Energy Retroﬁtting Technologies of Buildings: A Review-Based Assessment

: Demand for energy and resources is increasing day by day. The construction industry plays a major role in the consumption of energy and resources. Buildings that were built before energy-efﬁcient sustainable practices became popular consume a larger portion of energy as compared to the new buildings. As a result, enhancing energy performance through retroﬁtting of those old buildings is a major concern in the construction industry. In the modern built environment, there are many technologies available in the market to enhance the energy performance of such buildings. However, the body of knowledge regarding energy retroﬁtting technologies is still scattered. Therefore, in this study, a review-based assessment was undertaken to identify energy retroﬁtting technologies that could enhance energy performance in existing buildings. The Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) protocol was followed during the article screening and selection for this study. Following a systematic ﬁltering process, a total of 149 out of 643 research contributions have been considered for in-depth analysis of energy retroﬁtting classiﬁcation and respective energy retroﬁtting technologies. According to the review, energy retroﬁtting technologies are categorized into three main aspects; building envelope retroﬁtting, building system retroﬁtting, and renewable energy. The study found thirty-seven (37) energy-related retroﬁtting technologies in the current context. Further, 25 possible energy retroﬁtting technologies were identiﬁed under the six main subcategories, including façade, roof ﬂoor, window, door, and orientation. In terms of building system retroﬁtting, 10 possible energy retroﬁtting technologies were identiﬁed under the HVAC system and lighting system. The remaining two technologies identiﬁed under the renewable energy category were solar and wind technology. The review further conﬁrms that the application of energy-related retroﬁtting technologies has the highest contribution in terms of energy and cost saving of existing buildings. It is expected that this outcome would better guide stakeholders in decision-making with regard to the selection of energy retroﬁtting technologies in a given context.


