Building Retrofitting Process and Different Methods:A Comprehensive Review for Postgraduate Researchers

Abstract

Building retrofitting represents a critical strategy for improving energy efficiency, sustainability, and structural performance of the existing building stock worldwide. This comprehensive review synthesizes current research on building retrofitting processes and methodologies, providing postgraduate researchers with a thorough understanding of the field. The review examines retrofitting buildings for sustainability (RBS) as the upgrading and modification of existing buildings to improve their energy efficiency, sustainability, and overall functional performance [1]. Through systematic analysis of recent literature, this study categorizes retrofitting methods into four primary domains: building envelope retrofitting, structural retrofitting, mechanical systems retrofitting, and integrated retrofitting approaches. The paper analyzes decision-making processes, implementation strategies, and performance evaluation methods across different retrofitting types. Key findings indicate that in all types of buildings and climates, the most discussed retrofit strategies were building envelope insulation, improving the climatisation and lighting systems, and using renewable energy sources – representing approximately 70% of the strategies [2]. The review identifies significant research gaps in integrated retrofitting approaches and highlights the need for standardized measurement and verification protocols. Future research directions include developing artificial intelligence-driven decision-making tools, improving cost-effectiveness of deep retrofitting measures, and advancing integrated seismic-energy retrofitting techniques for climate-resilient buildings.

Keywords: building retrofitting, energy efficiency, structural retrofitting, decision-making, performance assessment, sustainability.

1. Introduction

The global building sector represents one of the most significant contributors to energy consumption and greenhouse gas emissions, accounting for 34% of final energy use and 37% of energy-related CO₂ emissions [3]. With 80% of existing buildings projected to still be in use by 2050 [4], the retrofitting of existing buildings has emerged as a crucial strategy for achieving sustainability goals and mitigating climate change impacts. The urgency of addressing building performance is further emphasized by the fact that in the European Union (EU), three-quarters of buildings are energy-inefficient, contributing significantly to carbon emissions [5].Building retrofitting, fundamentally defined as the modification or conversion (not a complete replacement) of an existing process, facility or structure [6], encompasses a wide range of interventions designed to improve building performance across multiple dimensions. These interventions extend beyond simple energy efficiency improvements to include structural enhancements, indoor environmental quality improvements, and aesthetic upgrades that collectively contribute to building sustainability and occupant well-being.The complexity of building retrofitting lies in its multidisciplinary nature, requiring integration of technical, economic, environmental, and social considerations. Different retrofit measures may have different impacts on associated building sub-systems due to these interactions, which results that the selection of the retrofit technologies becomes very complex [7]. This complexity necessitates systematic approaches to decision-making, implementation, and performance evaluation that can effectively navigate the trade-offs between competing objectives while ensuring optimal outcomes.Recent advances in retrofitting technologies, decision-support tools, and performance assessment methodologies have significantly expanded the possibilities for effective building upgrades. However, traditional retrofitting methods need a considerable upgrade; therefore, they often require significant time and financial investment, and can disrupt the building operations and occupant activities during implementation. Consequently, developing fast-response retrofitting solutions to save energy on urban and large scales is critical for city planners and policymakers [8].This comprehensive review aims to provide postgraduate researchers with a systematic understanding of building retrofitting processes and different methods, examining the state-of-the-art approaches, identifying current research gaps, and suggesting future research directions. The review addresses three primary research questions: (1) What are the current methodological approaches to building retrofitting across different building systems and components? (2) How do decision-making processes and implementation strategies vary across different retrofitting types? (3) What are the emerging trends and future research directions in building retrofitting research?

2. Literature Review and Research Methodology

2.1 Review Methodology

This systematic review was conducted following established guidelines for comprehensive literature surveys. The search strategy encompassed multiple scientific databases including ScienceDirect, Web of Science, and Scopus, focusing on peer-reviewed articles published between 2012 and 2024. Search terms included combinations of "building retrofitting," "building renovation," "energy efficiency," "structural retrofitting," "building envelope," "HVAC systems," "decision-making," and "performance assessment."The initial search yielded over 1,000 relevant articles, which were systematically screened based on relevance, quality, and contribution to the field. The final review corpus comprised 391 studies as referenced in recent comprehensive analyses, with particular emphasis on studies examining global trends in envelope retrofit techniques over the past decade, with 70% of studies published in the last five years [9].

