A Numerical and Experimental Study of Thermal Transmittance of Window Systems

Abstract

Highly glazed buildings are the trend in today?s architecture, but the glazing system is a weak barrier from the thermal point of view. The heat gain through window is a primary source of the cooling loads in air-conditioned buildings in the hot and humid climate of Singapore.

The thermal transmittance (U-value) is currently used in the calculation of the ETTV, which is a primary criterion in the energy performance standard adopted by the Building and Construction Authority of Singapore. However, the window U-value used in the ETTV calculation is the centre-of-glass U-value of the glazing unit alone, while it should be the overall U-value of the whole window system including the centre area of the glazing unit, the edge area of the glazing unit, and the window frame.

A numerical study has been undertaken on the thermal transmittance of window systems. The computations indicate that the overall U-value of common single glazing aluminium windows is 4 to 11% higher than the centre-of-glass U-value. For common double glazing aluminium windows without thermal break and with thermally broken aluminium frames, the overall U-values are 17 to 112% and 5 to 57% higher than the corresponding centre-of-glass U-values, respectively. The use of these overall U-values instead of the centre-of-glass U-values would enable a more accurate estimate of the energy performance of building envelopes in the standard.

In the current work, correlations have been obtained to allow building designers to easily convert the centre-of-glass U-values to the overall U-values for common window systems in Singapore. The range of environmental conditions simulated corresponds to the conditions in Singapore, which are completely different from the winter conditions in which the labelled properties are measured in North America and Europe.

A Guarded Hot Box facility has been constructed in compliance with standards 1363 and 1199 of the American Society of Testing and Materials (ASTM). While the instrumentation and calibration of the instrument have been completed, the hot box is pending ASTM certification. The U-values obtained by computations will be verified with the hot box testing in later work.