Variable U-value Approaches Toward Reproducible In-Situ Estimation of Linear Thermal Transmittance at a T-shaped Junction: A Simulation-Based Preliminary Study

As zero-energy building policies expand, reliable evaluation of high-performance envelopes has become increasingly important. In highly insulated buildings, reduced heat loss through planar walls increases the relative contribution of thermal bridges, highlighting the need for reliable evaluation of linear thermal transmittance (Ψ). Conventional steady-state methods, however, cannot represent time-varying environmental conditions and construction-related variations in actual buildings. This study presents a simulation-based preliminary assessment of post-processing procedures for future in-situ thermal-bridge evaluation. Transient heat-transfer simulations were conducted for an external wall–internal wall T-shaped junction and an adjacent reference wall under field-relevant meteorological boundary conditions. The simulated heat-flow responses were used to evaluate stable measurement window (SMW) filtering and two Variable U-value approaches: Case I replaces the fixed clear-wall U-value with the time-varying U-value of the reference wall, whereas Case II further normalizes the junction heat flow by the ratio of the average to the instantaneous reference-wall U-value. SMW filtering reduced the reproducibility error of the Baseline method from 129.16% to 41.14%, and Cases I and II further reduced it to 29.91% and 10.67%. Case II showed the lowest temporal variability and closest agreement with the steady-state benchmark (relative bias +0.75%, MAPE 3.82%), remaining robust under altered insulation performance and thermal inertia.