2.2 Scope and Classification Framework

Building retrofitting encompasses diverse interventions that can be classified across multiple dimensions. For this review, we adopt a systematic classification framework that organizes retrofitting approaches into four primary categories:

1. Building Envelope Retrofitting: Interventions targeting the building's thermal boundary, including walls, roofs, windows, and facades
2. Structural Retrofitting: Measures aimed at improving structural performance, particularly seismic resistance and load-bearing capacity
3. Mechanical Systems Retrofitting: Upgrades to HVAC, lighting, and other building services systems
4. Integrated Retrofitting: Holistic approaches that combine multiple retrofitting strategies to achieve synergistic benefits

This classification provides a comprehensive framework for understanding the diverse approaches to building retrofitting while acknowledging the interconnected nature of building systems.

3. Building Envelope Retrofitting Methods

3.1 Overview and Significance

Building envelope retrofitting represents the most extensively researched area in building retrofitting, with 55% of studies concentrating on reducing energy consumption, with wall system retrofits being the most common (47%), followed by glazing (37%) and shading (16%) [10]. The building envelope serves as the primary interface between interior and exterior environments, making it a critical determinant of building energy performance and occupant comfort.

3.2 Wall System Retrofitting

Wall system retrofitting encompasses various approaches to improving thermal performance, moisture management, and structural integrity. Traditional approaches focus on adding insulation layers, either externally or internally, while advanced methods incorporate innovative materials and integrated systems.External Insulation SystemsExternal thermal insulation composite systems (ETICS) represent one of the most effective approaches to wall retrofitting. These systems provide continuous insulation layers that minimize thermal bridging while protecting the existing wall structure from thermal stresses. Recent innovations in ETICS include the integration of vacuum insulation panels (VIPs) and aerogel-based materials that achieve superior thermal performance in minimal thickness.Advanced Materials IntegrationThe integration of advanced materials in wall retrofitting has shown significant promise for enhanced performance. The objective is to systematically evaluate the effectiveness, performance, economic and environmental impacts, retrofitting techniques and challenges of using advanced building envelope materials, phase change materials, aerogels, vacuum insulation panels, and heat-reflective coatings for energy retrofitting in residential buildings [11]. Phase change materials (PCMs) integrated into wall systems can provide thermal mass benefits and help regulate indoor temperatures, while aerogel insulation offers exceptional thermal performance in thin profiles.

Innovative Composite SystemsRecent research has explored the development of composite retrofitting systems that combine structural and energy performance improvements. A novel concept for the simultaneous seismic and energy retrofitting of Reinforced Concrete (RC) and masonry building envelopes, combining Textile Reinforced Mortar (TRM) jacketing and thermal insulation materials or systems. The hybrid structural-plus-energy retrofitting solutions examined are based on inorganic materials providing both cost effectiveness and fire resistance for the building envelope [12].

3.3 Window and Glazing System Retrofitting

Window retrofitting represents a high-impact intervention for building envelope improvement, given the significant heat transfer and solar heat gain associated with glazing systems. Modern retrofitting approaches range from simple glazing replacement to comprehensive window system upgrades.High-Performance Glazing SystemsThe evolution of glazing technology has provided numerous options for window retrofitting. Triple-glazed units, low-emissivity coatings, and gas-filled cavities represent established technologies that significantly improve thermal performance. Recent developments include dynamic glazing systems that can automatically adjust their thermal and optical properties in response to environmental conditions.Integrated Window SystemsAdvanced window retrofitting approaches integrate multiple performance aspects, including thermal, visual, and acoustic comfort. The photochromic film significantly improved visual comfort indicated by a 10%–17% improvement in the useful daylight illuminance and a 9% reduction in peak discomfort glare probability [13]. These integrated systems optimize the balance between energy efficiency and occupant comfort.

