In Situ Thermal Performance Assessment of a Wood–Aluminum Window: Case Study of Spatial Variability, Dynamic Effects, and U-Value Accuracy

Abstract

This paper presents an experimental evaluation of the thermal performance of a wood–aluminum window installed in a timber building exposed to real climatic conditions. The assessment is based on long-term in situ measurements of heat flux and temperature variations. The aim of the research was to determine the thermal transmittance of individual parts of the window assembly, analyze their dynamic thermal behavior, and identify critical areas in terms of heat losses. The results revealed a spatially heterogeneous distribution of heat fluxes through the window structure. The center of the glazing achieved values close to the declared Ug parameter, whereas the edge regions of the glazing and the window frame exhibited a significant increase compared with the declared values. The time-dependent analysis confirmed a distinct diurnal cycle of heat flux and a phase shift between the glazing and frame components. Furthermore, it was demonstrated that the accuracy of local U-value determination is significantly affected by the magnitude of the temperature difference, with the stability and repeatability of the calculated U-values improved at higher ΔT values, particularly above approximately 10–15 K. The findings highlight the need for detailed, spatially resolved assessment of window structures and emphasize the importance of experimental verification of their thermal performance under real conditions.