Development and validation of a non-contact method for U-value estimation in heritage and existing buildings

Improving the accuracy of in situ thermal diagnostics is critical for advancing building energy retrofits, particularly in heritage contexts where invasive approaches are not permissible. This work presents the development and testing of a cost-effective, fully non-contact measurement system designed to estimate wall thermal transmittance (U-value) in real buildings. The system, developed within the research framework of the “THE-METHER” project, implements an enhanced thermometric method to indirectly evaluate heat flux using temperature-based sensing. Its architecture integrates low-cost components (including infrared thermometry and ambient data acquisition) within a portable and modular platform. The proposed approach was validated through a two-stage experimental campaign. The laboratory phase focused on optimizing the infrared sensor positioning, identifying 10 cm as the optimal distance to ensure both accuracy and operational feasibility. Subsequently, in situ tests were conducted on a mid-20th-century building. Calibration activities highlighted initial inaccuracies in air velocity measurements from one of the low-cost sensors, which were mitigated through a dedicated correction procedure. The results show that the system yields U-value estimates with deviations in the range of 11–13% compared to standardized reference measurements, providing preliminary evidence of consistency under the tested conditions. While broader validation is still required, these findings support the method’s potential for non-invasive application in building diagnostics, particularly in contexts where conventional contact-based techniques are not feasible.