3.4 Solar Shading and Control Systems

Solar shading systems represent a cost-effective retrofitting strategy, particularly in cooling-dominated climates. Simple retrofit strategies such as solar shading, window glazing, air tightness then insulation can reduce energy consumption of an average of 33% [14][15].Fixed and Dynamic Shading SystemsShading systems range from fixed external shading elements to sophisticated dynamic systems that respond to solar conditions and occupant preferences. External shading systems generally provide superior performance compared to internal systems by intercepting solar radiation before it enters the building.Smart Shading IntegrationThe integration of smart controls and sensors in shading systems enables automatic optimization of solar heat gain and daylighting performance. These systems can significantly reduce cooling energy consumption while maintaining visual comfort for occupants.

3.5 Air Sealing and Infiltration Control

Air leakage control represents a fundamental aspect of building envelope retrofitting, often providing cost-effective energy savings. Comprehensive air sealing strategies address building envelope discontinuities and penetrations to minimize uncontrolled air infiltration and exfiltration.

4. Structural Retrofitting Techniques

4.1 Seismic Retrofitting Approaches

Structural retrofitting, particularly seismic retrofitting, has gained significant attention due to the vulnerability of existing buildings to earthquake hazards. The retrofitting methods are divided into two distinct categories: the local measures, which enhance the behavior of individual elements and the global ones, which operate on the structure as a whole [16].Local Strengthening MethodsLocal retrofitting measures focus on improving the performance of individual structural elements. These approaches include:

• Column and Beam Jacketing: RC jackets were provided to all basement floor columns [17] to enhance flexural and shear capacity
• Connection Strengthening: Improving beam-column joints and foundation connections to ensure adequate load transfer
• Element Replacement: Selective replacement of severely damaged or inadequate structural elements

Global Strengthening SystemsGlobal retrofitting approaches address the overall structural system performance:

• Shear Wall Addition: Installing new reinforced concrete or steel shear walls to improve lateral resistance
• Bracing Systems: Adding steel bracing elements to enhance lateral stiffness and strength
• Damping Systems: Installing energy dissipation devices to reduce seismic demand

4.2 External Strengthening Systems

External strengthening systems have emerged as innovative solutions for structural retrofitting. The ultimate goal of seismic retrofitting is to improve the overall seismic performance of the whole structure, thus a variety of external sub-structure retrofitting methods have been developed at home and abroad since the 1970s. The external sub-structure is connected with the existing structure as a whole on the structural-system-level, and it is of great significance for lifeline projects or non-interrupted buildings [18].

External Frame SystemsExternal frame systems provide additional lateral resistance while minimizing disruption to building operations. These systems can be designed as:

• Moment-resisting frames that provide both strength and ductility
• Braced frame systems that primarily enhance lateral stiffness
• Hybrid systems combining multiple structural systems

Base Isolation SystemsBase isolation represents an advanced approach to seismic retrofitting that decouples the building from ground motion. For the seismic retrofit of the existing structure an "all rubber" base isolation system (BIS) has been assumed by using ϕ600 HDRBs, allowing to reach a TISO = 4.0s. To maximize the effectiveness of BIS, the three bodies have been connected at each floor level and the upper structure has been strengthened previously, reducing to TFB=0.30s the fundamental period of the strengthened fixed base solution [19].

4.3 Masonry Structure Retrofitting

Masonry buildings, representing a significant portion of existing building stock, require specialized retrofitting approaches. Masonry structures comprise a very common structural material including most of the surviving historic structures worldwide. Due to their increased age and vulnerability, masonry buildings and structures are generally in greater need of structural retrofitting than newer ones constructed with more modern materials [20].Traditional Strengthening MethodsTraditional masonry retrofitting approaches include:

• Repointing and structural grouting to improve masonry integrity
• Addition of tie rods and anchoring systems for out-of-plane stability
• Installation of reinforced concrete bond beams

Advanced Composite SystemsModern masonry retrofitting increasingly employs composite materials:

• Fiber-reinforced polymer (FRP) systems for enhanced flexural and shear capacity
• Textile-reinforced mortar (TRM) systems providing improved ductility
• Steel reinforced grout (SRG) systems offering cost-effective strengthening

