The building sector is one of the sectors with the highest greenhouse gas emissions due to its high-energy consumption rate [1]. One-third of the harmful gas emissions worldwide are caused by the energy used in buildings [2] because buildings consume around 40% of the world’s total energy consumption [3]. Energy use in high-density cities accounts for a significant proportion of urban greenhouse gases (GHG) [4,5,6]. The residential sector is considered the second-largest consumer of Energy in Asia, with about 25% of the total energy consumed in South Korea coming from buildings [7]. Since the building sector is highly energy-intensive [3], methods to decrease greenhouse gas emissions by increasing building energy efficiency are critical issues that play a prominent role in energy saving and sustainability. It is paramount that the energy used by buildings is duly optimized [8] to achieve energy savings after building retrofits [9] via the adoption of Energy Efficiency Measures (EEMs), which decrease the amount of energy needed while providing the same level of comfort. To increase the building energy performance by reducing the energy consumption in residential buildings, energy-efficient materials, efficient Heating, Ventilation, and Air Conditioning (HVAC) devices, Light Emitting Diode (LED) lighting systems, Building Management Systems (BMSs), and solar photovoltaic are to be incorporated into the building during building retrofits [10,11].

An essential component of a successful retrofit project is using verified methods and tools such as energy audits, energy monitoring systems, infrared thermography, and blower doors to measure energy efficiency. Among these, the energy audit is an essential process that guides auditors in obtaining comprehensive knowledge of a building’s energy consumption profile and provides practical and systematic approaches to identify efficient energy usage and potential savings [1]. Through cost–benefit analyses and detailed reports, the energy audit assists in identifying and quantifying energy-saving opportunities, assessing the energy performance and carbon footprint of existing buildings, and evaluating the applicability of new energy-efficient technologies [11,12]. The audit process typically consists of four subgroups: walk-through assessment, survey, retrofitting, and data collection for controlling purposes. It is crucial to note that while an energy audit does not directly result in energy savings [13], it plays a crucial role in establishing areas that require improvement and uncovering opportunities for energy conservation, as well as assessing the applicability of new energy-efficient technologies [9]. Thus, selecting suitable Energy Efficiency Measures (EEMs) tailored to specific retrofitting cases becomes essential to optimize the retrofit process by considering energy and cost impacts.

Many researchers have demonstrated that energy efficiency measures in building retrofits can substantially reduce building energy use after energy audits [4]. Khalilnejad et al. [14] conducted virtual energy audits using a non-intrusive and automated method for buildings’ time-series smart-meter data. Mauriello et al. [15] evaluated a building via a one-week, in-home field study in five homes and a semi-structured interview with professional energy auditors. They used a temporal thermographic sensor system to analyze building envelope performance quantitatively. Woo and Moore [16] demonstrated a building energy audit process in a high-density multi-residential modular development in Melbourne, Australia, collecting extensive data on indoor air quality, occupant feedback, and utility usage. The analysis involved pre-survey, walk-through inspection, data collection, and formulation of energy efficiency solutions without software. Kerimray et al. [17] integrated a detailed building stock module into Kazakhstan’s 16-region TIMES energy systems model using statistical data and building energy audit reports. They found networked gas and district heating economically viable substitutes for coal in rural areas and flats, respectively, even with clean technology subsidies. These studies highlight the effectiveness of energy audits in identifying inefficiencies and proposing retrofit solutions.

Efforts are needed to gather reliable data on building energy performance and compare it across different locations and building types to advance energy efficiency practices. The energy consumption of Nigerian buildings remains uncertain. Mambo and Kebe [18] conducted an energy audit of 105 buildings in Nigeria. They identified prevalent issues such as using energy-inefficient products, inadequate daylighting utilization, the absence of building energy management systems, the low adoption of renewable energy systems, and the poor consideration of bioclimatic factors in building orientation. It is important to note that the objectives of improving buildings’ energy efficiency will result in (a) less energy consumption while still maintaining occupants’ comfort level, (b) saving energy and money, and (c) minimizing harmful emissions. However, focusing on constructing high-rise residential buildings in Korean metropolitan cities has led to neglecting energy retrofitting in existing residential buildings [19].

