A Systematic Review of Green Roofs’ Thermal and Energy Performance in the Mediterranean Region

Abstract

Due to ongoing climate change, urban areas face increasing challenges associated with rising temperatures and growing energy demand. Green roofs have emerged as a sustainable, nature-based solution to enhance urban resilience. This study presents a systematic review of the thermal and energy performance of green roofs in the Mediterranean region, and was conducted following the PRISMA framework. By identifying targeted research questions formulated using the PICO(C) structure, this review systematically evaluates the potential of green roofs to promote sustainable urban environments in Mediterranean regions. The findings highlight their effectiveness in mitigating heat stress, enhancing building energy efficiency, and counteracting urban temperature fluctuations, reinforcing their role as a key climate adaptation strategy in densely populated areas. The review also identifies critical research gaps that must be addressed to facilitate the large-scale adoption of green roofs. Specifically, the lack of long-term performance monitoring, the need for standardized assessment protocols, and the necessity of optimizing green roof configurations for Mediterranean subregions emerge as key areas for future investigation. This study bridges a crucial gap in the literature by providing a systematic, PRISMA-compliant evaluation. It offers the scientific community a robust knowledge base to inform policy, urban planning, and future research directions.

1. Introduction

It is well known that the increase in global temperatures has serious implications that impact on an urban scale, straining both the ecological and social spheres [1,2,3,4]. Cities are vulnerable to these climatic shifts, as densely built environments experience high temperatures and deteriorating air quality, promoting thermal discomfort and increased energy demand [5,6,7,8]. This trend is further intensified by the urban heat island (UHI) effect [9,10,11,12], which exacerbates localized warming and amplifies extreme weather events’ frequency, duration, and intensity. The combined effects of climate change and urbanization place unprecedented pressure on urban infrastructure, demanding immediate and effective mitigation strategies [13,14,15].

Mediterranean cities face unique challenges due to their hot, dry summers and wet winters, which increase the energy demand for cooling and heating. Moreover, the Mediterranean region has been identified as a climate change hotspot, experiencing warming rates significantly higher than the global average. Regional air temperatures have already exceeded the critical 1.5 °C warming threshold and are projected to reach a 2 °C increase within the next two decades, with extreme weather events becoming more frequent and severe [16,17,18,19]. The building sector plays a crucial role, as the high albedo and thermal inertia of commonly used construction materials and the related significant greenhouse gas emissions contribute to intensifying these climatic effects [20,21]. In response, urban planners and policymakers have prioritized strategies to mitigate urban overheating and its consequential impacts. Among the most effective approaches is adopting nature-based solutions, which enhance buildings’ thermal and energy performance and support broader climate adaptation and resilience efforts [22,23,24]. Green roofs have emerged as a highly effective passive strategy for improving urban sustainability [25,26,27,28,29,30,31]. These systems provide various benefits, spanning environmental, economic, and social dimensions, making them a key tool in mitigating the adverse effects of urbanization and climate change. Their integration into the built environment enhances thermal comfort, reduces the UHI effect, improves air quality, minimizes building energy needs, optimizes stormwater management, and increases urban biodiversity. However, ensuring long-term functionality and effectiveness requires tailored design strategies that account for the specific climatic and environmental conditions in which they are implemented [32,33]. The selection of green roof typology, layering composition, and material properties must be carefully optimized to maximize thermal and energy performance, particularly in regions characterized by extreme seasonal variability, such as the Mediterranean. Properly designed green roof systems can enhance a building’s thermal and acoustic insulation, regulate surface temperatures, and mitigate urban heat accumulation, making them a key component of climate-resilient urban planning [34,35,36].

Despite the widespread adoption of green roofs in temperate regions, their implementation in Mediterranean climates presents specific challenges. Existing guidelines from Europe and North America—such as the “Guidelines for Planning, Construction, and Maintenance of Green Roofs” [37], published by the Forschungsgesellschaft Landschaftsentwicklung Landschaftsbau (FLL) in Germany, and the “Guide to living terrace roofs and green roofs” [38], published by the Municipality of Barcelona, Spain—serve as valuable references for green roof implementation. However, their applicability to Mediterranean climates is limited due to specific climatic constraints, including high summer temperatures, prolonged droughts, and intense seasonal precipitation during winter. Green roofs in the Mediterranean area must be specifically designed to withstand these extreme weather conditions, ensuring long-term performance and resilience. Factors such as drought-tolerant vegetation, optimized substrate depth, and efficient irrigation systems are crucial in maintaining functionality and efficiency [39,40,41,42,43,44,45].

Despite these challenges, green roofs represent a promising strategy for urban climate adaptation [46].

However, their impact on building energy efficiency and thermal comfort in Mediterranean climates remains insufficiently summarized by the scientific literature. Therefore, this systematic review aims to provide a comprehensive and critical synthesis of the factors influencing green roofs’ thermal and energy performance in Mediterranean climates. By systematically analyzing existing methodologies and identifying key research gaps, the study advances current knowledge and supports future developments in sustainable building design and climate-adaptive strategies.

