Method to Diagnose Window Failures and Measure U-Factors on Site

Abstract

Windows are an essential part of buildings due to the requirement for natural light, views, and fresh air. However, windows are thermally the weakest bridge in a building due to their high thermal conductivity. Therefore, window U-factor (thermal transmittance) information is indispensable in calculating the overall energy load of a building. U-factors of windows, however, are difficult to obtain on-site because the label mounted on a window exhibiting its U-factor is typically removed after its installation. Further, it is almost impossible to detect any of a variety of window failures, such as the loss of insulating gases, leaky or cracked windows, and localized air leakage, simply by visual inspection. In this study, a novel technique to measure window U-factor in the field by measuring four temperatures (interior and exterior air temperatures, and interior and exterior window surface temperatures) is presented. Experimental and field tests on various types of full-scale windows have been performed to obtain their field-measured U-factors. Experimental results show that the field-measured U-factors match within 8% of the rated U-factors of the windows. Several assemblies combining storm windows with single- or double-pane windows were tested and the combined U-factors of the assemblies were measured and the readings were compared with the U-factors estimated by ASHRAE. In another test, argon from a double-pane window was removed deliberately and results confirmed the leakage using the proposed method. In addition, field tests at five different buildings were performed and the comparison between measured and rated (or estimated) U-factors is presented.