Abstract

Metal curtain walls are widely used in commercial buildings and offer many advantages including space saving, high quality in manufacturing, light weight, significant aesthetic freedom, and rapid construction. However, their thermal performance is still low due to the fact that metal curtain walls consist of a large portion of glazing, and glass and metal are high heat conductors. In practice, metal curtain walls are referred to as "heat sink" in heating-dominant climate. The relatively low thermal resistance results in low surface temperature in cold winter, and thus may cause condensation and thermal discomfort problems in addition to high energy consumption. Initially, metal curtain walls grew within metal window industry and the current methodology and standards developed for evaluating window performance are also used for curtain walls. However, metal curtain walls differ from windows in that they have a much larger continuous glazing portion, more complex configuration and heat flow at the joints. Their overall performance depends on the interaction and integration of individual constituents as well as the performance of each component. However, the assessment of curtain wall performance by existing standards is segmented and no study has addressed the overall thermal performance of curtain walls by treating them as integrated systems. The objective of this study is to evaluate the overall thermal performance of metal curtain walls using a developed holistic approach for the purpose of providing technical information on the improvement of curtain wall design. A comprehensive research program has been designed and implemented to establish the overall performance of curtain walls by experimental testing, analytical and simulation studies. For the first time, extensive experimental testing has been conducted on full-scale specimen under field conditions reproduced in a large-scale environmental chamber. The two-story full-size specimen (3.8m by 6.7m) includes two commercially available curtain wall systems with different design details. The experimental program includes: (1) full-scale air leakage test, (2) thermal performance test, (3) measurement of local convection film coefficient, and, (4) measurement of local draft induced by curtain wall cold surface. The analytical and simulation studies include: (1) effect of design details on thermal transmittance using the simulation program FRAME, (2) effect of local film coefficients on the condensation resistance prediction using the simulation program FRAME, (3) effect of thermal performance of curtain walls on occupant thermal comfort and on energy consumption. Results from extensive testing and simulations have revealed the intricate links among the components, the overall wall assembly performance and the impact on the energy use and indoor comfort; and, therefore, provided solid technical information for manufacturers on the productive direction of future R&D and for designers on the selection of curtain wall systems to achieve energy-efficient buildings with healthy and comfort indoor environment. The extensive testing provided a valuable set of experimental data to validate the current and future computer simulation programs.