## 1 Introduction  
Resource exhaustion and increasing energy costs have become the shackles of social development, raising the importance of energy conservation. In most countries, the building sector behaves as important energy consumer: it accounts for about 40% of the total world energy consumption [1]. In the US, the building sector consumed 72% of electricity and 36% of natural gas [2]. In mainland China, the building sector also accounts for about 28% of the annual national energy consumption [3]. Governments and professionals around the world have demonstrated the potential and necessity of energy conservation in buildings [4, 5]. Notably, some countries proposed green building certification standards, such as the Leadership in Energy and Environmental Design (LEED) as well as the Building Environmental Assessment Method (BEAM), to promote energy-efficient buildings [6, 7, 8, 9]. In addition to the promotion of sustainable behavior in the operation stage [10, 11], improving building energy efficiency is the mainstream approach to reducing energy use in buildings. There are two main strategies to enhance building energy efficiency: (1) active systems, and (2) passive design. The active solutions mainly focus on the employment of energy-efficient building service systems to reduce energy waste in the operation stage of buildings [12, 13]. The passive approaches, however, harness the internal environment conditions by taking advantage of the building design items, such as layout and form, rather than mechanical systems. The passive energy-saving solutions maximize the use of environmental sources, such as heating, cooling, ventilation, and lighting to create a comfortably built environment with a minimized energy demand [14; 15; 16]. Building envelop design can affect 20-60% of the building energy input, making it crucial for high-performance, sustainable buildings [17]. Compared with the active solutions, the passive designs generally require less life cycle cost [18], and present more significant energy-saving effects [19] as well as longer lifespans. For example, a proposed passive adaption solution for a residential case showed an 8.18-year payback and a carbon dioxide emission reduction of 11.674 million kilograms [20]. [PERSON] et al. [21] reported a passive design solution for a residential case in Beijing, which reduced the working time of the mechanical heating and cooling system by half, thereby saving 50% of energy consumption. In Hong Kong, a densely populated city, the electric energy consumption of the building sector accounts for over 90% of the total electricity consumption. [22; 23]. There are approximately 7.50 million residents in Hong Kong, where 44.8% of them live in high-rise public housing [24]. Hong Kong's extremely high-density space utilization [25] and hot and humid subtropical (HHS) climate pose significant challenges for constructing buildings that are both energy efficient as well as comfortable. Due to the heavy dependence on the heating, ventilation, and air conditioning (HVAC) system, Hong Kong's buildings are not designed in a sustainable way. Recently, passive design can be found in add-in features in buildings, but has not yet emerged as the main method to achieve building sustainability. Future buildings will need to focus on large energy reduction for global sustainable targets, which may pose a challenge for solely active design. Previous studies have proposed various passive design solutions and discussed their implications and potential benefits for buildings [26; 27]. For example, [PERSON] et al. [28] developed phase change material (PCM) solutions for buildings in different climate zones in China. [PERSON] and [PERSON] [29] discussed the window opening degree for natural ventilation in the residential case in Hong Kong. However, only a few published works focused on the comprehensive assessments of these passive cooling strategies in Hong Kong.