The construction of apartment neighborhoods on a massive scale in South Korea began with the start of the “Han River miracle” (1968–1997) [1
], the rapid economic growth initiated after the Korean War that elevated the country from a condition of poverty and external economic support to industrialization and a service-based economy [2
]. During the period of rapid economic growth, a multitude of national policies were developed to address the shortage of housing for the population displaced after the end of the Korean War [3
]. However, public reforms aimed at solving the housing shortage would in parallel aim to support the private construction business to foster economic growth, with the increasing liberalization of the real estate market [4
]. As such, housing in South Korea would become both a necessity for society and the public authorities, and an opportunity for speculation by the private sector [5
]. The typology of the apartment building was accordingly developed to minimize construction times, and maximize profits [6
]. The common apartment layout, originally developed at the end of the 1960s, is based on the traditional Korean house, the Hanok [7
], with multiple apartment units stacked on top of one another for more than 15 floors using reinforced concrete construction [8
]. The layout and structural system in reinforced concrete were used to increase the amount and facilitate the mass construction of apartment buildings in a short time span. The use and quality of construction materials for the realization of apartment buildings was minimized to reduce building costs [9
]. Strategies aimed at the maximum reduction of construction materials for apartments have also been supported by the lack of regulations in terms of building insulation, the regulations for which were only introduced in 1987 [10
]. Accordingly, the quality of apartments in South Korea built since the 1960s is relatively poor, causing apartment buildings to require integral renovation of the building envelope and technical systems after only 20 years [11
]. The constant deterioration of the housing stock in South Korea is further exacerbated by the high number of apartment buildings constructed during the period between 1968 and 2016, when 10 million apartment buildings were constructed. Approximately 50% of all South Korean households live in apartments, 40% of which have been in operation for more than 20 years [13
]. In just the period between 2005 and 2012, the percentage of apartment buildings in operation for more than 20 years rose steadily from 15.2 to 32.4% [14
]. Accordingly, by the end of 2020, the number of aged apartment buildings in South Korea will greatly increase, as a substantial share of apartment buildings has been realized between 1990 and 2000 (3.603 million apartment buildings, 44% of the housing stock as of 2010) (Figure 1
In the early 2000s, the historical purpose of the national policies and plans to provide housing coverage for all Korean households was fulfilled [17
]. Accordingly, demand for residential buildings declined, and speculative initiatives, as well as redevelopment initiatives aimed at demolishing existing apartment neighborhoods and replacing them with new residential complexes with an increased number of unit floors and rent prices (the so-called Joint Redevelopment Initiatives [18
]), were less effective in producing profits [19
]. The decline of the real estate market, also due to the Asian financial crisis of 1997, aligned with a change of national policies towards a more sustainable economy [20
]. In 2004, the South Korean Government started the “Low-Carbon Green Growth” [21
] plan for the reduction of Greenhouse Gases (GHG) across all economic sectors. The plan foresees the reduction of 30% of GHG emissions from the South Korean productive industry by 2030 [22
]. For the construction sector, the legal limits to the energy demand for the operation of new building projects have been consistently and progressively reduced by lowering the admitted U-values of construction components and the building envelope. Furthermore, all new public and 60% of private buildings will have to reach the zero energy standard by 2025 [23
]. Additionally, the Green Growth plan foresees the construction of 1 million “green houses” by 2020 [24
The directives contained in the Green Growth plan do not specifically address the concern of raising the wellbeing of building occupants. The main purpose of initiatives aimed toward the development of sustainable architecture in South Korea are tailored to reaching a gradual and substantial reduction of building energy consumption, without developing both design and assessment criteria for the improvement of thermal comfort in residential buildings. Furthermore, the analysis of the apartment building types being constructed from 1968 to the 2010s shows a conservative tendency with the construction of the same apartment layout types, which allow spatial features such as enclosed decks and internal buffer zones to be preserved as constant characteristics in projects of both new and aged apartment buildings [25
]. Accordingly, the impact of the standard apartment configuration on users’ behavior and thermal comfort remains unchanged. Furthermore, the widespread use of the ondol [26
The analysis of international literature on the research subject of quantifying the effect of building renovation concentrated mainly on energy efficiency and the reduction of heating and cooling loads through building renovations and building envelope improvements [27
]. Research results from the analysis of indoor comfort improvement by building envelope renovation with a modular system provide design parameters based exclusively on specific indicators, omitting an integrated design approach [29