5. Mechanical Systems Retrofitting

5.1 HVAC System Retrofitting

HVAC system retrofitting represents a critical component of building performance improvement, with heating, ventilation, and cooling (HVAC) accounting for 38% of building energy usage, and over 15% of all US energy usage [21]. The complexity of HVAC systems requires comprehensive approaches that address multiple system components and their interactions.Heat Pump Technology IntegrationHeat pumps have emerged as a dominant retrofitting technology. Heat pumps and upgrading the control system were most frequent retrofit measures. Heat pumps were the most frequent retrofit measure, quoted in 42 of the reviews, followed by upgrading the control system (41) [22][23]. Heat pump retrofitting offers significant potential for energy savings, particularly when replacing fossil fuel-based heating systems.Advanced Control SystemsControl system retrofitting represents a cost-effective approach to improving HVAC performance. A control retrofit approach, which is lightweight and replicable, to realize the improvement of energy efficiency of the existing building HVAC systems through integrating data-driven MPC into existing BASs [24][25]. Model Predictive Control (MPC) systems can optimize HVAC operation by anticipating thermal loads and weather conditions.Performance ResultsHVAC system retrofitting has demonstrated significant performance improvements. The evaluation results indicate that the control retrofit approach is effective in achieving energy efficiency and thermal comfort improvement. The system daily energy consumption was reduced by 24.5% in average and the percentage of the discomfort time was reduced from 70.2% to 5.7% in average after the control retrofit [26].

5.2 Lighting System Retrofitting

Lighting system retrofitting has been revolutionized by LED technology and advanced control systems. These retrofitting approaches often provide rapid payback periods and significant energy savings.

LED Conversion StrategiesLED retrofitting approaches range from simple lamp replacement to comprehensive fixture upgrades. Advanced LED systems incorporate dimming capabilities, daylight sensors, and occupancy controls to optimize energy performance.Integrated Lighting and DaylightingModern lighting retrofitting increasingly integrates natural and artificial lighting systems through sophisticated control strategies that maximize daylight utilization while maintaining appropriate illumination levels.

5.3 Renewable Energy System Integration

The integration of renewable energy systems in building retrofitting has become increasingly important for achieving net-zero energy performance goals.Solar Photovoltaic SystemsRooftop PV systems represent the most common renewable energy retrofit. The cost benchmark of PV systems between 2010 and 2020 as published by National Renewable Energy Laboratory (NREL) in 2021. This figure illustrates the considerable decrease in PV system costs between 2010 and 2020 [27], making solar retrofitting increasingly cost-effective.Building-Integrated Photovoltaics (BIPV)BIPV systems combine energy generation with building envelope functions, providing dual benefits of energy production and envelope performance improvement. BIPV, as multifunctional element, can both improve the energy performance of building envelopes and produce electricity from solar radiation in urban contexts [28].

6. Decision-Making Processes in Building Retrofitting

6.1 Multi-Criteria Decision-Making Frameworks

Building retrofitting decisions involve complex trade-offs between multiple competing objectives. The decision as to which retrofit technology (or measure) should be used for a particular project is a multi-objective optimisation problem subject to many constraints and limitations, such as specific building characteristics, total budget available, project target, building services types and efficiency, building fabric, etc. [29]Stakeholder ConsiderationsRetrofitting decisions must account for diverse stakeholder perspectives and priorities. The results demonstrated that stakeholders might make different selections of objective functions, affecting the final decisions on building retrofits [30]. Understanding stakeholder priorities is crucial for successful retrofit project implementation.Decision Support ToolsVarious tools have been developed to support retrofitting decision-making. Case-Based Reasoning (CBR) is valuable in providing references and avoiding possible failures, which is a promising approach for building retrofit [31][32]. These tools help decision-makers navigate complex trade-offs and select optimal retrofitting strategies.

6.2 Economic Analysis and Life-Cycle Assessment

Economic analysis forms a critical component of retrofitting decision-making, requiring comprehensive evaluation of initial costs, operational savings, and long-term benefits.Cost-Optimization ApproachesModern economic analysis approaches incorporate multiple financial metrics including net present value, payback period, and life-cycle cost analysis. The selection of the retrofit measures depends on many aspects such as meteorological conditions, thermal characteristics, indoor requirements, energy-end uses, total cost and even local normative restrictions. All these variables should be considered as input information into the decision matrix, setting constrains and limitations on the final results [33].Risk Assessment IntegrationRisk assessment has become increasingly important in retrofitting decisions. A building retrofit is subject to many uncertainty factors, such as uncertainty in savings estimation, energy use measurements, weather forecast, the changes of energy consumption patterns, system performance degradations, etc. These uncertainty factors result that investment in building retrofits is highly uncertain. Risk assessment is therefore essential to provide decision makers with a sufficient level of confidence to select and determine the best retrofit solutions [34].