Introduction
In the modernized world, a major concern is raised over heavy environmental impacts, depletion of resources, and difficulties in the supply of energy [1]. Demand for energy and resources is increasing day by day as they are considered the main driving forces for the larger portion of the global economy [2]. The authors further point out that technological advancement and population growth are the fundamental causes of it, and to overcome this, it is essential to enhance energy and resource efficiency [3].
The construction industry has a major impact on the natural environment [4]. The authors further say that the consumption of natural energy resources and emission of CO 2 is considerably higher in the construction industry. According to the Intergovernmental Panel on Climate Change (IPCC), the building sector is accountable for 40% of the total energy consumption and 25% of the total CO 2 emission globally [5]. This results in serious environmental consequences such as global warming, air pollution, acid rain, and climate change [6]. Therefore, most countries imposed various building energy regulations, standards, and codes to minimize the utilization of resources and energy [7].
Green buildings are part of an emerging concept in the construction industry that emphasizes the utilization of resources and energy in a sustainable manner [8]. This plays a vital role in achieving sustainable development (SD) by balancing the three pillars of the SD: social, economic, and environmental aspects [9]. Environmentally friendly buildings not only reduce resource and energy consumption but also use available recycling opportunities [10].
Instead of constructing new buildings, improving energy and environmental efficiency and decreasing water consumption enhance comfort and quality of a space related to natural lighting, air quality, and noise of existing buildings and play a vital role in achieving sustainable development [11]. Existing buildings with poor energy performance consume a larger portion of energy than new buildings [12]. A study by Liu et al. [3] also indicated that the construction of new buildings is responsible for a small percentage of the total energy consumption of the construction industry. Constructing a new green building by demolishing an existing building is a totally contrasting concept of energy conservation [9]. The authors also find that "by some estimates, it would take more than 65 years to regain the energy savings of demolishing an existing building and replacing it with a new green building". Moreover, it is not realistic to dismantle all existing structures in order to develop green buildings. As such, one appropriate alternative could be green retrofitting of buildings [13].
Green retrofitting buildings is the modification of existing buildings with an enhancement of energy efficiency, ease of operations, and improvement of environmental performance [6]. It focuses on energy savings by incorporating renewable energy sources, delivering fresh and filtered air within the building's interior, and increasing the overall efficiency of the structure [14]. A study by Jafari et al. [15] asserted that the emergence of retrofitting buildings tends to reduce energy consumption by 30-40%. Similarly, European Union countries also believe that through green retrofitting, 20% of building energy can be saved by 2030 [16].
Energy efficiency of buildings has been a focus of research for decades, as existing buildings account for a significant portion of the building sector's energy usage and carbon impact [17]. Further, the existing building stock is a prime target for energy-saving initiatives aimed at reducing the negative effects of buildings on the environment, human health, and the economy [18,19]. Therefore, one of the aims of building retrofitting is to analyze the building's energy use and economic fitness of using appropriate combinations of energy conservation measures [20].
For example, Chidiac et al. [21] researched the energy-saving potential of the application of energy retrofitting technologies in Canada including preheat upgrades, heat recovery, daylighting, boiler efficiency economizer, as well as lighting load reduction in Canada, and stated that the application of the aforementioned technologies saved 20% of energy consumption. In addition, Ascione et al. [22] evaluated the energy-saving potential and cost implications of the application of different energy retrofitting technologies (modifying the set point of indoor temperature, reducing the infiltration, increasing the thermal insulation of vertical walls, installing condensation gas heaters instead of old boilers) in Canada and concluded that there is a positive impact of the application of energy retrofitting technologies (i.e., 22% of energy saving and 11 years of payback period). Similarly, Fluhrer et al. [23] assessed the cost and benefits of some of the most commonly applicable energy retrofitting technologies in the USA and proved that identified technologies (upgraded windows; insulating reflecting barriers; daylighting, lighting, and plugs for tenants; retrofitting of chiller plants; ventilation controlled by demand; direct digital controls that are balanced) would account for 38% of energy saving equal to USD 22 million in cost savings.
Furthermore, Dascalaki and Santamouris [24] indicated that improvements to the building envelope; the use of passive systems and techniques; improvements to heating, cooling, and ventilation systems; modifications to the lighting and utilization of daylight are some of the common energy retrofitting options in Greece and those retrofitting measures account for 48-56% of energy saving. According to Al-Ragom [25], insulating the wall and roof area, upgrading the glazing system, and reducing the area of windows contribute to reducing energy consumption by 24-47% in Kuwait.
The foregoing review finds evidence that various green retrofit technologies with varying levels of contribution to energy saving are readily available for energy retrofit projects. Despite this, the body of knowledge in this regard is still scattered. Further, many studies demonstrate that the lack of knowledge and awareness of stakeholders about energy retrofitting technologies is one of the major barriers to the adaptation of energy retrofitting [26][27][28][29][30]. Hence, it is important to review and analyze what has already been reported in the literature regarding energy-related retrofit technologies and to widen the knowledge about this domain. With the aim of fulfilling the abovementioned research gap, it is necessary to perform a study to review the available energy retrofitting technologies that could enhance the energy performance of the building sector. To complete this, a systematic review was conducted with the following research question: What are the energy retrofitting technologies that could enhance the energy performance of the building sector?
It is expected that the outcome of this review will provide significant knowledge of energy-related retrofitting technologies available for existing buildings with low performance. The review results will be helpful to all stakeholders in the construction supply chain who are interested in accelerating the energy retrofitting implementation.
The paper is structured as follows: Section 2 briefly discusses the methodology adopted in this study; Section 3 examines the classification of energy-related retrofitting; Section 4 discusses energy-related retrofitting technologies; Section 5 presents the conclusions of the study.

