Field-based assessment of cavity thickness variation and thermal performance degradation in double-glazed windows

This study investigates the impact of outdoor temperature variations on cavity thickness in double glazing and its influence on thermal performance. In-situ measurements were conducted on windows in Korean buildings with different orientations, frame materials, and glazing types. Hourly winter measurements included cavity thickness, outdoor temperature, and glass surface temperature, with U-values calculated using the Temperature-Based Method (TBM). As the outdoor temperature decreased from 8.7 °C to −6.8 °C, the central cavity thickness was reduced from 10.1 mm to 8.7 mm, accompanied by an increase in U-value from 3.06 to 3.94 W/m²·K (22.3%). In cases where the cavity thickness decreased to below 5 mm, the U-value increased from 4.22 to 5.35 W/m²·K (21.1%), indicating a substantial reduction in thermal resistance. These results demonstrate that reductions in cavity thickness can lead to localized increases in heat transfer and degradation of thermal performance at the center of glazing units. The study provides real-time empirical evidence under dynamic outdoor conditions, highlighting the importance of considering central cavity behavior in window energy performance assessments to ensure reliable insulation performance and accurate building energy predictions. • Outdoor temperature drops reduce the air cavity thickness between glazing layers. • Cavity thickness reduction is most evident in north-facing curtain wall systems. • Central cavity thinning raised U-values, causing up to 22.3% insulation loss. • Field data show thermal degradation at glazing centers as well as edges. • Insufficient cavity thickness increased the U-value during nighttime cooling.