Optimal measurement timing for infrared thermography in building envelope assessment

Infrared thermography (IRT) is increasingly used for the quantitative assessment of building envelope thermal performance; however, its reliability under varying environmental conditions remains uncertain. This study evaluates the seasonal applicability of IRT for determining wall heat transfer coefficients (U-values) using continuous 24-h measurements conducted in summer, winter, and spring, with heat flow meter (HFM) data used as a benchmark. The results show that IRT-derived U-values exhibit large deviations in summer, with relative errors ranging from 16.7% to 88.5%, even during nighttime measurements, due to unstable heat flow caused by daytime heat storage. In contrast, measurements conducted during winter and spring heating periods yield substantially improved agreement with HFM results, with relative errors limited to 2.0%–17.6% and 2.9%–18.3%, respectively. Nighttime conditions after sunset, when solar effects are absent and heat transfer approaches a quasi-steady state, are identified as the most reliable time window for IRT-based U-value estimation. These findings clarify the physical limitations underlying seasonal inconsistencies reported in previous studies and provide practical guidance for the reliable application of IRT in building thermal diagnostics.