The general objective of this review is to critically assess the state of research on the thermal and energy behavior of green roofs within Mediterranean contexts, and to inform future scientific investigations, policy-making initiatives, and practical applications in urban resilience planning.

Specifically, the study seeks to:

• Identify the main design, material, and environmental factors influencing green roof performance;
• Analyze and compare the methodological approaches adopted in the current literature;
• Highlight existing research gaps and suggest directions for future studies.

In particular, the paper provides a structured background on green roof technology in Mediterranean climates, establishing a theoretical basis for the study (Section 1, “Introduction”). The first part examines the Mediterranean climate, highlighting its climatic conditions and their implications for building energy performance, urban overheating, and passive cooling strategies. Then, it analyzes the historical development of green roof technology in this region, emphasizing how policy frameworks, environmental challenges, and evolving urban sustainability goals have influenced its adoption and adaptation.

The discussion then shifts to the classification of green roofs, detailing different typologies and their functional performance regarding thermal insulation, water retention, and energy efficiency. Lastly, an overview of green roof components is provided, analyzing key design elements, materials, and layering configurations that determine long-term durability, hydrological behavior, energy efficiency, and thermal regulation capabilities.

This discussion sets the stage for Section 2, “Materials and methods”, which outlines the systematic qualitative and quantitative approach adopted in this review. As the first step of the methodology, a set of core research questions was defined:

• RQ1: “Have the thermal and energy performance of green roofs in Mediterranean climates been sufficiently analyzed in the existing literature?”
• RQ2: “Do green roofs significantly improve thermal and energy performance compared to conventional roofing systems in Mediterranean climates?”
• RQ3: “How do green roofs compare to other passive technologies in Mediterranean climates?”
• RQ4: “What are the primary factors influencing green roofs’ thermal and energy performance in Mediterranean climates?”
• RQ5: “What are the main research gaps and methodological limitations in current studies, and how can future research address these gaps?”

The methodology was structured step-by-step to ensure that each phase of the review process, ranging from literature identification to data analysis, contributes directly to answering these research questions in a systematic and traceable manner.

Section 3, “Systematic Review Process Outcomes”, presents the key results of this analysis, identifying major trends, performance evaluations, and knowledge gaps. The findings are then synthesized in Section 4, “Structured Response to Research Questions”, which systematically answers the core research questions guiding this review. Finally, Section 5, “Conclusions”, discusses the broader implications of the findings and outlines directions for future research and policy development.

1.1. The Mediterranean Climates

The Mediterranean region is characterized by pronounced seasonal variations in precipitation and temperature patterns, with hot, dry summers and mild, wet winters [47]. In recent decades, the eastern Mediterranean region has recorded a higher warming rate than its western counterpart [48,49,50,51], highlighting significant seasonal variability in air temperatures [52]. This climate makes the region’s urban areas highly vulnerable to the impacts of climate change [53], including rising temperatures [54], water scarcity [55], extreme weather events, and environmental degradation [56,57]. The Mediterranean area has experienced a higher warming rate than the global average [58]. Regional air temperatures have already exceeded the critical 1.5 °C warming threshold and are projected to reach a 2 °C increase within the next two decades [54]. Consequently, the region has observed significant changes in the frequency and duration of extreme events, such as heatwaves [59], droughts, and torrential rainfall. Mitigation strategies, such as green roofs, represent a practical approach to promoting more sustainable, resilient, and livable Mediterranean urban environments due to their ability to favorably influence local climatic conditions in cities.

1.2. Evolution of Green Roofs in the Mediterranean Area

The Mediterranean tradition of green roofs has a long history [60], dating back to ancient Mesopotamian and Roman civilizations. Early examples include the Mesopotamian ziggurats [61] and the fabled Hanging Gardens of Babylon [62], which featured verdant roof installations. Similarly, Roman architecture showcased green roof elements, such as the terraced garden atop the Villa of the Mysteries in Pompeii [63]. Throughout the medieval and early modern eras, green roof gardens remained a persistent feature in Mediterranean architectural design, found in different structures like Arab-influenced buildings, the Benedictine Abbey of Mont Saint Michel in France [64], the Guinigi Tower in Lucca [65], Italy, the Villa Medici in Florence, Italy, and the Piccolomini Palace in Pienza, Italy. This green roof tradition continued into the 19th and 20th centuries, as demonstrated by a sod roof installation in Paris and more recent projects like the Promenade Plantée and Jardin Atlantique roof gardens, underscoring the enduring influence of this Mediterranean architectural practice.

In recent decades, the implementation of green roof technology has emerged as a nature-based solution for energy retrofitting the built environment and designing sustainable new buildings. Figure 1 shows representative green roof systems in the Mediterranean regions.

1.3. Typologies of Green Roofs and Their Thermal Implications

Green roofs are classified according to key parameters determining their performance and suitability for various climatic and urban contexts. Each type delivers distinct functional, environmental, and aesthetic benefits. The literature identifies three green roof types (extensive, intensive, and semi-intensive), which are comprehensively analyzed in this section [68,69,70,71,72,73].