6.3 Performance-Based Decision Making

Performance-based decision-making approaches focus on quantifying and optimizing specific performance outcomes rather than prescriptive measures.Multi-Objective OptimizationA multi-objective optimization (MOO) is devised while considering uncertainties in building performance predictions and envelope parameters, and contextual constraints such as climate conditions or existing built forms [35]. These optimization approaches help identify optimal solutions that balance competing objectives.Uncertainty ManagementManaging uncertainty in retrofitting decisions requires sophisticated analytical approaches. Optimization methodologies are reviewed in the second part of the literature review, then the appropriate MOO under uncertainties is proposed [36]. These approaches help decision-makers make informed choices despite incomplete information.

7. Integrated Retrofitting Approaches

7.1 Holistic Retrofitting Strategies

Integrated retrofitting approaches seek to achieve synergistic benefits by combining multiple retrofitting strategies. Addressing seismic and energy performance by separate interventions is the common approach currently taken, however to achieve better cost-effectiveness, safety and efficiency, a novel holistic approach to building renovation is an emerging topic in the scientific literature. Proposed solutions range from integrated exoskeleton solutions, over strengthening and insulation solutions for the existing building envelope or their replacement with better materials, to integrated interventions on horizontal elements like roof and floor slabs [37].Seismic-Energy IntegrationThe integration of seismic and energy retrofitting represents a particularly promising approach. Integrating seismic with energy upgrading techniques is a promising research area [38]. These integrated approaches can achieve multiple objectives while reducing overall implementation costs and disruption.Exoskeleton SystemsExoskeleton retrofitting systems provide comprehensive building envelope and structural upgrades through external additions. Exoskeletons & improvement/replacement of envelopes comprise the most popular solution [39][40]. These systems can simultaneously improve energy performance, structural capacity, and building aesthetics.

7.2 Whole-Building Performance Optimization

Whole-building retrofitting approaches consider the integrated performance of all building systems and their interactions.System InteractionsUnderstanding system interactions is crucial for effective integrated retrofitting. The subsystems in buildings are highly interactive. Different retrofit measures may have different impacts on associated building sub-systems due to these interactions, which results that the selection of the retrofit technologies becomes very complex. Dealing with these uncertainties and system interactions is a considerable technical challenge in any sustainable building retrofit project [41].Performance SynergiesIntegrated approaches can achieve performance synergies that exceed the sum of individual improvements. Integrated strategies combining multiple retrofit procedures accounted for 13% of the reviewed studies. The benefits of retrofitting include not only enhanced energy efficiency and aesthetics but also improved indoor environmental quality, which contributes to occupant comfort and health [42].

7.3 Digital Integration and Smart Building Technologies

The integration of digital technologies and smart building systems represents an emerging trend in building retrofitting.Building Information Modeling (BIM) IntegrationBIM technology is increasingly used to support integrated retrofitting approaches. The research presented here overcomes these retrofit barriers by developing a novel step-by-step guideline for building retrofit. The proposed workflow combines the benefits of Building Information Modelling (BIM) with the Business Process Modelling (BPM) technique [43].Artificial Intelligence ApplicationsAI and machine learning applications are emerging as powerful tools for retrofitting optimization. This study integrates a Gaussian Process-based Deep Learning (GPDL) model to retrofit buildings on a metropolitan scale, aiming to accelerate the transition towards smart cities. Gaussian Process offers a probabilistic approach to assess uncertainty in data points, while deep learning captures complex data patterns. The hybrid approach enhances the accuracy and reliability of end use intensity (EUI) predictions, ultimately supporting the computation of the primary energy factor (PEF) for improved decision-making in energy management [44].

