The issues related to indoor air quality, energy cost and other environmental issues associated with energy generation has created an interest in the natural ventilation in buildings in the recent literature. In the hot and humid region, the intensive solar radiation and the higher humidity level create the requirement of using mechanical cooling mechanisms which contribute substantially to energy consumption in buildings [
1]. In most tropical countries, achieving thermal comfort without the use of air conditioning is becoming difficult due to poor building designs and global warming may make the issue worse. The thermal comfort of a poorly designed naturally ventilated building is difficult to control unlike in air-conditioned buildings, where thermal comfort can be achieved with the compensation of higher energy consumption.
Energy consumption in the household sector is governed by various factors including the building envelope characteristics and consumer behaviour [
2]. In most of the tropical countries, the energy demand is heavily affected by the energy poverty, with lack of access to energy or energy security. While the usage of energy-consuming equipment such as air-conditioning units is restricted due to energy poverty, the only option to achieve the required thermal comfort will be the proper design of the building envelope. For low-income households, thermal renovation is difficult due to poor economic conditions [
3‐
6]. Hence, considering the proper design aspects at the initial building design would avoid such costs and both thermal comfort and lower electricity requirement would be met. The effect of consumer behaviour on energy consumption has been widely discussed in various studies [
7‐
9] and hence this paper will only focus on the aspects of building envelope which affect the energy consumption of residential buildings in tropical climate.
Globally, a significant proportion of the building energy is consumed for achieving the required thermal and optical comfort [
10‐
12]. In addition to the building materials, the building form and the other associated factors heavily affect the indoor thermal comfort and the lighting energy of any air-conditioned or naturally ventilated building. The most important parameters affecting the thermal comfort and lighting energy requirement of the indoor environment are the building shape, orientation and the window to wall ratio (WWR) of the building [
13‐
20]. These parameters are inter-related and a proper combination is required to achieve the optimal thermal comfort and energy efficiency.
Several attempts have been made to identify the impact of the building form or building shape on the energy load of the building. Ourghi et al. [
21] provided a simplified analysis method to predict the impact of the shape of an air-conditioned office building on its annual cooling and total energy use. The simulations were conducted for four locations including Rome, Tunis, Cairo and Gabes. The results indicated a strong interdependency between the annual building energy use and various basic building features such as building shape, window size and glazing type.
Caruso and Kampf [
22] analysed the optimal three-dimensional form of buildings that minimise energy consumption (air-conditioning needs) due to solar irradiation using the evolutionary algorithm. The results indicate that the optimal forms are compact and oriented to a particular direction in the sky that depends on the site following a sort of self-shading concept. Further, Depecker et al. [
23] suggested that in a cold climate, the heating load is directly proportional to the shape coefficient. Depecker et al. [
23] defined the shape coefficient as follows.
$$\begin{aligned} C_{f}=\frac{S_{\mathrm{e}}}{V}, \end{aligned}$$
(1)
where
\(S_{\mathrm{e}}\) is the envelope surface area and
V is the inner volume of the building. The study has focused only on the building shape and has ignored the parameters such as climate, orientation and WWR.
On the contrary, AlAnazi et al. [
24] introduced an analysis method to estimate the impact of building shape on the energy efficiency of air-conditioned office buildings in Kuwait. The analysis has taken several building shapes and forms into account including rectangular, L shaped,
U shaped, T shaped, cross shaped, H shaped and cut shaped. For buildings with low WWR, the total energy use is found to be inversely proportional to the relative compactness of the building, independent of its form.
On the effect of WWR and orientation on the energy consumption, Alwetaishi [
25] conducted a research to identify the optimal WWR for educational buildings in various climatic regions and a WWR of 10% is recommended for a hot and humid climate region. A study in Teheran suggested that the orientation can save up to 105% of the annual energy of a building and the WWR has an important role in deciding the building orientation [
26]. Hence, investigating the impact of orientation on energy efficiency is important while determining the effect of building shape on energy use.