Materials and Methods
A literature review is a practical method for gaining a comprehensive knowledge of the current literature in a certain research area [31]. There are two types of literature reviews: narrative reviews and systematic reviews [32]. Without a specific methodological approach, a narrative review can cover a wide range of themes at varying comprehensive levels through existing literature [33]. A systematic review provides a wide range of literature evaluations that may be used to solve well-structured and specialized research questions using a specific methodological approach [34]. Compared to a narrative literature review, a systematic review is able to identify and screen the existing body of knowledge and thereby create new knowledge on a wider scale [35].
As such, the systematic review technique was used to carry out the literature search in this study. To conduct the systematic review, the preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) approach was adopted in this study. According to the PRISMA guidelines, four main steps should be followed in a systematic review: (1) identification, (2) screening, (3) eligibility, and (4) inclusion.
Prior to applying the PRISMA guidelines, it is essential to establish a search string based on the research question. Accordingly, a search string was developed to answer the research question: "What are the possible energy retrofitting technologies that could enhance the energy performance of the building sector?" During the development of the search strategy, keywords that describe the research question were considered. Energy retrofit, sustainable retrofit, green retrofit, and existing building are the basic search terms used in this study. When developing the final search strategy, the Boolean operator of "AND" was used to combine each keyword while "OR" was used to link synonyms. Further, wild cards such as question (?) and asterisk marks (*) were introduced to several terms to maximize the search results, and quotation marks ("") were used to identify the exact terms in this study.
As the next step, the developed search string was applied to three different databases; Web of Science, Scopus, and Science Direct. Though there are number of search engines available in the academic field, the aforementioned three databases provide adequate research outputs related to the established research question of this study. These databases can be considered the largest continuously updated academic databases, and they have been widely used in similar reviews [36]. Figure 1 illustrates the PRISMA flowchart that was used to screen and select the articles for the systematic review in this study. "AND" was used to combine each keyword while "OR" was used to link synonyms. Further, wild cards such as question (?) and asterisk marks (*) were introduced to several terms to maximize the search results, and quotation marks ("") were used to identify the exact terms in this study. As the next step, the developed search string was applied to three different databases; Web of Science, Scopus, and Science Direct. Though there are number of search engines available in the academic field, the aforementioned three databases provide adequate research outputs related to the established research question of this study. These databases can be considered the largest continuously updated academic databases, and they have been widely used in similar reviews [36]. Figure 1 illustrates the PRISMA flowchart that was used to screen and select the articles for the systematic review in this study.  As seen in Figure 1, the final search found 643 records: 183 from Web of Science, 310 from Scopus, and 150 records from Science Direct. Subsequently, records were narrowed down by considering relevant filters under search fields, publication year, subject/research area, document type, and language. From the identified total search, 252 records were excluded after the application of filters. Out of the remaining records, 96 duplicate records were removed, and 295 records were selected for the title, abstract, and keyword screening process. The preliminary screening identified that 98 of them had no relevance to the research question of this study. The full-text review of the remaining 197 records resulted in 149 research contributions for further in-depth analysis. Of those, 21 records discuss the classification of energy retrofitting while 131 records present energy retrofitting technologies.
As the final step, the search results were further analyzed and we synthesized their contents to extract the knowledge on energy retrofitting technologies as presented in the following sections. Accordingly, the review outcome is organized into two main sections: (1) classification of energy retrofitting and (2) respective energy retrofitting technologies.

Classification of Energy Retrofitting Technologies
This section presents the review outcome of 21 selected academic publications out of a total of 149 records identified in the systematic review.
The primary purpose of installing green retrofit technologies is to improve energy efficiency to a specific level [37]. Building construction, operation, and maintenance all necessitate the use of energy, which could be decreased by using energy-efficient technologies [38]. Ahmad et al. [39] defined energy-efficient technologies as techniques used to reduce the amount of energy required to supply products and services. To improve energy efficiency in existing buildings, energy retrofitting could be accomplished through well-prepared technologies and high levels of equipment efficiency on both the supply and demand sides [40]. On the other hand, energy retrofitting is a broad term that refers to energy efficiency techniques ranging from modest adjustments to substantial renovations. Energy-efficient retrofits of existing buildings are vital to meeting vital global energy and environmental goals [41]. As a result, there are many different types of energy retrofits for buildings [41]. Therefore, a wide range of energy retrofits is used for buildings. Energy retrofits are classified using a variety of categories, including the number of systems chosen for retrofitting, the amount of energy saved, the type of building system, the cost per square meter, the energy conservation methods employed, and the payback time [42]. Classification of green retrofitting measures based on the energy conversation method used and the amount of energy saved based on studies such as those by PNNL [43] and Ma et al. [44] is shown in Figure 2.
Energies 2023, 16, x FOR PEER REVIEW 5 of 16 As seen in Figure 1, the final search found 643 records: 183 from Web of Science, 310 from Scopus, and 150 records from Science Direct. Subsequently, records were narrowed down by considering relevant filters under search fields, publication year, subject/research area, document type, and language. From the identified total search, 252 records were excluded after the application of filters. Out of the remaining records, 96 duplicate records were removed, and 295 records were selected for the title, abstract, and keyword screening process. The preliminary screening identified that 98 of them had no relevance to the research question of this study. The full-text review of the remaining 197 records resulted in 149 research contributions for further in-depth analysis. Of those, 21 records discuss the classification of energy retrofitting while 131 records present energy retrofitting technologies.
As the final step, the search results were further analyzed and we synthesized their contents to extract the knowledge on energy retrofitting technologies as presented in the following sections. Accordingly, the review outcome is organized into two main sections: (1) classification of energy retrofitting and (2) respective energy retrofitting technologies.