8. Performance Assessment and Measurement & Verification

8.1 Measurement and Verification Protocols

Effective performance assessment is crucial for validating retrofitting effectiveness and informing future projects. Measurement and verification (M&V) is the process of using measurement to reliably determine the actual savings created within an individual facility by an energy management programme. The main purpose of M&V is to determine actual energy savings due to the implementation of retrofit measures [45].International Standards and ProtocolsThe International Performance Measurement and Verification Protocol (IPMVP) provides the foundation for most M&V approaches. The International Performance Measurement and Verification Protocol (IPMVP) underpins most M&V frameworks that are used in the industry. This protocol provides four options for M&V: Retrofit isolation (key parameter measurement), retrofit isolation (all-parameter measurement), whole facility, and calibrated simulation [46].Implementation ChallengesM&V implementation faces several challenges. Lack of reliable energy data and information about changes in building context before and after retrofit may jeopardize measurement and verification of savings [47]. Addressing these challenges requires comprehensive data collection strategies and robust analytical methodologies.

8.2 Performance Indicators and Metrics

Comprehensive performance assessment requires multiple indicators across different performance dimensions. 62 indicators were identified as applicable for measuring the performance of retrofit work. As some of these indicators bear the same meaning, they were consolidated and the number of resultant indicators amounted to 52. Grouped into four aspects, there are 16 economic indicators, 16 environmental indicators, eight health and safety indicators, and 12 users' perspective indicators.Energy Performance MetricsEnergy performance remains the primary focus of retrofitting assessment. Energy efficiency was the focus of 61 of the reviews, followed by energy modelling (13), indoor environment quality (11), and life cycle assessment (8) [48]. Standard metrics include energy use intensity, energy cost savings, and demand reduction.Advanced Assessment MethodologiesAdvanced assessment approaches incorporate multiple performance dimensions. This study proposes a novel method for evaluating building energy performance that combines energy signature analysis and hierarchical clustering. This symbolic hierarchical clustering method enhances the utility of open data and facilitates rapid decision-making. Additionally, it allows for a simple assessment of energy performance at the city scale.

8.3 Long-Term Performance Monitoring

Long-term monitoring is essential for understanding the sustained impacts of retrofitting interventions and informing maintenance strategies.Continuous Monitoring SystemsAdvanced monitoring systems enable continuous assessment of building performance. Retrofitting existing buildings is crucial for significantly reducing energy consumption in the building sector. The continuous monitoring and evaluation of retrofit building energy efficiency is necessary to maintain optimal energy performance.Performance Degradation AssessmentUnderstanding performance degradation over time is crucial for maintaining retrofitting benefits. Long-term monitoring enables identification of system performance issues and optimization of maintenance strategies.

9. Challenges and Barriers in Building Retrofitting

9.1 Technical Challenges

Building retrofitting faces numerous technical challenges that complicate implementation and limit effectiveness.System Integration ComplexityThe complexity of integrating new technologies with existing building systems represents a significant technical challenge. The works involved in retrofit are usually of complex and heterogeneous nature that require various specialties to be integrated in highly variable conditions. Furthermore, a thorough building's retrofit evaluation is quite difficult to undertake, because a building and its environment are complex systems regarding technical, technological, ecological, social, comfort, esthetical, and other aspects, where every sub-system influences the total efficiency performance and the interdependence between sub-systems plays a critical role.Performance Prediction UncertaintyAccurately predicting retrofitting performance remains challenging due to numerous variables and uncertainties. Identifying uncertainties in envelope parameters and performance predictions, developing a multi-objective optimization (MOO) model under uncertainties and built environment constraints, and testing the methodology applied to existing residential buildings [49].

9.2 Economic and Financial Barriers

Economic constraints represent significant barriers to widespread retrofitting adoption.High Initial CostsThe high upfront costs of comprehensive retrofitting can deter building owners despite long-term benefits. These barriers include low financial availability, user awareness, uncertainty in regulatory frameworks and fragmentation of the supply chain [50].Split IncentivesIn rental properties, the misalignment between those who pay for retrofitting and those who benefit from reduced operating costs creates barriers to implementation.

9.3 Regulatory and Policy Challenges

Regulatory frameworks often lag behind technological advances, creating barriers to innovative retrofitting approaches.Building Code LimitationsExisting building codes may not adequately address retrofitting scenarios, creating uncertainty for practitioners and limiting implementation of innovative solutions.Permitting ComplexityComplex permitting processes can delay retrofitting projects and increase costs, particularly for integrated or innovative approaches.