Similar researches have been carried out for cold climates with the heating load. Oral and Yilmaz [
27,
28] presented a methodology to determine the building form which provides minimum heat load. Further, Marks [
29] studied the optimum proportions of wall lengths, their angles and glazing parameters for multistory office buildings in Australia. The study was further extended by Jedrzejuk and Marks [
30] to present a multi-criteria optimisation method of the shape and structure of the buildings and optimisation of heat sources. For the buildings in tropical climate, Mangkuto et al. [
31] conducted a simulation study to investigate the effect of WWR, window orientation and wall reflectance on lighting energy demand and daylight metrics. The optimum solution derived from Pareto optimisation indicated WWR 30%, wall reflectance of 0.8 and south orientation as the optimum design.
The above-discussed studies are much focused on the non-residential buildings such as office buildings which heavily depend on heating and cooling. Residential buildings have a different occupancy pattern and the daytime energy consumption is lower compared to non-residential buildings. Several studies have been carried out to identify the effect of shape, WWR and orientation in residential buildings. Hachem et al. [
32] demonstrated that the number of shading facades and the ratio between the shading to shaded facade significantly affect the solar radiation on non-convex shapes. The study was based on residential buildings in cold climate including seven different shapes (square, rectangle, trapezoid, L,
U, H and T). Bichiou and Krarti [
33] conducted a research on single-family homes in the USA including five different locations. This research considered the building shape, WWR and orientation as important parameters for the optimisation. Three optimisation algorithms were considered and the optimal design reduced the life-cycle cost by 10–25 % depending on the type of homes and climate.
For naturally ventilated residential buildings, Liping et al. [
1] investigated the optimum thermal comfort by changing
U values, WWR, orientations and lengths of the shading device. The research was conducted for a typical residential building in Singapore and the results indicated that the
U value of facade materials for north and south should be less than
\(2.5\,\mathrm{W}/\mathrm{m}^{2}\,K\) and for east and west
U value should be less than
\(2.5 \,\mathrm{W}/\mathrm{m}^{2}\,K\). Further, the optimum WWR was found to be 24%. However, this research has neglected the effect of shape and the lighting energy. In Mirrahimi et al. [
11], the effect of building form was considered for the tropical climate in Malaysia and, in addition, other factors such as external walls, roofs, glazing area and natural ventilation were also evaluated. The research aimed mainly at the thermal performance and the total energy consumption of high-rise residential buildings and the lighting energy was not focused upon.
Most of the studies have been restricted to either air-conditioned buildings or buildings with one zone [
34] and the knowledge of the impact of building form on energy consumption of the naturally ventilated buildings is rare, especially for residential buildings and in hot humid climate. Bre et al. [
35] conducted a residential building design optimisation where some rooms are mechanically ventilated, while some are naturally ventilated. In their study, a typical residential building in Argentina was selected for the case study and, as per the overall results, solar absorptance of external walls, thermal transmittance of externals walls, orientation and window area fraction for natural ventilation were considered as the important factors for reducing the cooling demand.
However, in those previous works, the effect of building shape, WWR, orientation and the zones in residential buildings in hot and humid climate have not been sufficiently discussed. Further, the available studies on tropical residential buildings have not considered the effect of zone sizes or the locations. Furthermore, in most of the studies, the lighting energy demand of the residential buildings is neglected and the total energy demand is considered. Considering the effect of building envelope characteristics on the artificial lighting requirement, it is necessary to identify the effect on lighting energy separately. Given the lack of studies on the naturally ventilated residential buildings, our study aims to investigate the effect of shape, orientation and WWR on the lighting energy requirement and the thermal comfort of naturally ventilated houses, while giving a special emphasis on the zone sizes and the zone locations of the houses.
Hence this study is based on two research questions: (a) what is the effect of shape, zones, orientation and WWR on the lighting energy requirement of naturally ventilated residential buildings and (b) what is the effect of shape, zones, orientation and WWR on the thermal comfort of naturally ventilated residential buildings?