## 1 Introduction
[PERSON] et al. [29] calculated the actual U-value of two facades with theoretical U-values of 0.10 and 0.23 W/m\({}^{2}\)-K. Results had relative deviations between theoretical and measured U-values from 10 up to 65%. Finally, [PERSON] and [PERSON] [30] studied facades with a theoretical thermal transmittance of 0.22 W/m\({}^{2}\)-K, with relative deviations between U-values from 3 up to 59%. Infrared thermography (_IRT_) is a widely accepted NDT technique that allows users to inspect entire wall areas [31]. The employment of thermographic inspection can be divided into qualitative and quantitative studies [32; 33; 34]. In fact, [PERSON] et al. [35] highlighted that most studies were qualitative, to discover heterogeneities due to anomalies (moisture, thermal bridges, cracks, air leakages, etc.) below the plaster [36; 37; 38; 39; 40] and to define the geometry of a masonry, among other purposes [41]. Besides this, international standards (i.e., ISO 6781:1983 [42]), UNE EN 13187:1998 [43], ASTM E1311 [44], ASTM E1862 [45], and guidelines (i.e., RESNET [32]) only recommend boundary conditions for the use of qualitative _IRT_ tests to carry out energy audits in buildings [33; 46; 47]. In the last decade, few systematic attempts have emerged for developing accurate approaches related to the estimation of in situ U-values of facades by quantitative _IRT_. Nevertheless, the studies tended to analyze the convective and radiative heat-transfer processes of the wall from outside the building [27; 28; 48; 49; 50; 51; 52; 53; 54; 55; 56] instead of inside the building [57; 58; 59; 60; 61; 62; 63; 64; 65]. Some researchers pointed out two main constraints in the use of external thermography: (i) the tabulated value of the external convective heat transfer coefficient is considered a precautionary value, since it is computed to estimate the heat loss during the design stage of the building [51]; (ii) the calculated value of the convective heat-transfer coefficient based on [PERSON]'s equation may present a greater variability due to wind characteristics (angle, intensity, and direction) [66; 67; 68; 69; 70]; (iii) uncontrolled reflections indexes of surroundings may be given on the target [33; 61]. As regards internal thermography, [PERSON] et al. [71] proposed a numerical model where the equation of the specific heat flux by radiation was linearized. Moreover, the convective heat-transfer coefficient was focused on a dimensional approach that was reported in [PERSON]'s study [72] and [PERSON]'s study [73]. According to [PERSON] et al. [69], the calculation procedure with a dimensionless approach is more accurate. Along this line, [PERSON] et al. [74] carried out a comparative analysis among different numerical models based on quantitative internal _IRT_ in terms of radiative heat flux: [PERSON] [71], [PERSON] et al. [57], and [PERSON] et al. [61]. The results did not show significant differences in the use of the three equations. For determining the influence of internal convective heat transfer coefficients (hc) on the thermal characterization of building envelopes using _QIRT_, a cluster analysis was drawn up. The study took into account 25 correlations of temperature differences and 20 correlations of dimensionless numbers. The outcomes demonstrated that the use of dimensionless numbers for computing hc was the most efficient approach for the _QIRT_ from inside the building. Concerning the precision of the method, researchers obtained relative deviations between theoretical and measured U-values from 10 to 60% for external thermography and from 2 to 12% for internal thermography. [PERSON] et al. [51] stated that the deviation between _HFM_ and _QIRT_ measured data was greater for well-insulated walls, reaching \(>\)50%. As seen, the main benefit of internal _QIRT_ is that the practitioner can work under controllable test conditions, obtain a better precision, and avoid unknown reflections of surroundings on the target.