The transition towards a low-carbon future in the EU largely depends on the building sector ^[1][2][3][1,2,3]. Until the 1970s buildings were designed without optimized energy performance in terms of energy demands and consumption, and newly built energy-efficient buildings now make up a small share of the existing building stock. According to some estimates, at least 75% of EU buildings need to be made more energy efficient [4]. Energy renovations of the building envelope and its technical systems aimed at energy efficiency improvements, together with renewable energy technology installations, have been recognized as a key vehicle for achieving the EU energy efficiency target for 2030 and the transition towards climate-neutral Europe by 2050 [3].

An EU-wide definition of energy renovation does not exist. It can be described in terms of a set of intervention measures undertaken for the energy improvement of the building envelope, building technical systems, and renewable heat and electricity generation systems, and incorporation of ‘smart’ technologies. While it is widely acknowledged that an energy renovation should lead to certain energy savings after the intervention work is carried out, the link between the ‘depth’ of building energy renovation and the resulting energy savings is not clear [3]. The most commonly applied measures refer to energy improvement of the building envelope. This includes different technologies and measures applied to the transparent and non-transparent parts of the building envelope, i.e., the use of different thermal insulation systems of external walls, lofts, and roofs; installation of solar shading systems and ventilated facades; over-cladding and re-cladding using resource-efficient building materials; replacement of windows and doors; and introduction of natural ventilation and passive solar heating/cooling techniques.

Furthermore, the share of renewable energy sources (RESs) in gross final energy consumption (GFEC) across the EU has more than doubled in recent years, from 9.6% in 2004 to 21.8% in 2021 [5]. With RESs accounting for more than half of the energy in its GFEC, Sweden (62.6%) had by far the highest share among the EU Member States in 2021, ahead of Finland (43.1%) and Latvia (42.1%). These data reflect the fact that ‘these technologies have become more accessible and citizens have become more empowered’ [6]. In addition, the Clean Energy for all Europeans package [7] and the recast Renewable Energy Directive [8] make it easier for citizens to form energy communities, but also to produce, store, and sell their own renewable energy.

In the Republic of Serbia, energy efficiency is a high-priority matter. In the area of final energy consumption and energy sources in buildings, energy efficiency is regulated by the Law on Energy Efficiency and Rational Use of energy [9], the Law on Construction and Planning [10], the Rulebook on energy efficiency of buildings [11], and the Rulebook on the conditions, content, and manner of issuance of certificates of energy performance of buildings [12]. These regulations transpose the requirements of the EU Energy Performance of Buildings Directive (EPBD 2010/31/EU) [13], and the Energy Efficiency Directive (EED 2012/27/EU) ^[14][15][16][14,15,16] regarding energy efficiency of final consumption and energy services. The Directive amending the Energy Performance of Buildings Directive [15] introduced new elements and sent a strong political signal regarding the EU’s commitment to modernize the buildings sector in light of technological improvements and to increase building renovations. According to the data collected by the Statistical Office of the Republic of Serbia, approximately 55% of a total of 583,908 existing housing units in Belgrade were built in the period before the 1970s [16]. This figure reveals that Belgrade’s building stock has a significant number of buildings whose energy and environmental performance has to be improved. Therefore, significant energy efficiency can be achieved by an appropriate choice of energy renovation technologies.

The political aspect of feed-in tariff values for energy from renewable sources in Serbia is in line with the Directive (2009/28/EC) [17] and the Energy Community Ministerial Council Decision (D/2012/04/MC-EnC) [18], according to which Serbia (as a signatory of the Agreement of Energy Community) was obliged to achieve a very demanding and binding goal of a 27% share of RESs in GFEC in 2020. However, the results show that the share of RES in GFEC in 2019 was 21.44%, instead of the planned 25.6% [19]. In the previous period, the Republic of Serbia implemented the reform and drafted a large number of by-laws in order to align its policy in the process of European integration with the latest EU regulations in the field of RESs and their ambitious goals. The long-term goal requires global greenhouse gas emissions to be reduced by at least 80% below the 1990 levels by 2050 ^[20][21][20,21], while developed countries should reduce their emissions to 80–95% compared to 1990 levels within the same period [22]. Countries in transition, such as Serbia, and developing countries, need to continue to follow this guideline when it comes to the construction sector; it is necessary to introduce appropriate low-carbon technologies to build new energy-efficient buildings and to renovate the existing ones.