• Extensive systems: extensive green roofs feature lightweight designs with shallow substrates, typically less than 15–20 cm, optimized for water retention and drainage. Extensive solutions are designed to withstand climatic challenges like drought and warm summers while requiring minimal maintenance. Short plants, mosses, and herbs are optimal for this green roof typology. Sedum species are widely employed in extensive systems, constituting 20–40% of the stable vegetative cover, with temporal and spatial variations in the floristic composition. These plants are highly resilient, enduring high summer temperatures and prolonged drought. In some cases, Sedum species can be naturally irrigated by rainfall without additional irrigation systems. Figure 2 illustrates an example of an extensive green roof.
• Intensive systems: intensive green roofs are characterized by a deeper substrate, typically exceeding 15–20 cm, which results in higher structural loads and requires reinforced structural support. They have demonstrated superior runoff quality, water retention capacity, and insulation performance compared to extensive solutions. However, the intensive systems require significant maintenance. These green roofs can host many plant species, including shrubs, bushes, and small trees, providing several ecological and aesthetic functions. Moreover, this solution can give rise to accessible public spaces. Figure 3 illustrates a representative intensive green roof.
• Semi-intensive systems: semi-intensive green roofs integrate the advantages of extensive and intensive systems, balancing low-maintenance requirements and enhanced substrate depth. These green systems support a more diverse plant selection while maintaining structural feasibility. Supplemental irrigation is crucial in optimizing plant vitality, particularly in dry regions. Figure 4 illustrates a representative semi-intensive green roof system.

1.4. Key Design Elements Affecting Thermal and Energy Performance

Green roof systems consist of multiple interconnected layers, each playing a crucial role in the building envelope’s overall thermal behavior, energy efficiency, and environmental performance [77,78,79]. A well-optimized configuration enhances heat insulation during winter and cooling effects during summer and contributes to urban heat island mitigation. The key functional components are briefly summarized below.

• Plants and vegetation: the vegetation layer is the most visible and biologically active component of green roofs, playing a critical role in regulating microclimatic conditions, enhancing biodiversity, and contributing to stormwater management [39]. Its primary functions include thermal regulation, air purification, and water retention. Through evapotranspiration and shading, vegetation reduces heat island effects and enhances energy efficiency by lowering rooftop temperatures. The vegetation layer design varies according to the type of green roof. Plants should adapt well to the local climate, ensuring long-term viability, adequate coverage, and high albedo. Species combinations ensure year-round benefits while minimizing resource inputs. Native and climate-adapted plant species enhance ecosystem resilience and require fewer resources for maintenance.
• Growth medium: the growing layer influences vegetation establishment, stormwater management, and thermal performance [68]. Its primary function is to provide structural support for plant growth by supplying essential nutrients, retaining moisture, facilitating root development, ensuring proper aeration and drainage, and preventing waterlogging. Irrigation control is necessary for maintaining high moisture levels in the substrate and reducing the temperature of the layer.
  The depth and composition of the growing layer vary according to the type of green roof. Extensive green roofs typically require a substrate depth of up to 15 cm, intensive systems need depths of up to 60 cm, and semi-intensive solutions generally have a substrate depth of up to 30 cm. The growing medium is composed of a blend of inorganic and organic materials. Inorganic components, typically about 90% of the substrate, ensure structural stability and adequate drainage. Using inorganic materials with controlled porosity is crucial for plant survival in challenging climatic conditions.
  On the other hand, organic matter, usually constituting about 10% of the substrate, enhances biological activity and nutrient availability. The percentage of organic material must be carefully designed, as excessive content can overload the roof and potentially compromise its structural integrity. FLL guidelines [80] recommend a maximum organic material content of 8% for extensive green roofs and 12% for intensive ones.
• Filter layer: the filter layer is a fundamental component of green roofs [39]. It prevents fine particles from migrating into the drainage system, maintaining long-term permeability and improving stormwater management efficiency. To withstand prolonged exposure to moisture and environmental stressors, filter layers must exhibit high water permeability, mechanical strength, and chemical stability. Standard filter layers have a thickness ranging from 0.5 mm to 2 mm. Geotextile fabrics, particularly polypropylene and polyester fibers, are widely used due to their high tensile strength, flexibility, and resistance to decomposition.
• Drainage layer: the drainage layer regulates excess water flow, prevents waterlogging, and maintains an optimal air–water balance within the substrate [71]. By ensuring adequate aeration and moisture control, this layer enhances root health, plant growth, and the overall hydrological performance of the system. Additionally, it is crucial for stormwater management, significantly reducing peak discharge rates and mitigating urban flood risks. The thickness of the drainage layer typically ranges from 2 cm to 10 cm. Granular materials are widely utilized due to their porous structure, high durability, and water retention capabilities. However, these materials are generally limited to roofs with slopes of less than 5%, as steeper inclinations may require alternative solutions to prevent substrate erosion. Plastic-based panels are also widely employed. Anti-erosion elements, particularly for sloped green roofs, are often integrated into the drainage layer to prevent substrate displacement during heavy rainfall.
• Protection layer: the protective coating provides structural support during the green roof’s installation and operational phase [72]. Depending on the selected material, its thickness typically ranges from 3 mm to 10 mm. The protection layer is commonly made from geotextile materials, but polyester-based layers can also be effectively employed.
• Anti-root layer: the root barrier layer protects the underlying layers from potential damage caused by root intrusion, preventing water leakage and structural damage [81].
  The optimal thickness for this layer is about 4 mm, depending on the typology of green roofs. Polyethylene is one of the most common materials for this layer. However, alternative options like metal or plastic sheets can also be employed, depending on project requirements.
• Insulation layer: the insulation layer is a valuable solution for retrofitting buildings with minimal or no insulation, preventing heat loss during winter and escaping cool air during summer [39]. Some insulation materials also reduce sound transmission through the roof, thereby improving the acoustic comfort of the building. The optimal thickness of the insulation layer generally ranges between 4 cm and 10 cm in extensive green roofs and 10 cm and 20 cm in intensive systems. The most commonly used insulation materials include extruded polystyrene and expanded polystyrene. Polyurethane foam and polyisocyanurate panels are also effective, offering superior thermal resistance. Sustainable alternatives such as cork-based insulation boards and mineral wool panels provide additional benefits regarding fire resistance and environmental impact reduction.
• Waterproofing membrane: the waterproofing layer is a barrier against water infiltration into the building, ensuring the long-term durability of the entire system [79]. This layer must exhibit exceptional strength, flexibility, and the ability to withstand structural loads and environmental stresses. Among the most used waterproofing materials are synthetic rubber and bituminous membranes. The thickness of this layer generally ranges from 1.5 mm to 4 mm, depending on the specific material and system requirements. Adopting rigorous installation practices and diligent maintenance of the waterproof membrane is critical. During installation, it is essential to ensure a solid adhesion of the waterproof layer to the underlying structure to prevent potential infiltration problems.

