## 2 Materials and Methods
### Validation of the Proposed Self-Adjustment Approach  
For the independent validation, a series of additional high-sensitive heat flux sensors was acquired and installed on site. In particular, two flat-plate sensors Hukseflux HFP01 and one sensor HFP01 SC were mounted on the wall together with the surface temperature sensor Pt1000 TG7 (see Figure 5). To prove the efficiency and applicability of the proposed approach, a completely new dataset was obtained using high-sensitive HFP01 and HFP01 SC probes in the final stage of the field measurement. Although the sensitivity of HFP sensors is sufficiently high, the HFP01 SC was implemented in the measurement to confirm the data obtained by HFP01 during the validation procedure. Having the independent data from those sensitive sensors that stay apart of the sensitivity adjustment procedure described in the previous subsection, the independent validation could be performed by a simple confrontation of the proposed technique with sensor performances. Since the proposed technique potentially aims at the creation of a new alternative to the advanced type of sensors with self-calibration feature, it comes directly from the idea that independent data should be obtained from that kind of sensor. Therefore, additional smart sensors with integrated heaters were acquired, installed on the site, and self-calibrated to provide reference data for the validation. In principle, the validation is based on a direct comparison of \(q(t)\) (obtained using steps 1-4 of the proposed sensor adjustment technique as described in Section 2.3) with the time-resolved reference heat flux (recorded on HFP01 and HFP01 SC probes). Although the HFP01 ones are high-sensitive sensors, which is essential for obtaining reliable reference data, the HFP01 SC was implemented as well to confirm that reliability through on-site self-calibration. Such self-calibration applied to sensors with integrated heaters is based on measuring the difference in voltage output of the sensor before and after the heating period, and the heat flux generated by the heater to determine the new sensitivity. If the sensors need to stay mounted on the wall, the equal distribution of heat flux must be ensured allowing to use same approximation as dedicated for sensors buried in soils. To achieve this, the sensor was temporarily sandwiched by an AAC block, the same as those which the perimeter wall of the residential object was made of. Therefore, a temporary shelf was mounted on the wall and the sandwiching block was placed on top of it and gently pushed against the sensor. The good thermal contact between upper side of the sensor and temporary sandwiching block was achieved by using pressure-sensitive tape and heat sink grease. Also, the thermal conductivities of both wall and AAC block were checked using an ISOMET device to confirm that the thermal properties correspond to each other, so that Equation (6) can be applied directly without any further compensations. Assuming that the electrically generated heat flux is distributed equally to both sides of the sensor (i.e., half of the heat flux passes the sensor), the new sensitivity can be calculated [37] as  
\[S_{\text{SC}}=\frac{2U_{\text{SC}}}{q_{\text{SC}}}=\frac{2U_{\text{SC}}R^{2}_{ \text{current}}A_{\text{heater}}}{U^{2}_{\text{current}}R_{\text{heater}}}, \tag{6}\]  
Figure 5: Extension of the experimental setup for the validation of the proposed technique.