# Performance of Retro-Reflective Building Envelope Materials with Fixed Glass Beads
## 1 Introduction  
The urban heat island (UHI) phenomenon is a well-documented climatic change phenomenon in large cities worldwide [1; 2]. The temperature differences between urban regions and surrounding rural regions are continuing to rise due to the UHI phenomenon and climate change [3], especially in the summer period. This might lead to an increase in building energy consumption during the summer period of cooling demand and affect the quality of human life [4; 5]. Thus, many strategies of mitigating UHI are being carried out globally. Among these strategies for UHI mitigation, highly reflective (HR) building envelopes are considered as one solution for UHI mitigation and building energy savings [6]. Research has indicated that HR materials could reduce the surface temperature of buildings, with peak drop up to 11.5 \({}^{\circ}\)C [7]. \"Cool roof\" HR membranes applied to building rooftops have large potential in reducing surface temperature and energy savings [8]. Several inorganic materials applied to building exterior walls have been developed for improving the built environment. X-ray diffraction and differential thermal analysis were applied to verify the composition of these developed materials. The optical performance was evaluated through surface temperature measurement in the outdoor environment [9]. Most of these HR coating materials noted above are applied to building rooftops. Some researchers have suggested that it is possible to replace the HR coating materials with retro-reflective (RR) materials as building coating materials to mitigate the UHI effect, since the RR materials can reflect the incidentsunlight back towards the sky and will give less of an effect to the surrounding buildings and roads [10; 11; 12]. Currently, however, these RR materials are usually only employed for various safety and decorative purposes, e.g., traffic signs. They have not been applied to building exterior wall surfaces due to unknown durability and high cost of these RR materials. Therefore, there is interest in research on making the application of RR materials to building facades more practical and cost-effective. Furthermore, the retro-reflectivity typically varies depending on the incident angle of light, from perpendicular to the material (0\({}^{\circ}\)) to just before becoming parallel (at 90\({}^{\circ}\)). RR materials intended for traffic signs and safety clothing are desired to have the best retro-reflectance at low angles. Traffic signs must reflect strongly at oncoming vehicles. Strong retro-reflectivity at high angles is not a concern. However, for application to building surfaces to reflect sunlight, good reflectivity at high incidence angles is important. Midday sunlight on summer days can have incidence angles relative to vertical walls up to 90\({}^{\circ}\), depending on building latitude and solar position. Early morning and late afternoon sun may also have such high incidence angles reflecting light east-west in summer. Conversely, a sunlight incidence angle near zero is quite rare for vertical walls, but common at midday for roofs. For RR materials to be effective throughout the day as a building-surface UHI countermeasure, they should be effective over a wide range of incidence angles, rather than the narrower range needed for traffic signs and the like. In the current material market, RR materials are commonly made with one of two reflective components, glass beads or prisms. A simulation analysis of glass beads with different refractive indices of 1.5, 1.9, and 2.2 was carried out in our previous research [13]. It was shown that glass beads with refractive index of 1.9 were the most effective to mitigate UHI effect as RR envelope materials, compared to those with refractive indices of 1.5, 1.9, and 2.2. The RR performance of several glass bead-type RR materials has been investigated using an optical apparatus [14; 15].