Table 1. Summary of the main characteristics of green roof layers, including typical thickness ranges, commonly used materials, and the primary functional contributions to thermal and energy performance.

Layer	Typical Thickness	Typical Materials	Main Function
Plants and vegetation	Varies according to green roof typology	Species adapted to local climate (e.g., Sedum spp., grasses, shrubs, small trees)	Regulating microclimatic conditions through shading and evapotranspiration; enhancing biodiversity; contributing to stormwater management, thermal regulation, and air purification.
Growth medium	Up to 150 mm for extensive roofs; up to 600 mm for intensive systems	Blend of inorganic (≈90%) and organic (≈10%) components	Supporting vegetation growth; providing thermal mass; enhancing water retention and thermal inertia.
Filter layer	0.5–2 mm	Geotextile fabrics (polypropylene or polyester)	Maintaining permeability by preventing substrate particle migration; optimizing stormwater management.
Drainage layer	20–100 mm	Granular aggregates or plastic-based drainage panels	Regulating water flow; preventing waterlogging; maintaining an optimal air–water balance within the substrate.
Protection layer	3–10 mm	Geotextile or polyester-based materials	Providing mechanical protection to underlying layers during installation and operational life.
Anti-root layer	~4 mm	Polyethylene sheets, plastic composites	Preventing root intrusion; protecting waterproofing membranes and structural integrity.
Insulation layer	40–100 mm (extensive roofs); 100–200 mm (intensive roofs)	Extruded polystyrene (XPS), expanded polystyrene (EPS), polyurethane foams, polyisocyanurate, cork-based panels, mineral wool	Enhancing building thermal resistance; reducing heating and cooling loads; improving acoustic insulation.
Waterproofing membrane	1.5–4 mm	Synthetic rubber or bituminous membranes	Providing a durable, watertight barrier; resisting mechanical and environmental stresses.

provides a comprehensive overview of each green roof layer’s main characteristics, including typical thicknesses, materials, and main functions. Figure 5 illustrates a schematic representation of the typical technological composition of a green roof system.

2. Materials and Methods

This study employs a structured and replicable methodological framework to evaluate green roofs’ thermal and energy performance in Mediterranean climates.

To achieve these objectives, the review process was conducted according to a structured and transparent approach based on the Population, Intervention, Comparison, Outcome, and Context (PICO(C)) framework, as a guiding structure for the formulation of research questions and the overall design of the systematic review. This framework is commonly used in evidence-based research to ensure clarity, focus, and reproducibility. Its application in this study allows for the structured identification of relevant studies, consistent extraction of data, and transparent alignment with the review objectives, particularly in the context of performance-based evaluation of green roof technologies in Mediterranean climates.

The methodology follows a step-by-step approach, strictly adhering to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) 2020 guidelines, ensuring methodological rigor and reproducibility of the findings. Each phase provides a systematic, reproducible, and data-driven analysis. The aim is to develop a comprehensive and rigorously structured assessment progressively.

