Performance fundamentals of multilayer membrane building envelopes

Abstract

The contemporary building envelope must fulfil an increasingly complex array of performance demands, architectural objectives, and functional requirements. Multilayer membrane building envelopes represent one potential approach to the development of the facade as a dynamic material- and energy-efficient interface - using layered combinations of technical textiles and films, lightweight insulating materials, and functional components, they present an alternative to the traditional envelope in the form of a lightweight, translucent, and potentially adaptable building skin. The promise of these systems is, however, equalled by the challenge of their analysis and design. There remains a substantial deficit of knowledge surrounding their complex behaviour, and no suitable analysis techniques have been developed which are capable of correctly modelling their unique heat and moisture transfer characteristics. The objective of this dissertation was to improve the understanding of the thermal behaviour of these systems, and to develop an accurate and practical method for their analysis and design. This method was realized in the form of a custom-programmed numerical analysis tool which is able to predict thermal and moisture transfer parameters through assemblies of any number of parallel, planar layers of solid materials and gaseous fills. The analysis considers thermal transfer via conduction, convection, and radiation as well as moisture transfer due to vapour diffusion. Laboratory and field measurements of a series of multilayer membrane test specimens were used to validate the numerical analysis method, which was found to predict the thermal flux and surface temperatures of both the hot box and the roof enclosure test specimens with generally high accuracy and good consistency, in most cases with substantially lower error than the conventional thermal resistance approach. The outdoor test data confirmed the high sensitivity of membrane facade systems to changes in environmental conditions. Air temperature, solar incident radiation, wind speed, cloud cover, and the emissivity of the surrounding environment all appreciably affect the thermal behaviour of these systems and cannot be neglected if accurate analysis and design results are to be obtained. Both sets of experiments furthermore confirmed the inadequacy of the conventional performance metrics of U-value and g-value in characterizing transparent and translucent material assemblies, as these properties do not remain static across the range of boundary conditions seen in service. A “facade performance envelope” is therefore proposed as a more comprehensive and consistent approach to the performance assessment of contemporary high-performance building envelopes.