Enhancing Thermal Environment Quality with Voids and Indoor Gardens as a Passive Design Strategy towards Sustainable and Healthy Living

Subjective evaluation plays an important role in assessing indoor thermal quality (ASHRAE-55). This study assesses the physical and pyschological aspects of thermal comfort in two modified rooms, namely the Department of Architecture Universitas Muhammadiyah Jakarta (UMJ) Building Technology Laboratory (Room A) and the Architecture Student Community “Fathirista” Room (Room B). Both rooms have similar microclimate factors (humidity, temperature, radiation and air velocity) were distinguished as one of the room was modified with an additional void and indoor garden space, while another room would be only added indoor garden space. In the experimental process, four types of modification were applied to both rooms. 26 students, who were familiar with the indoor climate environment of the modified rooms, were then asked to experience the four thermal condition modifications and to describe them by completing a Likert scale questionaire as a subjective evaluation step. Through this semi-experimental research, the aim was to discover different levels of thermal comfort experience through a perceptional study as an interpretation of subjective evaluation. The second purpose was to establish whether voids and indoor gardens could significantly affect thermal comfort. The data were analysed with SPSS software, with the results showing that the modified rooms with a void and additional indoor plants (Room A) was the most comfortable room according to the respondents’ subjective evaluation. Even though the respondents had experienced the same thermal environment for years, they had different subjective evaluations towards the four modified conditions.

Healthy house; Indoor garden; Sustainable architecture; Thermal comfort; Void

As stated in ANSI/ASHRAE Standard 55 which later be revised by De Dear & Brager (2002), subjective human evaluation is important in asssessing thermal comfort, because results can only be meaningful by interpreting the condition of a person’s mind that expresses satisfaction with the thermal environment. Thermal neutrality is maintained when the heat generated by the human metabolism is allowed to dissipate, thus maintaining thermal equilibrium with the surroundings.

Bakar et al. (2011) states that several aspects must be considered in sustainable housing development. Aside from non-physical aspects, such as social issues, economics and communication, there are also physical aspects that may affect housing sustainability, including the environment in general, site/land use, transportation and the assessment of building forms for housing performance. Baker evaluated various sustainable rating systems in order to find the best one for housing in Malaysia in particular. His research emphasized that to have a healthier and more sustainable built environment, we must enhance the physical aspect quality of buildings, in which thermal comfort is included. Wong and Khoo (2003) analyzed the thermal comfort of a classroom in Singapore using the ASHRAE scale, Bedford scale, votes of preference and direct votes of acceptability in order to investigate occupants’ perceptions. Their research found that the classroom occupants generally accepted cool thermal sensations more readily than warm ones. They showed that the acceptable temperature range was from 27.1^oC to 29.3^oC, implying that the ASHRAE standard of 55–92 was not applicable in the free-running buildings in the local climate. However, they did not suggest any solutions to improve the indoor thermal quality in the classroom.

Expectations and perceptions of thermal comfort can be affected by hot and humid environmental conditions throughout the year and by personal adaptation (Feriadi & Wong, 2004). This is also supported by Humphreys et al. (2007) and Nikolopoulou and Steemers (2003), who state that thermal comfort is based on the context of people’s lives, because individuals who are accustomed to living in climates in different spaces will have different thermal comfort perceptions. Satwikasari (2018) confirmed that physical factors of the house can affect the occupant’s health state. She combined the perceptional study to measured the health quality of the occupants who suffered from Tuberculosis (TB) and also did field measurement to evaluated the thermal environment of their houses.

It is necessary for the room temperature to be averaged over a sufficient period of time, so that it can represent the usual conditions that people experience in their accommodation. The psychological dimension of thermal adaptation refers to an altered perception of, and reaction to, sensory information due to past experience and expectations. When the indoor thermal level of a building moves towards an uncomfortable level, people tend to take active strategies, such as turning on the air conditioning (AC), taking off clothes or opening windows. The greater the number of active strategies taken by the occupants, the higher the corresponding significant increase in energy consumption. To minimise energy consumption and to prevent deterioration of indoor thermal quality, passive designs have been chosen by researcher and architect in general as the best strategy to increase the quality of each thermal environment variable.