The methodological approach is based on the following steps:

Step 1. In the first step of the methodology process, clearly defined research questions were established using the PICO(C) framework, a commonly used approach in systematic and scoping reviews. This framework breaks down complex research problems into clearly defined and structured components, ensuring that all key dimensions are explicitly considered. Its systematic application improves transparency, facilitates replicability, and enhances the methodological rigor of the review. The accurate selection of research questions is the foundation for subsequent methodological phases. These questions inform keyword selection, literature retrieval, and inclusion/exclusion criteria while structuring the synthesis of existing knowledge on the thermal and energy performance of green roofs in the Mediterranean region.

Step 2. A hybrid strategy was employed to search for highly relevant records from well-known databases (Scopus and Web of Science), combining structured keyword-based searching with snowballing. This dual approach enables systematic and iterative research landscape mapping, capturing highly cited foundational studies and recent contributions. It reduces bias and ensures a balanced field representation.

For the search process using databases, a combination of core concepts related to green roof technology was identified to develop the search query. Boolean operators were applied to refine the search results. The PICO(C) framework outcomes guided keyword selection, ensuring that all relevant aspects were adequately represented.

The Web of Science (WoS) and Scopus search queries were designed using a structured Boolean logic approach to ensure a systematic, precise, and comprehensive identification of relevant literature. Both queries were restricted to the title, abstract, and keywords using the “TS = ()” command in WoS and the “TITLE-ABS-KEY()” command in Scopus, ensuring that only thematically relevant studies were retrieved.

To maintain logical coherence and completeness, each query was structured into three conceptual groups: green roof technology, thermal and energy performance, and Mediterranean climate context, enclosed within parentheses to preserve the integrity of Boolean logic. Boolean operators were applied strategically: OR captured synonyms and variations within each group, maximizing recall, while AND ensured that retrieved studies addressed all three dimensions simultaneously. The truncation operator (*) was used to retrieve multiple-word forms, broadening the search scope without sacrificing precision. Additionally, phrase searching (“) was employed to maintain term integrity, preventing the unintended separation of multi-word concepts such as “urban heat island”.

The complete search queries used for Scopus and WoS are presented in

Table 2. Complete search queries used for Scopus and WoS datasets.

Query for Web of Science

Query for Scopus

TS = ((“green roof *” OR “vegetated roof *” OR “green infrastructure”    OR “living roof *” OR “extensive green roof *”    OR “intensive green roof *” OR “semi-intensive green roof *”)   AND   (“thermal insulation” OR “thermal properties” OR “cooling potential”    OR “heat transfer” OR “building energy efficiency” OR “energy efficiency”    OR “energy demand” OR “energy saving *” OR “thermal inertia”    OR “thermal comfort” OR “thermal performance” OR “passive cooling”    OR “water efficiency” OR “building energy”    OR “cooling capacity” OR “stormwater management” OR “runoff retention”    OR “urban heat island” OR “urban heat island mitigation”    OR “climate adaptation” OR “climate resilience” OR “urban resilience”    OR “sustainable urban design” OR “seasonal variability”    OR “passive system” OR “reduce energy demand” OR “reduce energy needs”)   AND   (“Mediterranean” OR “Mediterranean climate” OR “Mediterranean basin”    OR “Mediterranean region” OR “Mediterranean cities”    OR “hot-dry climate” OR “hot summer climate” OR “warm temperate climate”))

TITLE-ABS-KEY((“green roof *” OR “vegetated roof *” OR “green infrastructure”    OR “living roof *” OR “extensive green roof *”    OR “intensive green roof *” OR “semi-intensive green roof *”)   AND   (“thermal insulation” OR “thermal properties” OR “cooling potential”    OR “heat transfer” OR “building energy efficiency” OR “energy efficiency”    OR “energy demand” OR “energy saving *” OR “thermal inertia”    OR “thermal comfort” OR “thermal performance” OR “passive cooling”    OR “water efficiency” OR “building energy”    OR “cooling capacity” OR “stormwater management” OR “runoff retention”    OR “urban heat island” OR “urban heat island mitigation”    OR “climate adaptation” OR “climate resilience” OR “urban resilience”    OR “sustainable urban design” OR “seasonal variability”    OR “passive system” OR “reduce energy demand” OR “reduce energy needs”)   AND   (“Mediterranean” OR “Mediterranean climate” OR “Mediterranean basin”    OR “Mediterranean region” OR “Mediterranean cities”    OR “hot-dry climate” OR “hot summer climate” OR “warm temperate climate”))

, detailing the specific keywords, and Boolean operators applied to refine the selection of relevant studies.

A snowballing technique that involved backward and forward citation tracking was utilized to refine the dataset further and reduce the risk of omitting highly relevant studies. Key papers identified through the initial keyword-based search were analyzed for additional references, ensuring that significant studies were included. This iterative process was guided by benchmark studies [39,68,69,70,71,72,79,81,82] selected for their status as widely cited reviews of green roof technology. These references acted as starting points for further exploration, enabling the identification of important thematic trends, methodological frameworks, and recent advancements in green roof research, especially in Mediterranean climates.