Classification of Energy Retrofitting Technologies
This section presents the review outcome of 21 selected academic publications out of a total of 149 records identified in the systematic review.
The primary purpose of installing green retrofit technologies is to improve energy efficiency to a specific level [37]. Building construction, operation, and maintenance all necessitate the use of energy, which could be decreased by using energy-efficient technologies [38]. Ahmad et al. [39] defined energy-efficient technologies as techniques used to reduce the amount of energy required to supply products and services. To improve energy efficiency in existing buildings, energy retrofitting could be accomplished through well-prepared technologies and high levels of equipment efficiency on both the supply and demand sides [40]. On the other hand, energy retrofitting is a broad term that refers to energy efficiency techniques ranging from modest adjustments to substantial renovations. Energy-efficient retrofits of existing buildings are vital to meeting vital global energy and environmental goals [41]. As a result, there are many different types of energy retrofits for buildings [41]. Therefore, a wide range of energy retrofits is used for buildings. Energy retrofits are classified using a variety of categories, including the number of systems chosen for retrofitting, the amount of energy saved, the type of building system, the cost per square meter, the energy conservation methods employed, and the payback time [42]. Classification of green retrofitting measures based on the energy conversation method used and the amount of energy saved based on studies such as those by PNNL [43]and Ma et al. [44] is shown in Figure 2.

Types of Retrofits Based on Energy Conservation Method
Energy retrofits can better manage building energy through a variety of methods. These approaches include regulating energy supply using renewable energy sources, controlling energy demand through energy-efficient renovations, and lowering energy waste through human behavior management [45].

Supply Side Management
Renewable technologies such as solar photovoltaics, biomass, wind energy, and geothermal power systems are used in supply-side management retrofits [46,47]. Furthermore, supply-side retrofits equip buildings with alternative electrical and thermal energy [44].

Demand Side Management
Demand-side management refers to the use of energy-efficient technology, initiatives, or policies to reduce power usage [48]. Instead of merely increasing supply, demand-side management enables a utility to manage the balance of its resources and demands for energy by controlling the needs of its customers [49].

Human Factors Management
In terms of design, delivery, and use, a successful retrofit must be "human-centered." Evidence reveals that a lack of understanding of human factors might cause retrofit projects to fail in terms of delivery or long-term performance against their goals [50]. As an example, the authors discovered that variable occupant energy use patterns can change building energy usage with a sensitivity analysis of a building energy model [51]. Comfort requirements, occupancy regimes, occupant activities, and access controls can be modified to save energy [44].

Existing Building Commissioning (EBC)
Commissioning is a thorough quality assurance procedure that ensures a building operates as planned and that the facility's crew is ready to operate and maintain its systems and equipment. Further, EBC can be divided into recommissioning, retro-commissioning, and ongoing commissioning [52]. Consequently, by optimizing building operations and restructuring maintenance methods, significant energy savings can frequently be obtained with minimum risk and capital outlay [53]. Additionally, commissioning is the process of ensuring that buildings function effectively, which involves identifying and implementing energy-saving options on existing equipment and processes but excludes substantial investments in capital equipment [54].

Standard Retrofit
Standard retrofit measures are cost-effective for building owners who are only able to make incremental capital improvements. Equipment, system, and assembly retrofits are comprised of standard retrofit measures [43]. Furthermore, these retrofits usually improve the system to maximize each building's potential energy saving with low investment cost, which is more cost-effective for the system users [55].

Deep Retrofit
A building owner can reduce energy consumption significantly beyond the savings from operation and maintenance and standard retrofit procedures with a deep retrofit project [43]. When applied to buildings with poor overall efficiency and multiple building systems, deep retrofits are the most cost-effective [56]. Even so, the synergy between the various systems should be examined before any deep energy retrofit measures are implemented to achieve better performance [57]. According to Jermyn and Richman [58], deep energy retrofits are whole-building retrofits that include building envelope and HVAC system upgrades to achieve large energy efficiency gains. Consequently, PNNL [43] further stated that deep energy retrofits are a mix of several EBCs and standard retrofits which address several systems of the existing building.
Of the above classifications, EBC has been identified as the lower energy-saving and the most straightforward type of energy retrofit [59].