2. Multi-Criteria Decision Making (MCDM) Methods for Energy-Sustainable Renovation of Building Envelopes

Around 75% of the total building stock in Europe comprises residential building, of which 36% are multi-apartment housing blocks, and more than half (57%) were built in the period before 1970 [23]. As they typically rely on low-cost technologies, most of these multi-family housing blocks are characterized by poor energy performance [24]. It is estimated that approximately 85% of the 160 million buildings within the European Union (EU) are energy-inefficient [25]. New construction accounts for only 1% of the annual addition to the total gross floor area in the EU [23], and in most industrialized countries by 2050 new buildings will only contribute 10%–20% to additional energy consumption, whereas more than 80% will continue to be consumed by the existing building stock [26]. Therefore, renovation is considered to be the primary factor in achieving the EU sustainability goals of becoming climate neutral by 2050 [27]. A number of studies have attempted to relate the ‘depth’ of renovation to relative energy savings. According to the European Building Performance Institute (BPIE), minor renovations account for 0–30% of the final energy savings, moderate renovations for 30–60%, and ‘deep’ renovations for 60–90%, while renovations of near-zero energy buildings (NZEBs) account for savings of more than 90% [28]. Conducting a cross-regional analysis, the authors in the study [29] concluded that ‘deep’ renovations can be associated with improvements of at least 75% and post-renovation primary energy consumption of less than 60 kWh/m² per year. This research focused mainly on the end uses of heating, cooling, ventilation, and hot water. Furthermore, an extensive body of studies assesses the energy consumption for space heating and domestic hot water, either in residential or public building stock, considering available technologies (heating and electric power systems), combined with renewable energy supply [30]. The analysis shows that significant progress is needed in order to increase the annual rates of building energy renovation (by 3% instead of the expected 2%) and that the NZEB principle needs to be respected in order to achieve the highest level of energy efficiency and meet the national and EU goals by 2050 [27].

2.1. Application of MCDM Methods in Sustainable Building Design and Construction

Recently, the MCDM process has become increasingly prominent in the field of construction sustainability, both in practice and in the academic community ^[31][32][31,32]. MCDM represents one of the most important fields of operations research dealing with problems that involve multiple and conflicting objectives [33]. MCDM is both an approach and a set of techniques, with the aim of providing an overall ordering of options, from the most preferred to the least preferred option ^[33][34][33,34]. MCDM approaches provide a systematic procedure to help decision makers choose the most desirable and satisfactory alternative in an uncertain situation [35]. From a technical-scientific point of view, decision-making support needs to justify its choices clearly and consistently, especially for addressing issues in connection with sustainability [36]. Furthermore, these methods are able to handle both quantitative and qualitative criteria and to manage tension between conflicting criteria and stakeholders’ interests ^[37][38][37,38]. The use of MCDM and the method of multi-criteria decision analysis (MCDA) allows the problem to be considered at two different levels: at the managerial level, objectives are defined and the final optimal alternative is selected, while at the engineering level, alternatives are designed, a multi-criteria assessment of alternatives is performed, and the consequences of the choice are analyzed [39]. These elements have contributed to the increased use of MCDM and MCDA methods in building assessment procedures, providing a framework for evaluation and selection of sustainable building technology options in recent years.

