Thermal performance characterization of supply-air double windows: A new guarded hot box protocol and numerical modelization

Abstract

The window area constitutes a significant part of the façades of heritage buildings in French cities. An effective way to improve window performance as well as preserve the appearance of heritage buildings is to retrofit old single-glazed windows into supply-air double windows by putting a secondary window inside the existing window. For window renovation, the U-value is an important indicator. To accurately identify the U-values (i.e., the Ueq value, Uuse value and Uloss value) of supply-air double windows, a new guarded hot box (GHB) method was firstly proposed which improved the GHB setup and calibration/testing procedure. Based on GHB experiments, the thermal performance of different window configurations was identified and compared. The results showed that U-value reductions were in the range of 59.2%-80.8% by renovating a single-glazed window into a supply-air double window. Moreover, the window U-value was compared to the center-of-glass U-value, which is often used in building codes to simulate building energy consumptions. It was found that the deviations between the window U-value and the center-of-glass U-value were in the range of 21.6%-45.5% for different window configurations. The GHB experimental data were further utilized to validate a three-dimensional computational fluid dynamic (CFD) model. Based on the validated model, a parametric study was performed to investigate the impacts of different window configuration parameters on U-values. The results showed that the location of Low-E coating, coating emissivity and air flow rate had significant effects on U-values. In addition, it was found that improving the thermal conductivity performance of the internal window frame was more important than the external window frame.