Energy Retrofitting Technologies
Apart from the above classification, based on the application, energy retrofitting technologies can be classified under three aspects: (1) building envelope-related, (2) building system-related, and (3) renewable energy-related measures [60]. Façade, roof, floor, window, door, and orientation belong to the main building envelope categories [61,62]. Under the building systems, heating ventilation, and air conditioning (HVAC) systems and the lighting system were identified, whereas solar and wind technologies are grouped under the renewable energy category [63,64]. Figure 3 illustrates these classifications. implemented to achieve better performance [57]. According to Jermyn and Richman [58], deep energy retrofits are whole-building retrofits that include building envelope and HVAC system upgrades to achieve large energy efficiency gains. Consequently, PNNL [43] further stated that deep energy retrofits are a mix of several EBCs and standard retrofits which address several systems of the existing building. Of the above classifications, EBC has been identified as the lower energy-saving and the most straightforward type of energy retrofit [59].

Energy Retrofitting Technologies
Apart from the above classification, based on the application, energy retrofitting technologies can be classified under three aspects: (1) building envelope-related, (2) building system-related, and (3) renewable energy-related measures [60]. Façade, roof, floor, window, door, and orientation belong to the main building envelope categories [61,62]]. Under the building systems, heating ventilation, and air conditioning (HVAC) systems and the lighting system were identified, whereas solar and wind technologies are grouped under the renewable energy category [63,64]. Figure 3 illustrates these classifications.

Building Envelope-Related Technologies
Façade retrofitting is the most effective passive retrofitting design strategy due to two reasons: (1) external walls are the major surface exposed to direct sunlight, thus contributing to obtaining the biggest cooling and heating demand for the building; and (2) as a result of that, façade retrofitting can increase the energy efficiency of buildings and thereby reduce the cost of energy and contribute to maintaining the indoor environmental quality as well [65]. The authors further identified five (05) façade retrofitting applications that are more focused on energy saving: low-emissivity glazing, wall insulation, shading

Building Envelope-Related Technologies
Façade retrofitting is the most effective passive retrofitting design strategy due to two reasons: (1) external walls are the major surface exposed to direct sunlight, thus contributing to obtaining the biggest cooling and heating demand for the building; and (2) as a result of that, façade retrofitting can increase the energy efficiency of buildings and thereby reduce the cost of energy and contribute to maintaining the indoor environmental quality as well [65]. The authors further identified five (05) façade retrofitting applications that are more focused on energy saving: low-emissivity glazing, wall insulation, shading devices, light shelves, and surface reactivity. Among those five strategies, wall insulation and shading devices show more significant benefits in energy saving [66]. However, when considering the cost perspective, wall insulation and low-emissivity glazing are comparatively economical [67]. Low-emissivity glazing used in green buildings and green retrofitting generally cut emissions by 4% and thereby help to increase energy efficiency indirectly [65]. Wall insulations act as a thermal buffer to protect the building from the exterior environment [65]. Expanded polystyrene, mineral wool, and polyurethane foam are the materials most commonly used in wall insulation. Hosseini et al. [68] stated that the application of super-insulation aerogel instead of a traditional marble envelope also enhances the thermal resistance of wall insulation. Further, Luo et al. [69] highlighted that the selection of insulation materials that have less thickness will encourage better performance in terms of energy usage and cost. When referring to shading devices, shading devices reduce the direct solar radiation falling onto the building facades. Application of awnings, vertical and horizontal fins, recessed windows, overhangs, and vegetation are some commonly used shading strategies [65]. Green walls are the most common façade retrofitting technology that reduces cooling energy demand in both winter and summer seasons [70]. In addition, green walls contribute to maintaining indoor and outdoor air quality improvement as well as water efficiency [71][72][73]. According to He et al. [74], the integration of overhangs into residential buildings reduces energy consumption by up to 5.3% in hot climates, whereas applying both overhangs and side fins in the same building in the same climates can save a further 1.4% of energy. Light shelves refer to the horizontal architectural devices that allow deep penetration of natural light into the building and reflective surfaces refer to light color building materials that have high solar reflective indices, such as integral color treated concrete. Further, external wall finishes can also be considered one of the major options for reflective surfaces [65].
Similar to façade retrofitting, the incorporation of insulation layers is the most common roof retrofitting strategy. However, Dall'O et al. [75] point out that at least a minimum of 0.15 U-value (thermal transmittance) should be available in the chosen insulation materials for the roof insulation. In addition, a green roof can act as a thermal insulation layer of the existing roof structure and increase the thermal performance. For example, Niachou et al. [76] revealed that green roofs have 45-46% and 22-45% annual energy-saving potential for heating and cooling, respectively, compared to non-vegetated roofs. Similar to green wall implementation, a green roof improves air quality and water efficiency too [61,77]. Application of roof painting, installation of a suspended insulated ceiling with good thermal properties, and installation of low-impact cool or high reflective clay tiles in the roof are some other roof retrofitting technologies [62,78]. The application of floor retrofitting technologies also increases energy efficiency. The addition of floor insulations such as extruded polystyrene and gypsum board is the main way of floor retrofitting [79].
Hube [80] stated that the heat transmission through windows is five times greater than that of the other parts of the building envelope and, as a result, accounts for 20% of energy loss. However, this negative effect can be mitigated with the introduction of retrofitting measures. For example, in the United States, 7-16% of energy consumption in a residential building can be saved through window retrofitting [81]. Moreover, Sartori and Calmon [82] stated that retrofitting windows in residential buildings in Brazil saves about 15% of energy. Further, inefficient window glazing increases energy consumption in buildings [83]. Further, the performance level of the windows can be decided based on their U-value; 1.11 is the best value [75]. According to Hosseini et al. [68], the usage of low-emissivity triple-glazing windows instead of existing two-glazing systems enhances the energy saving of a building. The authors further state that double or triple low-emissivity glazing has the potential of reducing energy requirements by 40% per unit area. Moreover, triple-glazed windows show small payback periods [69]. Similarly, Blom et al. [84] mentioned that installing high-efficiency double glazing instead of single or double glazing would reduce the poor thermal performance of existing windows. According to He et al. [85], double-glazed windows are mainly used in cool climate zones whereas single-reflective glazing is highly used in relatively warm climate zones. In addition, the literature has identified many other window retrofitting options, such as installing diffusers/louvers on the windows, application of low-emissivity coating, and maximizing the deployment of natural light and ventilation [78,86]. In terms of the building envelope, doors also play a significant role in energy efficiency and maintaining indoor environmental quality. Adding a seal, caulk, gasket, or weather strip to prevent air leakages is the most popular retrofitting strategy for doors [63,78].

