## 1 Introduction  
In the thermal assessment of buildings, various types of heat gains and losses constitute the overall balance of the object. Its character, whether positive or negative, depends mostly on the thermal performance of the building enclosure [1; 2]. Therefore, the heat transfer analysis of this building element should receive special attention to provide an essential part of the data that forms a considerable contribution to the overall heat balance of buildings. Heat flux probes and sensors considered for applications in the building sector need to meet specific requirements, such as relatively low values of heat flux, alternating directions of heat flux, surface roughness of investigated walls, or exposure to radiative heat flux. Various types of sensors are available on the market, differing from each other by measuring techniques and/or construction principles. Gardon [3; 4; 5], Schmidt-Boelter gauges [6; 7; 8], thin plate sensors [9; 10], or radiometers [11; 12] represent the most common types of non-optical sensors. An extensive review of heat flux measuring techniques was provided by [PERSON] et al. [13]. Another report [14] provided a review of non-optical detectors for heat flux measurement in fires. Assuming that heat flux can be induced by different origins, the essential criterion of sensor selection should be made with respect to the environment, in which the sensor is intended to be placed in. Basically, the heat flux mode or possible combination of multiple modes (convective, radiative, or conductive), including the range of anticipated quantities, should be known in advance to provide a background for making a qualified decision on sensor selection. For example, Gardon and Schmidt-Boelter gauges were designed for the measurement of both convective and radiative heat fluxes. As such, they are suitable for fire- or flame-induced heat transfer measurement. Pure radiative transfer, such as solar radiation, can be measured by pyramometers [15]. The conductive heat transfer is typical for building physics applications in the case that opaque building elements are the subject of investigation. As the sensors are mostly mounted on the wall surface, flat plate sensors or foil sensors suit best for these kind of applications. Those sensors are represented by thermopiles working on the principle of the Seebeck effect; that is, they generate an electrical output signal that is proportional to the heat flux passing through the sensor. Before application, each transducer needs to be calibrated to determine its sensitivity, that is, the electrical output to the passing heat flux ratio. In most cases, the initial calibration is performed by the manufacturer and the user is provided with the calibration protocol. Also, the default sensitivity is very often printed somewhere on the sensor body to be easily located and adopted by the user. The common practice for the calibration of heat flux transducers is based on inducing steady-state conductive heat flux in the controlled environment by means of the guarded hot plate [16] or the heat flow meter apparatus [17; 18]. Procedures for calibrating heat flux probes that are relatively thin in comparison with their planar dimensions are well described in ASTM C1130 [19]. This practice in conjunction with other test methods serves as a valuable handbook for the determination of the sensitivity of heat flux transducers under both laboratory and in-situ conditions. For the laboratory calibration, the conjunction with Test Methods C177 [16], C518 [17], C1363 [20], C1114 [21], or ISO 8302 [22] is well appreciated, while three different techniques are recognized: (i) ideally-guarded, (ii) embedded, and (iii) surface mounted. As described in Section 4.1 of ASTM C1130 [19], the ideally protected technique (i) is used for determining the baseline calibration assuming that only homogeneous and thermally characterized materials are used in the test stack (series of layers) promoting one-dimensional heat flow.