]. Integrated studies on the increase of buildings’ indoor comfort and energy-efficiency concentrate on case studies of new constructions. Such studies provide no solutions for renovations and the development of construction components on the basis of indoor comfort and energy efficiency-related parameters [30
]. Recent research efforts in the field of the simulation analysis of thermal comfort in the Korean apartment separately address the impact of hybrid as well as natural ventilation systems [31
] and technical cooling and heating on occupants’ wellbeing during different seasons of the year [33
]. In particular, the research literature proposing solutions to alter the indoor temperature of apartment units and increase occupants’ thermal comfort concentrates on evaluating the use of active systems, such as blinds and interior shading systems [35
]. Additionally, the implementation of improvement measures for the outdoors in apartment neighborhoods decreases the effect of the urban heat island on indoor comfort [36
]. Accordingly, a more comprehensive approach, taking into account mandated legal requirements for a lower building envelope U-value in existing constructions and producing sustainable buildings in both social and technical terms, is yet missing. Furthermore, both simulation-based and statistical studies do not concentrate on individuating the causes of lower thermal comfort in apartment buildings but focus on providing an assessment of the status quo of apartment dwellers’ thermal comfort. Accordingly, the relation and causality between energy consumption for technical heating and cooling and thermal comfort is not investigated. The existing studies do not discuss solutions to reduce energy consumption for technical heating and cooling or increase thermal comfort through the use of passive strategies and the correction of user behavior through the alteration of the building envelope and apartment unit standard layout. Recent studies have investigated the relation between different apartment building typologies and the cumulative solar radiation received according to their orientation. Previous studies are focused on identifying specific features in residential constructions that could maximize passive solar gains [37
]. However, no study has yet discussed the influence of passive solar gains on building comfort. In particular, solutions to take advantage of the higher solar exposure of apartment units in residential buildings to increase comfort are missing.
This study provides an assessment and design framework for the individuation of comfort issues and their causes in apartment buildings, and its application to an exemplary apartment building located in Seoul. The BIM-parametric renovation framework allows the definition of environmental and building construction-related variables that both directly and indirectly influence the indoor thermal comfort of apartment buildings’ occupants. The building energy parametric simulations allow the influence of environmental variables, such as solar gains from sun exposure and heat losses/gains due to ventilation through window openings on the indoor thermal comfort of apartment building occupants, to be quantified. Additionally, user behavior through the setting of operative schedules defines a further criterion to assess thermal comfort in realistic conditions. Accordingly, thermal comfort defines an important aspect that determines the development of the enhanced building envelope, influencing parameters such as the insulation thickness and the g-value of windows components in the modular panels of the enhanced envelope system. The simulation of the energy requirements and the analysis of the thermal comfort of occupants after the installation of the enhanced building envelope system on the existing construction allow calculation of the increase in comfort of apartment building dwellers. Comfort analyses executed on the renovated apartment establish a direct connection between the reduction of energy for technical heating and cooling and indoor thermal comfort. The iterative process of the BIM-parametric renovation framework provides the possibility of weighing the impact of multiple renovation solutions on thermal comfort. Therefore, the renovation framework establishes thermal comfort as a relevant and quantifiable element to be considered in the development of refurbishment projects for a wide range of buildings in South Korea and Asia. The BIM-parametric renovation framework reduces the time and cost involved in the development of renovation solutions, as it operates on changeable variables, and does not require the simulation models to be rebuilt for each renovation variant. Furthermore, the BIM-parametric renovation framework allows a realistic analysis on the existing comfort condition of apartment dwellers to be conducted. The renovation framework takes into account environmental variables based on the statistical climate data of the apartment location. Accordingly, the results of environmental and energy simulation are improved in accuracy and allow the development of more successful renovation projects by addressing specific issues with tailored cost-effective solutions. Therefore, this research presents an effective method for the renovation of aged buildings through an enhanced BIM-building parametric simulation integrated design framework. This framework facilitated a comprehensive and integrated analysis of the building performance regarding service energy demand, as well as visual and thermal comfort. The case study analyzed demonstrates the effectiveness of the framework and identifies potential improvements to the development of BIM-parametric software integrated renovation design development. The BIM-parametric integrated framework allows controlling and dimensioning the design of building components directly through accurate building simulations, improving the efficiency of the digital-to-realization chain of renovation projects.