Building System-Related Technologies
In general, the major portion of energy consumption in buildings is used for HVAC applications [87] and it is about half of the energy used in buildings [1]. Hence, it is important to enhance the HVAC performance for better functioning. Fine and Touchie [88] revealed that by retrofitting HVAC systems alone, the energy required for space heating in residential buildings in Canada can be reduced by 14%. There are many retrofitting technologies available in the current market. According to Sobti and Singh [87], the use of earth heat exchangers instead of conventional AC systems can be considered as the one of most feasible and economical solutions. However, the authors further stated that this earth heat exchanger technology is not suitable for climates with a more humid nature due to the damage of pipes in systems with condensation. The most common HVAC retrofitting technology is the replacement of inefficient systems with efficient systems; split air conditioning systems, solar air conditioning systems, and hybrid air conditioning systems [78]. Dall'O et al. [75] stated that the incorporation of heat recovery systems into mechanical air ventilation systems can have 70% heat recovery efficiency. The authors further identified another retrofitting measure that shows maximum performance in energy efficiency as the installation of local control systems (i.e., thermostatic valves). According to Sun et al. [77], most existing buildings have poor HVAC system performance due to the absence of a proper management system and authors further suggest the installation of a building management system as a timely solution for that. Updating the chiller plant, tuning up the HVAC system, installing variable devices (i.e., variable frequency drives (VFD), variable speed drives (VSD), variable refrigerant volume (VRV), variable air volume (VAV)) and allowing more natural ventilation are some other retrofitting applications related to the HVA system [89][90][91].
According to Al-Kodmany [63], most of the existing buildings do not have energyefficient lighting systems, and upgrading the lighting system with energy-efficient fixtures will provide significant energy savings of approximately 70%. Sun et al. [77] recorded that light emitting diode (LED) lighting is the most feasible and economical lighting retrofitting option due to the following features of LEDs: semiconductor light source, low power consumption, and long life span. In addition, LED lights are more durable and offer better light quality compared to other lighting types [92]. For example, the life span of LED lights is longer than 50,000 h, and fluorescent lights and incandescent lights have lifetime limits of 8000 h and 1000 h, respectively [63]. In addition, the replacement of T12 linear fluorescent with T8, and the replacement of T8 with T5 also increase the energy efficiency [63]. Furthermore, the integration of lighting controls (i.e., daylighting control, lighting occupancy control, constant lighting control) sensors (i.e., time or occupancy sensor) avoid energy waste when there are no occupants and there is sufficient natural light [60,78,93].