10. Emerging Trends and Future Research Directions

10.1 Technological Innovations

Several technological trends are shaping the future of building retrofitting.Advanced Materials DevelopmentNew building materials, such as the 3D printed building envelopes, the aerogel for insulation, the radiative sky cooling skins etc., may help reduce building energy consumption. The rise of new technologies may bring some opportunities in the building energy sector [51]. These materials offer enhanced performance characteristics and new possibilities for retrofitting applications.Artificial Intelligence IntegrationAI applications in retrofitting are expanding beyond design optimization to include predictive maintenance, performance monitoring, and adaptive control systems. AI (Artificial Intelligence) or ML (Machine Learning) to realize smart building envelopes [52] represents a growing area of research and development.

10.2 Integrated Assessment Approaches

Future research is moving toward more integrated assessment approaches that consider multiple performance dimensions simultaneously.Multi-Domain Performance AssessmentThe research suggestions found in the reviews were grouped into four main topics: reducing uncertainty in energy savings, developing calculation tools for building retrofit, more complex retrofit evaluation, and digitisation [53]. These multi-domain approaches better capture the full value of retrofitting interventions.Life-Cycle Sustainability AssessmentComprehensive sustainability assessment approaches that consider environmental, economic, and social impacts across the building life cycle are becoming increasingly important for evaluating retrofitting strategies.

10.3 Scale and Implementation Challenges

Future research must address the challenge of scaling successful retrofitting approaches to achieve broader market transformation.District and City-Scale RetrofittingDeveloping fast-response retrofitting solutions to save energy on urban and large scales is critical for city planners and policymakers. This study integrates a Gaussian Process-based Deep Learning (GPDL) model to retrofit buildings on a metropolitan scale, aiming to accelerate the transition towards smart cities [54].Standardization and ReplicationDeveloping standardized approaches that can be replicated across different building types and contexts while maintaining effectiveness represents a critical research need.

10.4 Climate Resilience Integration

Climate change adaptation is becoming increasingly important in retrofitting strategies.Extreme Weather ResilienceFuture retrofitting approaches must consider increasing frequency and intensity of extreme weather events, requiring integration of resilience measures with traditional performance improvements.Adaptive Building SystemsDynamic and adaptive building systems that can respond to changing environmental conditions represent an important area for future development.

11. Discussion and Analysis

11.1 Current State of Building Retrofitting Research

The analysis of current research reveals a mature but rapidly evolving field with significant opportunities for advancement. The dominance of energy efficiency focus, with 55% of studies concentrating on reducing energy consumption [55], reflects both the urgent need for energy savings and the relative maturity of energy retrofitting technologies.However, the relatively limited attention to integrated approaches, with integrated strategies combining multiple retrofit procedures accounting for only 13% of reviewed studies [56], suggests significant opportunities for development of holistic retrofitting strategies that can achieve multiple objectives simultaneously.

11.2 Methodological Gaps and Opportunities

Several methodological gaps emerge from this review:Decision-Making Tool DevelopmentWhile various decision-making approaches exist, at present, no standard method exists that can systematically support the decision-making process of building stock retrofit plans [57]. This represents a significant opportunity for developing standardized, widely applicable decision-support tools.Performance Assessment IntegrationThe need for more comprehensive performance assessment approaches is evident. Reducing uncertainty in energy savings, developing calculation tools for building retrofit, more complex retrofit evaluation, and digitisation [58] represent key research priorities identified across multiple studies.

11.3 Implementation Challenges and Solutions

The implementation of building retrofitting faces several persistent challenges that require systematic attention:Disruption MinimizationTraditional retrofitting methods often require significant time and financial investment, and can disrupt the building operations and occupant activities during implementation [59]. Developing less disruptive retrofitting approaches remains a critical need.Cost-Effectiveness OptimizationThe challenge of achieving cost-effective retrofitting solutions requires continued innovation in both technologies and implementation approaches. Methods to identify the most cost-effective retrofit measures for particular projects is still a major technical challenge [60].