Application of MCDM in ESR of Building Envelopes

The method proposed in [40] for optimizing the building envelope and technical equipment, the Weighted Sum Method, has been used to achieve a reduction in global investment cost, primary energy index, and carbon dioxide emission in relation to the basic scenario. Similarly, in [41], by using a genetic algorithm, NSGA-II, the authors analyzed the relationship between the initial characteristics of residential buildings and the optimal retrofit solutions in terms of either maximum economic performance or energy consumption reduction in NZEBs for the lowest achievable thermal discomfort. Giurca et al. in [42] developed a method for selecting technical solutions for the rehabilitation and thermal and energy modernization of buildings, using the TOPSIS method. In addition, by deeply investigating MCDM design methodologies and processes in the building renovation field, Kamari et al. in ^[43][44][43,44] introduced three sustainable retrofitting frameworks based on (1) application of MCDM including either multiple-attribute decision making (MADM) methods, (2) multiple-objective decision making (MODM), and (3) Holistic Multimethodology for Sustainable Building Renovation (HMSR), to help stakeholders in the renovation process make transparent decisions in a rational order. Unlike previous studies, Dražić and Laban [45] proposed an evaluation and decision-making methodology for the selection of a specific building element—the most suitable type of window—which, in addition to economic-financial, technical, technological, and environmental assessments of considered window types, includes a decision flow algorithm and an optimization method. Similarly, by using multi-criteria analysis, a decision procedure for the most resilient design of a residential wall system was considered in [46] while maintaining the required thermal comfort under global warming and even during an extreme climate event. An overview of the most recent studies regarding the use of MCDM methods in ESR of building envelopes, published in 2020 or later, is given in Table 1. It can be noticed that almost all different methods are applied, across a variety of locations and building types selected for the case studies. According to the authors’ knowledge, the latest example of use of the AHP method was on a cultural heritage building in Italy, where it was used to evaluate the restoration score and to create priorities among different alternative designs of the thermal envelope [47]. In addition, in order to discover opportunities for local clean solar energy production and utilization, by integration of solar thermal collectors (STCs) and PVs into the building envelope, including the facade and the roof, the renewable sources were analyzed using MCDM methods ^[48][49][48,49]. Some other studies [58] discussed the application of MCDA methods in selecting energy supply systems, such as combined cooling, heating, and power (CCHP) systems together with renewable energy systems, from the technological, economic, and sustainability aspects. In addition, considering a large number of different criteria relevant for energy systems (i.e., (a) technical: energy efficiency, primary energy ratio, safety, reliability; (b) economic: investment cost, operation and maintenance cost, fuel cost, electric cost, net present value, payback period, service life; (c) environmental: emission of different gases, non-methane volatile organic compounds, land use, noise; and (d) social: social acceptability and social benefits), the authors [58] concluded that fewer criteria are more beneficial for sustainable energy decision making and proposed the methods for selecting the ‘major’ criteria.

2.2. Application of Hybrid MCDM Methods in Sustainable Building Design and Construction

Although the above-listed approaches provide an insight mainly into individual methods, an increasing use of hybrid tools, i.e., integration of different methods, can be observed recently, owing to their complementarity in fulfilling different tasks in complex design processes. The outcome of the integrated approach helps in prioritizing challenges and also in exposing the interrelationships among the challenges [59]. For example, Pinto et al. [60] describe the use of a hybrid method that integrates AHP and EVAMIX multi-criteria approaches to evaluate design alternatives with a view to improving a building’s performance while preserving heritage identity. In another study [59], an integrated multi-criteria decision making (MCDM) technique comprising the Best–Worst Method (BWM) and Decision-Making Trial and Evaluation of Laboratory (DEMATEL) is used to evaluate the challenges to LCA. BWM is used to prioritize the challenges to LCA, indicating problems with the quality of data, lack of inventory data, difficulty in comparison, absence of a dedicated LCA method, and scale-up issues as the top five critical challenges to LCA adoption. By comparison, DEMATEL is used to reveal interrelationships among the challenges, according to which 7 challenges come under the cause category and the remaining 13 challenges come under the effect category. Considering material selection as a typical multiple-criteria decision making (MCDM) problem, and decision progression with the intention of decreasing the number of possible material alternatives to the final choices, several studies have been conducted integrating different (MCDM) methods. A study [61] was aimed at developing the Combinative Distance-based Assessment (CODAS) method with target-valued attributes to achieve practical and functional applications, particularly in engineering design problems. Consequently, the Step-Wise Weight Assessment Ratio Analysis (SWARA) method has been combined with the proposed multi-attribute decision making (MADM) approach, as one of the extensions of MCDM techniques, to calculate the weights of the criteria. Along with the proposition of a novel and hybrid SWARA-CODAS TB approach, this redearch has also tackled a material selection problem in dam construction. Furthermore, Zolfani and Chatterjee [62] compare the results of variability between the criteria priorities for Step-Wise Weight Assessment Ratio Analysis (SWARA) and the Best–Worst Method (BWM) for weight derivation and make suggestions about the conditions of using these two methods for furnishing material selection problems in sustainable interior design.