The combination of highly cited reviews as benchmark references and forward/backward citation tracking enables the identification of both seminal works (which define the foundational knowledge of the field) and emerging studies (which reflect the latest advancements and innovations).

By combining database searches with snowballing, the methodology ensured comprehensive coverage, high precision, and methodological reproducibility, establishing a solid foundation for a rigorous and well-balanced literature review.

Step 3. The selection process followed the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines [83], ensuring transparency, replicability, and methodological rigor. Precise inclusion and exclusion criteria were established to filter the identified literature systematically, ensuring that this review effectively addresses the research questions derived from the PICO(C) framework. To strengthen the robustness of this phase, two independent authors of this work conducted a double-check process to mitigate selection bias and ensure consistency in study inclusion.

Clear inclusion and exclusion criteria were meticulously defined based on the PICO(C) framework (see

Table 3. Inclusion and exclusion criteria applied to all studies during the search process.

Criteria	Inclusion	Exclusion
Study Type	Peer-reviewed research articles	Review articles, conference papers, data articles, editorial letters, book chapters, patents, white papers, non-peer-reviewed reports
Study Scope	Research specifically focusing on green roof technologies, and their thermal and energy-related benefits	Studies focusing on green façades, vertical gardens, or other green infrastructure without addressing green roofs
Climatic Relevance	Studies conducted in Mediterranean climates	Studies in cold or tropical climates unless they provide comparative insights relevant to Mediterranean conditions
Methodological Rigor	Studies with clear methodology, experimental tests, numerical simulations, or validated models.	Studies lacking clear methodology, poorly documented results, or without quantifiable performance indicators

), allowing for systematic and unbiased filtering of the identified literature. This process enhanced transparency, replicability, and methodological rigor by following the PRISMA guidelines, ensuring a robust foundation for the subsequent analysis.

No temporal restrictions were applied during the literature search. This choice allowed a comprehensive exploration of how research interest in the thermal and energy performance of green roofs in Mediterranean climates has evolved, capturing early foundational studies and more recent advancements.

The search phase began with executing structured queries in WoS and Scopus. Specific filters were applied to each database to refine the identification of the record and exclude irrelevant studies.

For Scopus, the applied filters included:

• Subject areas: Environmental Science, Engineering, Energy, Social Science, Earth and Planetary Sciences, Physics and Astronomy, and Materials Science.
• Document type: restricted to articles to ensure peer-reviewed, high-quality sources.
• Language: limited to English.

For WoS, the following filters were applied:

• Subject areas: Environmental Science, Energy Fuels, Construction Building Technology, Engineering Civil, Green Sustainable Science Technology, Environmental Studies, Engineering Environmental, Urban Studies, Ecology, Water Resources, Meteorology and Atmospheric Sciences, Thermodynamics, Engineering Chemical, Regional Urban Planning, and Architecture.
• Document type: restricted to articles to ensure a focus on peer-reviewed research.
• Language: limited to English.

After these filters were applied, 143 records were retrieved from Web of Science and 130 from Scopus, resulting in an initial dataset of 273 records.

The screening phase consisted of two steps. In Phase 1, duplicate records were identified and eliminated, reducing the dataset to 169 unique records. In Phase 2, a full-text analysis was performed by applying the predefined inclusion and exclusion criteria, ensuring only studies relevant to the review objectives were considered. After this final filtering stage, 36 reports were selected for inclusion in the review [84,85,86,87,88,89,90,91,92,93,94,95,96,97,98,99,100,101,102,103,104,105,106,107,108,109,110,111,112,113,114,115,116,117,118,119], marking the transition from records to thoroughly screened reports.

An additional 20 reports were identified using the snowballing technique to enhance the dataset’s comprehensiveness and reliability [120,121,122,123,124,125,126,127,128,129,130,131,132,133,134,135,136,137,138,139]. This technique tracked the backward and forward citations of highly cited and authoritative articles. This iterative approach allowed for the inclusion of seminal studies and emerging contributions, ensuring that the review captures both historically significant and recent advancements in the field.

To ensure the reliability of the study selection process, an inter-coder reliability analysis was conducted using Cohen’s Kappa coefficient. This step was crucial for quantifying the consistency in applying the PRISMA protocol and minimizing subjectivity in inclusion/exclusion decisions. Two independent reviewers performed the screening process in parallel. Each of the 169 unique records was assigned a code and evaluated independently by both reviewers, categorizing them as “include” or “exclude”. The results were then processed using MATLAB_R2024b, where the “exclude” judgment was assigned a numerical value of 0, and the “include” judgment was assigned a value of 1. A contingency table was constructed to calculate the observed agreement ($P_o$) and the expected agreement $(P_e$) values. Finally, the Cohen’s Kappa coefficient ($K$) was then computed as:

$$K={\displaystyle \frac{P_o-P_e}{1-P_e}}$$

(1)

Using the obtained agreement values (e.g., $P_o$ = 0.80 and $P_e$ = 0.35), Cohen’s Kappa was calculated at 0.69, indicating a substantial level of agreement between reviewers. Subsequently, classification discrepancies were resolved through discussion, ensuring a consensus on each record’s final inclusion or exclusion.

