Solar-Induced Surface Temperature Deviation and Thermal Time Lag in Daytime IRT-Based U-Value Estimation

Infrared thermography (IRT) provides a rapid and non-contact method for evaluating the thermal performance of exterior walls; however, daytime U-value estimates can be influenced by solar radiation and transient wall thermal responses. This study investigated the effects of wall heat capacity and orientation on solar-induced exterior wall surface temperature deviation, thermal time lag, and IRT-based U-value error. Transient heat-transfer simulations were conducted for one lightweight wall and two heavyweight walls with the same design U-value of 0.581 W/(m 2 ·K). For each wall, a no-solar reference case was compared with orientation-specific solar-exposure cases under identical weather conditions. The lightweight wall generally showed larger and faster surface-temperature changes, whereas the heavyweight walls exhibited smaller peak deviations and more delayed responses. For south-facing walls under high-solar conditions, the thermal time lag was approximately 1.3 h for the heavyweight walls. Under the no-solar reference condition, the IRT-based U-value error varied with time of day, particularly for the heavyweight walls. Current wall solar radiation was positively correlated with solar-induced relative error across all wall types and orientations. When the estimated thermal time lag was applied, the correlations generally increased for the heavyweight walls. These results suggest that both current solar exposure and delayed wall thermal response should be considered when interpreting daytime IRT-based U-value estimates.