Multi-Functional Building Envelopes: Key Properties, State-of-the-Art Technologies and Visions for the Future
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A broad and original definition of the building envelope is given as a shelter aiming at controlling various fluxes in the building. This paper reviews the underlying key properties ruling the phenomena enabling the completion of the multiple functions ascribed to the building envelope. The functions of flux control are emphasized. Indeed the thermal conductivity of a material, thermal inertia of buildings, transmittances of a window, solar energy production, sound attenuation or absorption of a concrete wall or moisture permeability of a protection sheet are all key properties ruling the control of fluxes such as heat, sun, sound or moisture and will be explained here. A non-exhaustive review of the principle state-of-the-art multi-functional technologies using these key properties to achieve the different functions is given. Hence the review of building integrated photovoltaic solutions, phase change materials, smart windows, double skin façades, green envelope, aerogels, multi-functional concretes and photocatalytic materials, among other technologies. The focus is put on the technologies explanation and the performances, in the multiple roles they have, are highlighted. Through a definition and a description of the ideal multi-functional building envelope as assuring safety, providing comfort and any other desired functions without any drawbacks this paper aims to orientate technological breakthrough of the future towards multi-functionality and to a total control of all the fluxes. Eventually, based on the association and the understanding of existing technologies, original ones achieving multiples functions are proposed as possible visions for future developments.