Optimizing the thermometric method to assess the thermal transmittance of faÇades in energy audits

Greenhouse gases emitted to the atmosphere due to the high energy consumption have contributed to climate change and the degradation of the ecosystem. Under these circumstances, the European Union has established a series of objectives to obtain a low-carbon economy [1]. In this context, building sector plays a fundamental role. The energy consumption related to such sector is among the highest because it is responsible for the 40% total energy consumption at a global level [2]. Likewise, such consumption generates 38% of greenhouse gas emissions [3]. For these reasons, the European Union has established that the existing buildings are needed to reduce their emissions by 90% [1]. This is due to the deficient behaviour of the existing buildings' envelope because its thermophysical properties significantly influence the energy consumption [4– 6]. From the different elements of the envelope, the façade is where the buildings suffer greater energy losses, since façades are the element of major surface having heat transfer with the external air [7], whereas thermal transmittance is among those thermophysical properties with the highest impact on the energy demand [8,9] because it allows heat losses with the external air to be reduced [10,11]. Thus, determining the thermal transmittance correctly is essential to suggest adequate energy conservation measures, thus reducing the energy consumption and greenhouse gas emissions. To determine the U-value, there are many methods, both theoretical and experimental. One of the most used experimental methods by professionals is the thermometric method [12]. Such method consists of determining the U-value of an element by measuring external and internal ambient temperatures, as well as the internal surface temperature of the element. To do this, the equation of the average method from ISO 9869-1 is adapted by applying the Newton's Law of Convective Cooling, obtaining therefore the equation for the thermometric method. There were no research studies 175 developing such method, but its potential and limitations have been highlighted in the last year [12–14]. However, despite the progress of this method in the research field, there is disparity concerning the type of analysis used. This study analyses the feasibility of using different theoretical approaches for the method by analysing 20 case studies. To do this, 9 formulations were suggested by using various approaches for the internal convective heat transfer coefficient. Moreover, two approaches of data analysis (arithmetic mean of the instantaneous measures and mean of the sum of the numerator and denominator) and the data filtrate required to be applied were assessed. The results determined the most acceptable analysis configuration to apply the method.