Impacts of façade openings' geometry on natural ventilation and occupants’ perception: A review
Introduction
Climate change as a result of greenhouse gas (GHG) emissions from human activities is one of the critical issues in the current century. It is estimated that buildings consume around 40% of global energy and contribute more than 30% (between 30% and 40%) of GHG emissions [1]. Since a large percentage of these harmful emissions comes from Heating Ventilation and Air Conditioning (HVAC) [2,3], finding low or zero-carbon alternatives to HVAC is a persuasive idea to reduce GHG emissions. One of these alternatives is natural ventilation (NV) [4], which can be used for cooling as an alternative for Mechanical Ventilation (MV) all around the world.
NV, in general, is a process whereby fresh air is introduced to indoor spaces without using any mechanical system [5]. In most cases, NV employs natural forces such as wind and buoyancy, to move fresh air into an indoor area that occurs as results of pressure and density differences [6]. These forces are usually affecting the NV simultaneously and can act in isolation, or opposition, or reinforce each other and sometimes operate in synergy [7]; however, one of these forces is always predominant, determined using Archimedes number (less than one is wind-induced) or Froude number (less than one is buoyancy forces) [8,9]. Applying the NV strategy to modern architecture and quantifying its impact on modern expectations of comfort, could demonstrate a significant potential to reduce Mechanical Ventilation (MV) usage and persuade occupants of its usefulness in meeting their comfort needs. Numerous studies show the remarkable potential of NV strategies for ventilation purposes [[10], [11], [12]] and the critical priorities of these strategies compared with MV.
In modern architecture, various architectural elements such as windows, ventilation grills, wind catchers, solar chimneys, wing walls, and overall façade system are typically considered by designers for NV purposes in buildings [[13], [14], [15]]. Among these elements, façade design such as the provision of windows, overhangs and balconies remains one of the leading technologies for NV purposes to reduce energy consumption [[16], [17], [18], [19], [20]]. A balcony, as a private outdoor space, is perceived by residents as one the most desired features for providing fresh air, particularly in cooling dominant climates such as tropical and subtropical regions [21]. Furthermore, other typical applications of balconies such as entertaining and drying laundry suggest a balcony as an essential design element, particularly in residential apartments [22]. Consequently, it seems necessary to investigate balconies’ impacts on NV utilisation and performance in both social and technical aspects.
The provision of balconies, in general, changes the pressure around building façades and subsequently affects the NV driving forces, including wind and buoyancy [23,24]. Balconies mostly affect outdoor and indoor airflow profiles and Indoor Air Velocity (IAV) that lead to changing indoor thermal comfort. Over the last decade, results of some studies that focused on the influence of the balconies on NV performance, reveal the critical role and importance of balconies' features, such as depth, on the indoor NV and thermal comfort in its attached room [[25], [26], [27], [28]]. Undeniably, the design of balconies appears to be a determinant factor of NV performance at the purely technical view. Thus, an investigation and classification of the balconies’ geometric features and their impacts of the related parameters to the NV performance, such as IAV, could extract critical information in the sustainable design context.
On the other hand, since NV utilisation has a social nature that might be affected by technical design decisions, there is a need to address socio-technical factors (i.e. comfort and satisfaction) and their effects on the quantity and quality of the NV utilisation through balconies to improve design decisions based upon these parameters [29]. For instance, an exploration of the literature on Post-Occupancy Evaluation (POE) revealed some design-related disruptive impacts such as balcony scale on NV utilisation [22,30,31]. Subsequently, there is a significant opportunity for design improvement through investigation of socio-technical factors to firstly realise the critical design-related factors that have been focused in previous studies, and then find the degree of effectiveness through a detailed review of these POE studies in the concept of NV through balconies.
