## 1 Introduction
The (ii) embedded technique is based on the same principle as (i); however, material layers identical or comparable to the building construction to be studied under application are used, instead. The surface-mounted calibration technique (iii) represents the most complex approach as it needs to address and incorporate all environmental effects that may cause the lateral heat flow in the vicinity of the applied sensor. The conjunction of ASTM C1130 with C1046 [23] allows the calibration results to be used for in situ measurement of heat flux through opaque building elements with subsequent analysis of measurement data according to ASTM C1155 [24]. The in-situ measurement practice provides practical information, recommendations, and tips to ensure potential simplicity and ease of application of heat flux sensors. A comprehensive study to support the continuous development of precision and bias statements for ASTM Practice C 1130 was published by [PERSON] et al. [25], in which the authors provided a complex analysis of common issues that are typical for building applications. In that study, sensor equivalency, linearity and temperature effects, uncertainty analysis, and suitability test method were analyzed, which can partially address some of the typical issues associated with heat flux measurements in the building sector. According to the practice [19], the heat flux transducer that is to be mounted on the building envelope construction should be calibrated using a hot box that is oriented similarly to the measuring site. Additionally, this approach requires a specific configuration and sample preparation involving a homogeneous specimen of known thermal resistance, temperature sensor, and guarded mask to obtain reliable results. Also, the calibration should be performed at levels of temperature and relative humidity consistent with the end-use conditions. Varying conditions during in-situ measurement may bring significant uncertainties to the measurement, therefore periodic performance checks should be carried out to reflect those fluctuations and to minimize measurement errors. However, such checks, according to the above-described practices, require unmounting the sensor thus interfering with the ongoing experiment. Some initiatives to address the varying sensitivity has come directly from the sensor producers, which documents the awareness of this issue. This initiative resulted in the introduction of new commercial gauges that were capable of self-calibration without the need for sensor removal in certain applications [26; 27]. The advanced solution is based on a novel design of the sensors with integrated heaters in sensor bodies to generate a controlled heat flux allowing a subsequent auto-calibration. Moreover, it is worth mentioning that some of the heat flux plates for measurement in buildings were originally developed for meteorological applications to measure heat fluxes in soils, which predetermines the self-calibration practice. Such practice presumes that the sensor is surrounded by soil during measurement. Typically, when the built-in heater is activated to generate the prescribed heat flux, the surrounding soil with equal properties ensures that half of the flux will be transferred upwards, while the other half will go downwards. This is an essential assumption for sensor calibration [28]. If the advanced sensors are mounted on the wall, surface-specific conditions must be ensured, similar to those in laboratory calibration using ASTM practices. This can bring some serious limitations or drawbacks or even exclude the in-situ self-calibrations of advanced sensors in the building sector. An alternative to the standard calibration procedures might be represented by the approach proposed in this paper, which turns the traditional sensors applied in the building sector into advanced ones. The approach allows for the continuous measure of the heat flow without a need to interfere with the ongoing experiment.