The final selection included 56 studies, 36 of which were found through database searches, and 20 additional articles identified through the snowballing technique. This study ensures that the final dataset consists of methodologically robust, thematically relevant, high-impact studies by adhering to the PRISMA methodology and employing a systematic multi-step filtering process. Thus, it forms a solid foundation for an evidence-based systematic review. Figure 6 presents an overview of the applied search process through the PRISMA framework.

Step 4. Then, each selected study was meticulously examined through careful full-text reading, ensuring a thorough extraction of all relevant information. A systematic approach was employed to extract, categorize, and organize key information from the selected studies, ensuring a comprehensive and structured synthesis of findings. Relevant data were meticulously recorded in a structured Excel spreadsheet, facilitating the analysis of bibliometric, methodological, technological, and performance-related aspects.

The first category of extracted data included basic bibliometric information, such as the authors, title, abstract, year of publication, author keywords, source title (journal), number of citations, and DOI. Additionally, the study’s geographical location was recorded to assess the regional distribution of research on green roof technology in Mediterranean climates.

The second category focused on methodological aspects, capturing key details such as the study’s aim and scope, type of research (experimental, numerical simulation, or hybrid study), analytical methods, software employed, monitoring time frame, climatic data sources, and model calibration assessment. These parameters ensured a comprehensive understanding of how each study was conducted and the reliability of its findings.

The third category addressed the technological characteristics of green roof systems, detailing the type of green roof implemented, stratigraphy and materials used, vegetation characteristics, substrate specifications, and irrigation methods. These details provided critical insights into the diverse configurations of green roof technology and their potential impact on thermal and energy performance.

Finally, the last category encompassed findings, synthesizing the key results reported in each study. The categorization of findings facilitated a structured comparative analysis, identifying performance trends, recurring challenges, and knowledge gaps across studies. By organizing the extracted data, this approach enabled a quantitative and qualitative synthesis, strengthening the robustness of the review and supporting data-driven conclusions on the thermal and energy performance of green roof technologies in Mediterranean climates.

Step 5. A bibliometric analysis was performed once all the reports included in the review had been meticulously selected. This analysis provides a comprehensive overview of the scientific production of green roof technology in Mediterranean climates, capturing publication trends, geographical distribution, citation impact, and keyword analysis.

Step 6. A trend analysis was performed to identify the evolution of research themes, author collaboration networks, and thematic coverage in green roof technology in Mediterranean climates. This approach enables the detection of emerging trends, knowledge gaps, and underexplored research areas, ultimately supporting the advancement of green roof technologies. By integrating quantitative and qualitative bibliometric approaches, this review ensures a more comprehensive representation of the research panorama.

VOSviewer 1.6.20 (0) software was employed to analyze the temporal evolution of research topics by examining keyword usage trends across published studies. Additionally, it was used to identify the most active contributors and assess the extent of scientific collaboration within the field. Following this, a detailed topic analysis was conducted for each record, systematically categorizing studies based on key research themes. To quantify the prominence of these themes, a heatmap analysis was performed, mapping the frequency of topics across studies and cross-referencing them with thermal and energy performance evaluations.

Step 7. A structured methodology, following Braun and Clarke’s (2006) approach [140], was used to conduct the thematic analysis. This method was selected due to its flexibility in capturing explicit and implicit themes, offering a more nuanced interpretation of the data than alternative qualitative approaches. In particular, the primary objective of this process was to identify and systematically organize key themes emerging from the analyzed literature. To enhance clarity and coherence, a thematic map was developed to visually represent the connections between the various themes identified throughout the study.

The thematic analysis followed a structured five-step process: (i) generation of initial codes, (ii) grouping codes into themes, (iii) theme refinement, (iv) theme definition and description, and (v) summary table and thematic map development.

A coding system was established in the first step to categorize key concepts extracted from the reviewed studies. A dedicated column was added to the Excel file used in the data synthesis phase, assigning each record a set of concise codes. This approach summarized the core ideas identified in the findings. In the second step, similar or related codes were consolidated into broader thematic categories, ensuring that each theme condensed a central concept consistently emerging across multiple studies. This step allowed for a more structured and meaningful interpretation of the data.

The third step involved critically reviewing the identified themes to ensure coherence and analytical rigor. Each theme was assessed for its relevance and consistency with the dataset, and any redundancies or overlaps were addressed through refinement and reorganization. In the fourth step, each theme was further elaborated with a clear and detailed description, ensuring a precise definition aligned with the insights derived from the literature. Finally, a summary table (

Table 4. Application of the PICO(C) framework to the review’s topic.