This study attempts to provide an assessment of indoor comfort in Korean apartment buildings through the analysis of a standard apartment unit and the application of a BIM-parametric renovation framework. In particular, the execution of energy simulations partially takes into account local conditions. Local conditions included the cumulative solar radiation incident on the building envelope, shading from neighboring buildings, and the average U-value and transmission indexes of Korean construction components. Conversely, outdoor dry bulb temperature and humidity data are retrieved from the Energyplus database for the climatic zone of Seoul. Climatic data offer only an approximation of the real thermal condition of the area selected for the execution of the case study analysis. Accordingly, the BIM-parametric system should be expanded to take into account local variations in terms of temperature and humidity depending on phenomena such as traffic concentration, urban heat island effect, soil sealing, and water runoff, among others. The introduction of local variables into the simulation of building energy demand and occupants’ comfort would allow a higher degree of precision in modeling the indoor and outdoor comfort condition of existing apartments. Accordingly, simulations of the energy demand of apartment units could be more accurately produced. Local variables determined by natural and anthropogenic elements exert a specific effect on the condition of the building envelope. External agents could greatly affect the extension and range of the interventions required for the in-situ refurbishment of the analyzed building and similar apartment construction, such as the amount of insulation or the construction materials utilized for the design of the enhanced building envelope modules.
Furthermore, the analysis of local conditions can improve the refinement of hybrid adaptive comfort models, which have only recently found application in the scientific literature (see Section 2
, “Materials and Methods”). The individuation of dynamics influencing user behavior in actively altering their comfort conditions allows the correction of thermal comfort thresholds in the hybrid adaptive comfort model. The identification of external dynamics allows even prediction of the user behavior, if dwellers were compelled to select between active (technical heating and cooling) and passive (natural ventilation, increased clothing) measures to reach their desired comfort conditions. In the case of building 103, the exemplar case of traffic particulate concentration in the Gireum neighborhood defines a specific agent influencing the willingness of users to open windows to provide their apartments with natural ventilation and passive cooling. Accordingly, such dynamic could greatly influence the indoor comfort of apartments, and the acceptable comfort thresholds for apartment dwellers.
Additionally, local tendencies and characteristics in terms of the choice of heating and cooling technical systems can greatly influence the resulting preferences in terms of indoor comfort for the case of South Korea. For the setting of comfort indoor temperatures and schedules in the Energyplus simulations of both existing and renovated apartment unit conditions presented in this research, a benchmark study [57
] that collected statistical data on preferred heating and cooling temperatures for indoor comfort in South Korean apartment has been consulted. Accordingly, the use of the ondol traditional floor heating system in South Korea influences the mean radiant temperature of the floor surfaces in apartment buildings. Specifically, the benchmark study shows that the required ondol floor heating mean radiant surface temperatures tend to be higher than the acceptable comfort threshold calculated for the EN 15251 Class II model. Higher temperature values depend on the functioning of the ondol heating system as the air temperature of indoor spaces would reach a comfortable temperature levels through convection. Moreover, the introduction of outside air through natural ventilation during winter would contribute to enhancing the difference between air and mean radiant surface temperatures. Therefore, specific adjustments to the comfort model used to evaluate indoor user conditions should be introduced. Temperature adjustments would reflect the specific technologies and use modes for technical heating and cooling.
For the execution of the Energyplus simulation, two schedules, one for winter and one for summer, have been defined. Accordingly, the definition of the two schedules is based on the relatively short duration of spring and autumn seasons in South Korea, in terms of mean temperatures and precipitation concentration [63
]. However, the months between March and June, as well as between September and November, show a comparably higher average temperature difference between daytime and nighttime in respect of the entire year. Variations in terms of outdoor temperatures can greatly influence the comfort condition of apartment occupants, and their adaptive behavior in changing their comfort conditions. Ventilation, and heating and cooling set-point operative schedules for both active and passive solutions could be altered extensively, even only for one week or daily cycle. Accordingly, sharp local temperature variations can contribute to reducing the overall annual amount of comfort hours, altering therefore results for the indoor comfort analysis of apartment units. Since daily temperature transitions exceeding variations of 5 °C in less than 3 h, such as those that occur during spring and autumn in South Korea, can contribute to altering comfort solutions and strategies, and different scenarios in terms of operative schedules and user interventions should be defined. Each scenario should be defined by the combination of active and passive schedules in altering the indoor comfort conditions in South Korean apartments. Adaptive scenarios can vary based on the alteration of clothing and intensity of natural ventilation, as well as heating and cooling set-points. Specific adaptive scenarios determine a more detailed and precise method to define the real behavior of apartment occupants than the statistical analysis of a significant pool of apartment buildings in South Korea. Moreover, the energy consumption for technical cooling and heating in apartment buildings can be compared according to different adaptive scenarios. The best performing case could be selected as the one providing the lowest energy consumption and highest thermal comfort, both impacting the wellbeing of users, and resulting in a more sustainable energy consumption for apartment buildings.