11.4 Future Research Priorities

Based on the comprehensive analysis, several research priorities emerge:

1. Integrated Retrofitting Methodologies: Developing comprehensive approaches that simultaneously address multiple performance objectives while minimizing costs and disruption.
2. AI-Driven Decision Support: Advancing artificial intelligence applications for retrofitting optimization, performance prediction, and adaptive control.
3. Climate Resilience Integration: Incorporating climate adaptation measures into traditional retrofitting approaches to address increasing extreme weather risks.
4. Standardized Assessment Protocols: Developing standardized methods for comprehensive performance assessment that can be applied across different building types and contexts.
5. Scale-Up Strategies: Creating approaches for scaling successful retrofitting strategies from individual buildings to districts and cities.

12. Conclusion

This comprehensive review of building retrofitting processes and methods reveals a dynamic field characterized by significant technological advancement, methodological innovation, and growing recognition of the critical role retrofitting plays in achieving sustainability goals. The analysis of current research demonstrates that while energy efficiency remains the primary focus, there is growing attention to integrated approaches that address multiple performance objectives simultaneously.Key findings from this review include:Methodological Diversity: Building retrofitting encompasses diverse approaches across building envelope, structural, and mechanical systems domains, with building envelope insulation, improving climatisation and lighting systems, and using renewable energy sources representing approximately 70% of strategies [61].Decision-Making Complexity: Retrofitting decisions involve complex multi-criteria optimization problems that require sophisticated decision-support tools. The lack of standardized decision-making approaches represents both a current limitation and future opportunity.Performance Assessment Challenges: While measurement and verification protocols exist, lack of reliable energy data and information about changes in building context before and after retrofit may jeopardize measurement and verification of savings [62]. Comprehensive performance assessment remains challenging but essential.Integration Opportunities: Integrated retrofitting tackles seismic & energetic issues cost effectively [63][64], suggesting significant potential for holistic approaches that achieve synergistic benefits.Emerging Technologies: Advanced materials, artificial intelligence, and smart building technologies offer new possibilities for enhanced retrofitting performance and reduced implementation barriers.

12.1 Implications for Practice

For practitioners, this review highlights several key considerations:

• The importance of adopting systematic decision-making approaches that consider multiple objectives and stakeholder perspectives
• The value of integrated retrofitting strategies that can achieve synergistic benefits
• The need for comprehensive performance assessment and long-term monitoring to validate retrofitting effectiveness
• The potential of emerging technologies to enhance retrofitting performance and reduce implementation challenges

12.2 Implications for Policy

Policy implications include:

• The need for updated regulatory frameworks that facilitate innovative retrofitting approaches
• The importance of addressing financial barriers through appropriate incentive structures
• The value of supporting standardization efforts to enable widespread adoption of effective retrofitting practices
• The necessity of integrating climate resilience considerations into retrofitting policies

12.3 Limitations and Future Work

This review has several limitations that should be acknowledged. The rapid pace of technological development means that some findings may become outdated quickly. The focus on peer-reviewed literature may not fully capture emerging industry practices. Additionally, the geographic distribution of research may not represent all global contexts and building types.Future research should address these limitations through:

• Regular updates to capture emerging technologies and practices
• Integration of industry practice and case study evidence
• Expansion to include diverse global contexts and building typologies
• Development of standardized research methodologies to enable better comparison across studies

12.4 Final Recommendations

For postgraduate researchers entering this field, several recommendations emerge:

1. Interdisciplinary Approach: Building retrofitting requires integration of technical, economic, environmental, and social considerations. Researchers should develop competencies across multiple domains.
2. Systems Thinking: Understanding the complex interactions between building systems is crucial for effective retrofitting research and practice.
3. Stakeholder Engagement: Successful retrofitting requires consideration of diverse stakeholder perspectives and priorities.
4. Performance Validation: Rigorous measurement and verification of retrofitting performance is essential for advancing the field.
5. Innovation Integration: Emerging technologies offer significant potential but require careful evaluation and validation.

The field of building retrofitting represents a critical area for addressing climate change, improving building performance, and enhancing occupant well-being. The continued development of effective retrofitting approaches requires sustained research effort, technological innovation, and policy support. This review provides a foundation for understanding current knowledge and identifying opportunities for future advancement in this vital field.

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