Assessment of heat loss in building envelopes: a method for evaluation, grading and comparison using UAV-based infrared thermography

Assessing heat loss in existing building envelopes is essential for implementing energy-efficient retrofitting projects. However, evaluating envelope thermal performance using infrared thermography (IRT) remains challenging because of limited detection range and the lack of standardized metrics and comparative methods. This study proposes an unmanned aerial vehicle (UAV)-based IRT method for heat-loss assessment. First, infrared digital orthophoto maps (IR-DOMs) of building facades are generated using photogrammetric techniques, enabling extraction of infrared (IR) temperatures from external walls and windows. A dynamic prediction model, referred to as the ‘standard temperature’, is then developed from numerically generated reference samples and regression analysis to provide reference temperatures for comparison with measured IR data. Finally, by analyzing the difference between the measured average IR temperature and the standard temperature, together with the average heat-loss discrepancy, the method introduces quantitative indicators for grading and evaluating envelope thermal performance. The methodology was examined using heating energy consumption data from five buildings in a cold region of China. The resulting heat-loss grades were generally consistent with both annual and 24-hour heating use, indicating that the proposed indicators can support comparative evaluation across multiple buildings and help prioritize retrofit candidates.