Renewable Energy-Related Technologies
In most cases, day-to-day fuel requirements depend on fossil fuels. However, due to the cost and the scarcity of these resources, most researchers are focusing on renewable sources of energy [63]. To this end, the application of renewables is another dominant area in the retrofitting of existing buildings. According to Tan et al. [60], the most common available retrofitting technologies for renewable energy are solar water heating, building-integrated photovoltaics (BIPV), and building integrated wind turbines (BIWT). Similarly, He et al. [92] identified the solar domestic hot water system as the frequently used renewable technology in residential buildings. As per Luo and Oyedele [69], the lifecycle cost saving of the installation of wind turbines is higher than that of photovoltaic panels. However, the application of retrofitting technologies related to renewable energy is problematic due to their high initial cost [60]. According to Al-Kodmany [63], though the initial investment in those technologies is comparatively high, running cost savings of those measures can offset the negative effect of high initial cost in most cases. Table 1 presents the retrofitting technologies identified from the review of relevant literature sources. Table 1. Energy retrofitting technologies identified from the review.

Conclusions
This systematic review determined that energy-related retrofitting technologies can be categorized based on main two criteria: energy conservation and the amount of energy saved. The review confirms that energy conservation retrofitting technologies are more in demand in the current society. Further, there are three types of energy retrofitting technologies based on energy conservation as supply side, demand side, and human factor management. Supply-side management mainly focuses on the generation of renewable energy whereas demand-side management refers to the technologies that can reduce a building's energy use. The third category is human factor management, which focuses on changes in energy consumption. In the case of retrofitting technologies based on the amount of energy saved, three categories are categorized as technologies that save energy up to 15%, 15-45%, and above 45%: EBC, standard retrofits, and deep retrofits, respectively.
The study further found thirty-seven (37) energy-related retrofitting technologies under two main categories of building envelope retrofitting measures and building system retrofitting measures. Out of 37 energy retrofitting measures, 25 measures are discussed under the building envelope context, 10 measures are identified as energy retrofitting technologies that can be applied to the building systems, and the remaining 2 technologies are identified in terms of renewable energy. Façade, roof, wall, floor, window, door, and orientation are identified as the main building envelope measures that can be considered as the main contributors responsible for the internal and external heat transmission of the building. When considering building system retrofitting, HVAC and lighting systems receive more attention due to their contribution to building energy consumption. Wind and solar technologies are identified under the renewable category. Besides, the review further pointed out that the performance of green retrofitting of existing buildings can be increased by the incorporation of many retrofitting technologies rather than being limited to one technology. In addition, review results concluded that the choice of retrofitting technologies for a particular existing structure may vary with different factors including region, climate, building age, building type, and occupants.
Due to the well-documented nature of the energy crisis and the energy consumption issues in the current world, the systematic review of this study mainly focused on energy-related retrofitting technologies. However, other sustainability criteria including water efficiency, indoor environmental quality, resources and materials used, and the sustainable site, also influence the achievement of sustainability of existing building stocks. Therefore, it is expected that future studies would assess other retrofitting options that could be integrated into existing buildings. Moreover, this study suggests further future directions to establish the proper decision-making basis to choose suitable retrofitting technologies for a given context. This is mainly due to the availability of a large number of retrofitting measures.
However, there are still several limitations in this study. The study considers journal articles as the document type because usually they go through a systematic peer review process and have a higher recognition in the selected domain. Further, language was limited to English as it is the universal language, though some articles can be found in other languages. A further study could address the abovementioned limitations.
This systematic review can enhance the familiarity of stakeholders with the currently available energy-related retrofitting technologies that could improve the inefficient energy performances in existing buildings. Further, these identified technologies will help in the decision-making of selecting energy retrofitting measures for a given context.

Data Availability Statement:
The data that support the findings of this study are available within the article.

Conflicts of Interest:
The authors declare no conflict of interest.