Contents lists available at ScienceDirect Progress in Organic Coatings journal homepage: www.elsevier.com/locate/porgcoat A study on the durability of a glass bead retro-reﬂective material applied to building facades Jihui Yuan a,b, ⁎ , Kazuo Emura b , Craig Farnham b a Dept. of Architectural Engineering, Graduate School of Engineering, Osaka University, Suita, 565-0871, Japan b Dept. of Housing and Environmental Design, Graduate School of Human Life Science, Osaka City University, Osaka, 558-8585, Japan ARTICLE INFO Keywords: Urban heat island Glass bead RR materials Durability Thermal measurement Optical analysis ABSTRACT Heat from buildings contributes about 1/2 of the city’s anthropogenic heat to the urban heat island (UHI). The ratio of heat emitted from building external walls occupies about 1/3 of the anthropogenic waste heat total in Japan. Retro-reﬂective (RR) materials applied to building facades instead of highly reﬂective (HR) materials for UHI mitigation are being studied. Glass bead RR material is one of the main RR material types. In this study, several glass bead RR materials with diﬀerent refractive indices and diﬀerent color reﬂective layers were de- veloped. The long-term change in solar reﬂectivity and angular distribution of reﬂection intensity for these glass bead RR materials was evaluated for purpose of durability veriﬁcation. The result showed that both the solar reﬂectivity and angular distribution of reﬂection intensity for these developed glass bead RR materials almost have no signiﬁcant change after 368 days of exposure in the outdoor environment. Moreover, it is shown that the glass bead RR material with a refractive index of 1.9 has much better RR performance, compared to that with a refractive index of 1.5. 1. Introduction 1.1. Highly reﬂective materials for building envelopes The urban heat island (UHI) phenomenon is a well-documented climatic change phenomenon in lots of large cities worldwide [1,2]. Especially in the summer period, the urban temperature is continuing to rise due to the UHI phenomenon and climate change [3], and leads to a vast increase in building energy consumption for cooling demand and aﬀects quality of human life [4,5]. Thus, many strategies of mitigating UHI are carried out globally. A research has shown that the surface temperature of buildings could be reduced by approximately 10 °C when the highly reﬂective (HR) coating materials were used [6]. HR envelope materials are still being researched as one solution to mitigate UHI eﬀect. For the purpose of lowering the outdoor environment high temperatures and reducing the UHI eﬀect in hot summer period, a new type of building rooftop membrane was selected and an experiment of surface features and optimization on the membrane were carried out by Pisello et al. [7]. Gobakis et al. [8] have developed several inorganic materials used in building envelopes for improvement of built environments. In order to verify the composition of these developed materials, X-ray diﬀraction and diﬀerential thermal analysis were used. In addition, the optical characteristics of developed materials were evaluated through surface temperature measurement in the outdoor environment. Several HR paints were developed for application to building external walls to mitigate UHI eﬀect and save energy in buildings. The experimental veriﬁcation of these HR paints was also carried out by Cozza et al. [9]. 1.2. Retro-reﬂective materials for building facades Most of the HR envelope materials noted above are applied to rooftops of buildings. However, some scholars have suggested that it is possible to replace the HR envelope materials with retro-reﬂective (RR) materials as building envelope materials to ﬁght the UHI eﬀect, since the RR materials can reﬂect the incident sunlight back towards sky and will not aﬀect the surrounding buildings and roads [10–12]. However, currently these RR materials are usually only employed for various safety and decorative purposes. They are not used as building envelope materials, because the durability of these RR materials is still unknown and their costs for construction are relatively expensive. Thus, there is interest in research on RR materials for application to building facades. RR materials are commonly made with one of two reﬂective com- ponents, glass beads or prisms. For glass bead RR materials, Rossi et al. [13,14] investigated the RR capacities of ﬁve selected RR materials using an optical apparatus, and showed that the RR capacity of these RR https://doi.org/10.1016/j.porgcoat.2018.03.009 Received 28 July 2017; Received in revised form 1 March 2018; Accepted 10 March 2018 ⁎ Corresponding author at: Dept. of Architectural Engineering, Graduate School of Engineering, Osaka University, Suita, 565-0871, Japan. E-mail address: yuanjihui@hotmail.co.jp (J. Yuan). Progress in Organic Coatings 120 (2018) 36–48 0300-9440/ © 2018 Elsevier B.V. All rights reserved. T