The design of balconies should also be carried out with consideration of vital standards of the health and comfort of attached indoor spaces. This is because a conflict between occupant satisfaction, comfort, and health on the one hand, and energy saving on the other hand, is usually a critical issue, which sometimes sparks controversy over the impacts of energy saving on occupants' comfort and health. The importance of efficient and sustainable design, as well as various standards, criteria, and rules for designing, are attempting to reduce the gap between the occupants' comfort and higher usage of energy in buildings. One benefit of these national or international standards such as American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) [32] is to recommend criteria of living spaces to satisfy residents' health and comforts with regards to energy saving matters. Occupants also play a crucial role since they carry out the act of energy-saving. A review on buildings' design, as well as users’ forgiveness factor, which depends on overall comfort and average satisfaction scores of ventilation or air and temperature in summer [[33], [34], [35]], could determine the interaction between these parameters.
This review article aims to explore the impacts of designing balconies' geometry on NV performance and perceptions and behaviour of users, as two vital technical and socio-technical factors, respectively. For this, the current paper firstly summarises the application of the most considered passive design elements, for NV utilisation in building. The article explores the number of studies that have focused on the application of balconies for NV utilisation, the principal focus of the current research, compared to other elements for passive design in Section 2. Section 3 focuses on studies that have investigated the influence of design features of balconies on NV performance and classified the crucial parameters based on the degree of impacts reported in the literature. Socio-technical factors such as occupants' comfort are considered through an exploration of the application of POE on NV utilisation through different façade openings, and specifically through balconies, to find the degree of comfort, critical obstacles, and incentives, under NV mode instead of MV (Section 4). The most common methods that have been employed to identify the critical design elements of balconies and impacts of geometry's features on users are investigated in Section 5. Results, including research gaps, are summarised and lead to a comprehensive discussion and recommendations for possible future studies to fill the gaps (Section 6). Finally, the current article draws an overall conclusion in Section 7.
Section snippets
Natural ventilation (NV)
Ventilation is employed to supply fresh air, maintain comfort (temperature and humidity), and eliminate or dilute contamination [36]. There are two primary types of ventilation, MV and NV, which are usually applied by occupants, particularly for cooling purposes. Energy consumption concerns and global warming due to GHG emissions are the critical reasons causing NV to be considered as an appropriate alternative for MV. NV uses outside air movement and buoyancy pressure differences to refresh
Design parameters of balcony
Studies on how balcony design features affect NV performance and thermal comfort inside apartments are limited in the literature, and these parameters have not been adequately explored, unlike other openings such as windows. Among the limited number of publications, a few articles focused on the impacts of balcony design on NV performance. These studies mainly explored the effects of the availability of the balcony and then investigated the impact of various balcony design parameters on NV
Socio-technical parameters
Because the purpose of NV is to assist in occupant comfort, it is critical to investigate the impact of balcony design features on occupant sensation, comfort, and behaviour due to the social nature of NV utilisation [87]. Passive designers should always be aware of drawbacks, as well as incentives, that could affect the utilisation of NV through balconies. For instance, violating thermal comfort and privacy are crucial factors that sometimes stop users from utilising NV, while having access to
Methods for evaluation NV effectiveness
The literature reveals several methods or techniques used to evaluate the impact of balcony geometries on NV performance and occupant perceptions [107]. This section aims to investigate the most practical approaches through the literature and compare their advantages and limitations.
Summary and suggested future studies
Although designers and researchers have increasingly considered NV to reduce GHG emissions and energy consumption, there are not comprehensive review studies to investigate the impacts of design-related parameters of passive elements on NV performance and occupations’ perception. The current article, therefore, has comprehensively reviewed the articles on NV through façade openings, particularly balconies that are the key focus of the current study. For this purpose, the current article focuses
Conclusion
The current article focuses on the technical and socio-technical impacts of designing façade openings, specifically balconies, on NV performance and thermal comfort through a comprehensive review. The early results show that the application of balconies for NV utilisation has not been adequately investigated yet. The detailed review also recommends that more studies are required to reveal the impacts of design-related parameters of balconies. The literature review also revealed that there is an
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