One of the most common passive design strategies in a low rise building is a void. This is basically an empty space located in between the upper and lower floors. It usually serves as a air circulation regulator, so that the temperature and natural light intake of a building can be maintained. Adding a hole in the upper floor allows occupants to let more air flow naturally inside the building from any air inlet in the top level of the building. A void also gives more chance for the occupants to adjust the local air quantity they need in order to balance their body temperature. Moreover, the existence of voids is very important to support the comfort and health of a building, especially for buildings that are close to their neighbours, because the hole of the void can also act as a wind tower, draining the hot air up, and giving a perfect cross-ventilation effect inside the building. Besides voids, another strategy to increase indoor air and thermal quality is to create an indoor green space. By adding indoor plants, oxygen availability inside a building can be increased. Aside from filtering dust within 3 km of dust sources, more than 75% of dust can be filtered by dense vegetation. Indoor plants can also reduces heat, as they serve as shade, and cool the room from the effects of direct solar UV and radiation. Klemm et al. (2015) suggests that adding more green space could be perceived as a better thermal environment.

Voids and indoor gardens were chosen by our team as the main modification aspects, as they can be two practical and affordable solutions for small to low-rise residential buildings. The funds needed to renovate an upper floor area with voids and to add indoor plants are considered to be quite low compared to the purchase of mechanical equipment for artificial ventilation, such as air conditioners, fans or exhaust fans. Furthermore, the addition of vertical air holes can also be a solution for a low-rise houses located in dense residential areas, which do not have residual land next to them. Therefore, an experiment with both passive design strategies is urgently needed in order to convince the community about the function of both voids and indoor gardens.

Prior research related to this semi-experimental project by Hakim (2015; 2009) analysed the measured data with ANOVA methods for each thermal comfort variable, namely temperature (T), absolute humidity (AH), relative humidity (Rh), air velocity (v), and Bedford Comfort Level (S), in the case of the existence of a void. However, the results showed that only AH, Rh, v and S were significantly influenced by the existence of the void. These results were confirmed by the DMRT (Duncan Multiple Ranges Test) method, especially the significant differences in air velocity (v) in both modified rooms.

In this advanced research, a perceptional study was conducted to measure the personal thermal comfort by employing a Likert scale questionnaire, with a scale derived from Bedford Comfort Levels (S), with selected respondents, with analysis using SPSS software to observe the percentage of each comfort level. By stabilizing the indoor air and temperature with the addition of void and indoor gardens, we aim to discover different thermal comfort experiences through a perceptional study as an interpretation of subjective evaluation. The second purpose is to prove whether voids and indoor gardens, as passive design strategies, can significantly affect thermal comfort quality in low-rise residential buildings. The paper will explain the assessment process and results of the thermal experiences in the modified experiment rooms with four different conditions.

On analysis of the results, the participants felt more comfortable in a room with an additional void, inicating the ‘cool’ or ‘cold’ options on the questionnaire. This shows that ‘cool’ or ‘cold’ sensations had become the standard comfort level for the respondents in the research. This means that by adding a void and installing some indoor plants below the void area, can successfully create the best thermal experience. Even though the respondents were accustomed to a certain microclimate in the modified rooms, they felt different individual thermal sensations. This might be because they had different thermal comfort tolerance levels, depending on their activities or daily life outside the building. This fact adds a new finding to the previous statement about perceptual opinions regarding thermal comfort. Subjective evaluation was not only affected by the quality of ventilation design to improve the comfort level, but also the context of individual life, which may affect sensitivity levels to changing conditions.

Through this experiment, there is strong evidence that by adding an appropriate void inside a narrow and dark room, especially in low-rise buildings, it is possible to effectively increase indoor thermal quality. By adding an ‘additional hole’ above, an additional air outlet is also created, resulting in cross-ventilation. With faster air velocity (V) inside a room, relative humidity (RH) will move towards its ideal percentage, thus also increasing the Bedford Thermal Standard (S) point. Moreover, indoor plants will add more green space inside a building, which could improve the oxygen banks and subsequently occupants’ health.

Suggestions for further research include finding the ideal void area percentage to floor area, as this can affect the Floor-Area-Ratio (FAR). Voids can also be an effective alternative passive design for the dense settlements in urban areas, as vertical ventilation helps building occupants to get more natural ventilation and lighting. Hopefully, this research will initiate more projects related to healthy and green built environments by optimising passive strategies.

We would like to thank LPPM Universitas Muhammdiyah Jakarta (UMJ), which provided research funding, the Department of Architecture UMJ, and also the Faculty of Engineering UMJ, for giving us chance to conduct this research and use their facilities.

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