## 2 Literature Review  
Hong Kong is hot and humid for most of the year and has a long cooling period [30]. Passive building systems are fundamental to achieve high energy efficiency and comfort in built environment [31; 32; 33]. The current development of the passive cooling design shows high potential for it to achieve high energy efficiency in buildings with access to its proper use [34; 35]. The Hong Kong climate, however, is not optimal for many techniques such as direct natural ventilation due to occupancy comfort. The average temperature is around 33 \({}^{\circ}\)C with 80% relative humidity in summer and 12 \({}^{\circ}\)C in winter. The temperature in winter can reach 3 \({}^{\circ}\)C in extreme conditions. The climate is cooling-dominated. According to the local sustainable building design guide, BEAM Plus [6, 7, 8], the air conditioning period is for seven months from April to October. Several pieces of research focusing on passive design techniques in a climate with a warm temperature, a dry winter and a hot summer show potential energy-saving with acceptable occupancy comfort [30, 36, 37]. Features such as window to wall ratio and orientation can be crucial for success in passive design. The widely employed passive design technologies can be categorized into four groups: (1) geometry, (2) passive envelope, (3) solar shading, and (4) natural ventilation. A building's geometry determines its self-shading, daylight availability, solar gain, natural ventilation, noise control, and pollutant dissipation [38]. For residential buildings, a certain amount of self-shading can improve the total energy performance, as the cooling load reduction outweighs the slight increase in lighting energy [30]. Geometry also affects the availability of natural ventilation inside the building, especially the wind-driven ventilation. An unobstructed layout is favorable as it can effectively drive the airflow through the building [32]. Carefully chosen building orientation and location can maximize wind availability. Comparisons of simulation results among different public residential buildings in Hong Kong show clearly that certain types of layout (e.g., concord type) have fewer hours of discomfort in Hong Kong, even under the free-running condition [33]. Additionally, passive design in geometry and layout involves the early stage of design and thus usually requires no extra investment. The properties of building envelopes significantly affect the building thermal performance [38]. Three passive building envelope design techniques are widely-employed: (1) thermal insulation layer, (2) PCM, and (3) fenestration. The thermal insulation layer is made from materials with low thermal conductance, such as wool and polystyrene. Thermal insulation reduces the energy intake from the envelope by reducing the U-value of the external wall. [PERSON] et al. [39] found in an experiment that the best insulation performance occurs when the innermost layer on the external wall shows a 35-86% reduction in heat flow rate. The optimum thickness for insulation varies largely depending on the building location and materials [40]. In addition, PCM is also widely used in passive building envelope design and presents strong high energy-saving potential in a warm climate. For example, a case in Ankara improved energy efficiency by 7.2% [41]. In Hong Kong, PCM might save 2.9% of annual energy consumption under 28 \({}^{\circ}\)C with a long energy payback period of 23.4 years [42, 43]. However, PCM might add energy load to the AC at night. Precise PCM enthalpy curve simulation is required to present a more accurate result. [PERSON] et al. [44] validated the method of conduction finite difference (CondFD) to the simulation of PCM in EnergyPlus with empirical results, and they suggested that considerable savings can be achieved with PCM. Fenestration takes up a large amount of heat input to of building's cooling system [30].