The Energyplus simulation executed for the analysis, comparison, and quantification of indoor comfort hour improvements between the existing and renovated apartment unit conditions identifies the influence of non-conditioned buffer enclosed deck zones on indoor comfort. The solution proposed for the renovation of the apartment unit analyzed is the extension of indoor spaces into the buffer zones. The extension of the apartment unit indoor floor space creates de facto new areas that require additional energy for technical cooling and heating. However, the overall reduction in U-value for the building envelope allows energy loads for technical heating and cooling to be substantially reduced. The addition of new zones requiring technical cooling and heating could influence the energy demand of renovated apartment buildings. Increased technically heated and cooled area could prevent further optimization of energy consumption and indoor comfort. Additionally, the presence of buffer zones is considered beneficial for the dissipation of excessive heat from solar gains, reduction of temperature decrease/increase due to filtration of air introduced in the apartment through natural ventilation. Furthermore, passive temperature control in apartment units is depending on outdoor temperature variations, through the increase or decrease of the exposure of indoor spaces to non-conditioned areas. Accordingly, the definition of minimal alternative renovation strategies, such as the replacement of enclosed deck windows with better-performing components, and the quantification of indoor comfort improvement according to the principle of minimum intervention, should be additionally analyzed. The definition of low-intensity renovation strategies allows determination of the potential benefit buffer zones could improve indoor comfort with the minimum material and economic expense for the renovation of apartment buildings.
The renovation of the apartment building analyzed in this research proposes the installation of a solar shading system, both for functional and comfort purposes. The influence of the shading system on the indoor comfort of the renovated apartment condition has been evaluated through the inclusion of Bidirectional Scattering Distribution Function (BSDF [64
])-based window lighting models. The BSDF-based method factored in the calculation of solar gains and resulting air temperature for the zones of the Energyplus apartment unit model. Accordingly, the operative schedules defined for the deployment of the shading system determine an active intervention of users to alter their comfort condition by effectively diminishing the amount of light penetration indoors. Therefore, the operative model based on user intervention on the orientation of solar shading should be accounted for in the evaluation of the adaptive comfort model for the calculation of indoor comfort hours through the EN 15251 Class II comfort model. However, the calculation of temperatures according to pre-set operative conditions is executed through the BIM-parametric renovation framework in a precedent step, before the mapping of indoor comfort for apartment spaces. Therefore, the use of passive methods, such as solar shading and natural ventilation, are not included in the same model, but separately included in the assessment of indoor comfort. The adaptive comfort model defines in fact comfort zones that take into account the potential action of users in changing their comfort condition through natural ventilation only. However, the comfort analysis according to the adaptive comfort model is executed after the calculation of indoor temperatures, following the dynamic simulation of solar gains. Different execution procedures for specific passive strategies can alter the comfort model and produce unclear results. Accordingly, a more comprehensive model should be defined to take into account different passive strategies and their effects on indoor comfort, without the need to subdivide the analysis process into specific modules.
To solve issues with the alternate phasing of the comfort analyses of data produced by energy simulations depending on the behavior of users, a recursive indoor comfort evaluation model could be established within the BIM-parametric renovation system. A recursive comfort analysis system calculates the comfort condition of occupants in multiple instances by introducing corrections in repeated energy simulation and comfort analysis iterations. The comfort analysis iterations evaluate the impact of different passive and active strategies in altering the indoor temperature, humidity, and daylight. Each recursion allows change in the user-defined variables, such as the heating and cooling set points, ventilation schedules, shading orientation, and operation to minimize energy demand, as well as maximize indoor comfort. The recursive comfort method could therefore define a specific tool to assess indoor comfort with more accuracy, depending on detailed local conditions for the apartment units analyzed.