Improving the energy performance of old residential buildings in Korea is imperative to address their high energy consumption, resulting in high utility costs and inadequate indoor comfort. With buildings accounting for 56% of Seoul’s total energy and 87% of the city’s electricity consumption [19], the Korean government initiated the Building Retrofit Program (BRP) in 2008 to reduce greenhouse gas emissions by 40% by 2030. The program enhances energy efficiency by installing new or retrofitting existing equipment, insulation improvements, heating and cooling enhancements, and lighting upgrades.

Green buildings and architecture are necessary for sustainable building development [20]. Buildings are responsible for global high-energy consumption and carbon emissions [21]. Retrofitting measures mitigate the effect of climate change on buildings by improving their energy performance at beneficial cost-effectiveness [22]. The most critical aspect of retrofitting is structural refurbishment, which aids in added strength, stability, and safety [23]. Although retrofitting existing buildings offers significant opportunities to reduce global energy use and greenhouse gas emissions [24], an insight into the applicable building retrofit measures within a climate zone will guide the optimization framework to attaining sustainability in architecture and the built environment [22]. Retrofitting measures are essential for reducing energy consumption in residential and commercial buildings and cooling and heating requirements in hot and cold climates [25]. Many researchers in the past have worked on different retrofitting measures to improve the energy efficiency of buildings [26,27,28,29,30,31].

In 2021, Rabani et al. [32] studied optimizing energy use and improving building thermal and visual comfort conditions. They employed an optimization method that allowed for the simultaneous optimization of various aspects, such as the building envelope, energy supply, fenestration, shading devices, and control methods. Qu et al. [33] examined three types of passive interior retrofits, namely internal wall insulation, glazing upgrade, and airtightness improvement, for a historic building renovation. Their evaluation considered energy-saving potential, affordability, and thermal comfort performance and proposed five assessment indicators, including energy reduction rate and specific initial cost. Fina et al. [34] investigated the profitability of implementing active and passive retrofitting measures in buildings in 2021. They developed an optimization model to quantify the impact of renovation measures on the heat load. Their findings revealed that the profitability of passive retrofitting measures, specifically building envelope renovation, depends significantly on additional costs associated with CO₂ emissions and the default heating system.

Furthermore, Shirazi and Ashuri [35] explored retrofitting measures to enhance the operational energy consumption of different building categories in the United States (U.S.) in 2020. They assessed the embodied impacts associated with these measures and compared their environmental efficiency. The study identified that retrofitting residential buildings, mainly through attic/knee insulation and HVAC unit replacement, had the highest environmental impacts. This underscored the significant environmental effects of retrofitting foundation wall insulation and upgrading windows in dwellings constructed before the 1970s.

In a study by Pallonetto et al. [36], the energy savings achieved via retrofitting measures on Irish residential buildings were investigated. By progressively retrofitting detached dwellings to become all electric, energy savings of up to 45% and CO₂ reductions of approximately 29% were achieved compared to the pre-retrofitted buildings. Andrade-Cabrera et al. [37], in 2017, developed an automated calibration methodology for retrofitted buildings using parametric Energy Conservation Measures (ECMs) functions. Through Particle Swarm Optimization, retrofit functions were calibrated based on a baseline model representative of the building before the retrofit. The analysis demonstrated that the proposed methodology could effectively calibrate retrofitted building models with acceptable accuracy, generating lumped parameter building models with similar dynamics for different ECMs across various building energy models. In another study by Pallonetto et al. [38], conducted in 2016, the impact of building retrofit measures on the carbon footprint of dwellings was examined using EnergyPlus. The retrofit measures resulted in an overall reduction in carbon footprint from 43.3 to 30.8 kg/m² CO₂, considering a pre-retrofitted dwelling as the baseline. The case study accounted for a mix of energy supply sources, including fossil fuel for space heating, electricity for household equipment, and conventional gasoline cars for transportation.