## 1 Introduction  
Efficiency in terms of energy and resources is required by the European Green Deal when building and renovating the European building stock [1] for achieving the European Union's energy efficiency targets [2]. To meet the 2050 long-term climate and energy targets, 97% of the residential building stock, which is responsible for around 36% of CO\({}_{2}\) emissions in the European Union [3], needs to be upgraded to become highly energy efficient and obtain the Energy Performance Certificate (EPC) label A [4]. This high percentage is due partly to the average age of European residential building stock: more than 80% of residential buildings are over 25 years old (Table 1) and, therefore, most of them were built without thermal regulation. Construction is considered the sector with the most potential for energy saving. Buildings represent 40% of the energy used in the European Union [5]. Almost 26% of Europe's total final energy consumption is used in residential buildings [6]. Space heating represents a large amount of energy consumption in the housing sector [7; 8] and, therefore, has a significant capacity for energy saving. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Region**} & \multirow{2}{*}{**Floor Space**} & \multicolumn{3}{c}{**Average Age of Residential Floor Space**} \\ \cline{3-5}  & & **Pre 1960** & **1961–1990** & **1991–2010** \\ \hline North and West & 50\% & 42\% & 39\% & 19\% \\ Central and East & 14\% & 35\% & 48\% & 17\% \\ South & 36\% & 37\% & 49\% & 14\% \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Distribution of residential floor space by year of construction in the European Union 27, Switzerland and Norway [9]. According to the IEA EBC Annex 71-project [10], measuring the actual energy performances discloses a notable gap between actual and expected energy performances of buildings. The actual thermal behaviour of building envelopes could contribute to this energy performance gap in buildings. Deviations between the predicted and actual behaviour of envelopes are related to aspects of the design and construction stages [11]. Moreover, assumptions about energy efficiency improvements resulting from building refurbishment are not always met [12; 13]. Consequently, accurate in situ measurements of the actual U-value of facades are necessary. The thermal performance of building facades is a fundamental parameter that should be evaluated to obtain an accurate energy diagnosis of buildings [14; 15; 16; 17; 18; 19; 20; 21]. Successful decision-making during energy renovation processes of existing buildings requires precise characterization of thermal properties of building components [22; 23]. Classifying the opaque part of the facades is essential to precisely analyse the thermal performance of facades in the housing sector. In recent years, the thermal behaviour of the materials that compose facades had been studied in depth by several authors such as [PERSON] et al. [24] measuring the thermal properties of brick materials, [PERSON] and [PERSON] [25] or [PERSON] [26] measuring the properties of thermal insulation materials. However, when analysing the thermal transmittance of walls, assumed U-values have been a meaningful source of error in estimations of energy savings and carbon emissions [27]. Evidence suggests that assumptions concerning heat loss from a dwelling pre-retrofit and post-retrofit are not correct [12]. Therefore, accurate on-site measurements are required to provide information on the actual thermal transmittance of facades. To effectively quantify the actual performance of buildings, optimized on-site measurements combined with dynamic data analysis techniques are needed [28]. Several methods can be used for the in situ measurement of U-value of existing buildings' facades [29; 30]. One of the most common is the heat flow meter method, standardised by ISO 9869-1:2014 [31]. This method obtains the thermal transmittance by measuring the heat flow rate that passes through a wall and the inside and outside environmental temperatures.