passive house before its operational stage. Indeed, they allow us to quantify the gap between the design or modeling and the actual work. However, it is a proof of concept for both non-invasive techniques, and, consequently, further research is required. For the first time, thermography was implemented in a quantitative way, to determine the in situ thermal transmittance of passive house facades. Previous studies demonstrated that the optimal \(\Delta\)T of quantitative _IRT_ ranged between 7 and 10 \({}^{\circ}\)C for conventional heavy multi-leaf walls (U \(>\) 0.362 W/m\({}^{2}\cdot\)K) [63]. In the current research, the above statement could also be assumed. Besides this, this technique is characterized to be faster than _HFM_, since the _IRT_ test only takes 2 h. In contrast, _HFM_ offers better results for PH facades in terms of accuracy and dispersion of the instantaneous measurements. The deviation between theoretical and measured U-values was found to be 1.05% (_WI_) and 3.06% (_W_2) for the _HFM_, compared to ~5% for _QIRT_. The coefficient of variation ranged from 1.39 (_WI_) to 6.80 (_W_2) for _HFM_, while the values for the thermographic analysis were found to be 8.06% (_WI_) and 9.69% (_W_2). Notably, the good _HFM_ results of the experimental mock-up could be attributed to the high temperature gradient (~19 \({}^{\circ}\)C) that was achieved during the measurement campaign. Determining the actual thermal transmittance of low U-value facades when weather conditions are favorable and temperature differences between the inner and outer environments are greater than 10 \({}^{\circ}\)C requires a minimum test duration of 72 h, using the _HFM_ average method, and 2 h of using the _QIRT_ method. The first condition implies that practitioners should check the short-term weather forecast before starting the test. The second condition represents a limitation in the period of time in which the test can be performed, especially when the _HFM_ method is used. In the Mediterranean  
Figure 9: Confidence intervals at 95% for quantitative IRT measurements. climate, due to the need to obtain large temperature differences between environments, the test should be scheduled for the coldest months (normally from October to April). In terms of execution of the method, the practitioner should have experience in conducting measurement campaigns in the real built environment. In both methods, the tests should be performed on northern facades and preferably in the early morning, with a low wind speed (\(<\)1 m/s), since wind and incident solar radiation are adopted as external thermal stimuli that could alter convective factors or lead to a time lag of a few hours [61; 75]. Despite this, some differences between _HFM_ and _QIRT_ were highlighted. The installation of the equipment for the _HFM_ monitoring process may be easier for professionals to carry out by following the ISO recommendations. However, a prior inspection, with the help of qualitative _IRT_, is needed to detect the ideal location of heat-flow meters. In addition, _HFM_ could be considered more invasive. To guarantee proper thermal contact between the transducer and the wall, a layer of thermal interface material (with a thickness of less than 0.01 m) should be applied. As regards the installation of equipment for the _QIRT_, the monitoring process requires practitioners with specific training to calibrate the IR camera in accordance with the target and to achieve a reliable sequential video recorded during the test. Parameters such as the emissivity of the wall or the reflected ambient temperature can be adjusted during post-processing with the specific software, but the wall area or the distance between the IR camera and the specimen cannot be changed later.