PICO(C)	Definition	Application to the Study Objective
P (Population)	Peer-reviewed papers on green roof technology	Analysis of peer-reviewed research articles addressing the implementation of green roofs on residential, commercial, and public buildings in Mediterranean climates
I (Intervention)	Systematic literature review	Analysis of the thermal and energy benefits of green roofs, their limitations, and design strategies
C (Comparison)	Comparison between green roofs and other passive technologies	Comparative analysis of green roofs versus: (1) conventional roofs (bitumen, tiles, concrete) (2) cool roofs (high-albedo reflective roofs) (3) ventilated roofs (4) white roofs (5) green roofs combined with well-known passive technologies
O (Outcome)	(1) Qualitative and quantitative benefits in terms of thermal comfort and energy efficiency (2) research gaps	Identification of key aspects regarding the thermal and energy performance of green roofs. Assessment of design challenges, research gaps, and areas requiring further investigation. Examination of the main performance metrics used across studies
C (Context)	Green roofs in Mediterranean regions	Focus on green roof implementation in Mediterranean areas, characterized by hot, dry summers and mild, wet winters

) was compiled in the last step to concisely present the identified themes and their corresponding descriptions, facilitating interpretation and discussion. Additionally, a thematic map was constructed to visually depict the relationships and interdependencies among themes, providing a structured representation of the findings.

Step 8. Building upon the qualitative thematic analysis, a quantitative evaluation assessed the prevalence, distribution, and impact of key themes identified across the reviewed literature. This approach allowed for systematically quantifying green roof performance in Mediterranean climates, offering a data-driven perspective on thermal and energy efficiency. Each study was coded based on its thematic focus, enabling assessment of the dominant research trends and their associated performance outcomes. Integrating both qualitative and quantitative analyses provides a robust, multidimensional field assessment.

The flowchart shown in Figure 7 outlines the entire methodological process.

3. Systematic Review Process Outcomes

3.1. Research Questions’ Identification

The five elements of the PICO(C) framework were carefully adapted to align with the specific objectives of this study, ensuring a structured and precise approach to literature selection and analysis. Table 4 provides a detailed breakdown of these elements, illustrating how each component was defined and applied to systematically identify and evaluate relevant studies on green roof thermal and energy performance in Mediterranean climates.

This review formulates five key research questions (RQ) that align with the study objectives and ensure a structured inquiry into the existing literature.

• RQ1: “Have the thermal and energy performance of green roofs in Mediterranean climates been sufficiently analyzed in the existing literature?”

  ○

  Justification for RQ1: derived from the Population (P) and Outcome (O) elements, RQ1 thoroughly evaluates past studies and methodological trends to analyze the existing field knowledge comprehensively.

• RQ2: “Do green roofs significantly improve thermal and energy performance compared to conventional roofing systems in Mediterranean climates?”
• RQ3: “How do green roofs compare to other passive technologies in Mediterranean climates?”

  ○

  Justification for RQ2 and RQ3: rooted in the Comparison (C) dimension, RQ2 and RQ3 assess the relative effectiveness of green roofs against alternative passive cooling strategies.

• RQ4: “What are the primary factors influencing green roofs’ thermal and energy performance in Mediterranean climates?”

  ○

  Justification for RQ4: formulated using the Intervention (I) and Outcome (O) dimensions, this question investigates how design features, materials, and environmental conditions impact green roof technology performance.

• RQ5: “What are the main research gaps and methodological limitations in current studies, and how can future research address these gaps?”

  ○

  Justification for RQ5: this question, derived from a holistic consideration of all PICO(C) elements, ensures that the review identifies unexplored areas, methodological limitations, and potential directions for future research.

By structuring research questions through this approach, this study ensures that the systematic review comprehensively addresses key knowledge gaps, methodological inconsistencies, and practical challenges in green roof technology for Mediterranean climates. The insights derived will provide evidence-based guidance for researchers, policymakers, and urban planners, ultimately supporting the future development of sustainable, resilient, and energy-efficient green roof solutions.

3.2. Bibliometric Analysis

As mentioned, a bibliometric analysis was performed to comprehensively examine publication trends, geographical distribution, citation impact, and keyword patterns. This analysis identified research dynamics, influential contributions, and emerging areas of interest, providing valuable insights into the field’s evolution and guiding future research directions.

The analysis of publication evolution (Figure 8a) highlights a fluctuating yet overall increasing interest in green roof technology within Mediterranean climates. While the early 2010s saw limited research output, a notable increase in scientific reports was observed from 2014 onwards, reaching a peak in 2020. Despite the overall rise, fluctuations were observed, with declines in 2019 and 2021, followed by a resurgence in 2020 and 2023, indicating heightened and reduced research activity cycles. The latest data for 2024 and 2025 indicate a drop in publications, which may be attributed to ongoing research projects yet to be published.

The citation analysis (Figure 8b) indicates that while the number of reports has increased over the years, the research impact has fluctuated significantly. Notably, 2012, 2016, and 2020 stand out as peak years for citations, likely driven by the publication of influential studies and increased global attention on sustainable urban development. A decline in citations in more recent years (2023–2025) may be due to the time lag required for new publications to accumulate citations. These variations highlight the importance of high-impact publications in shaping the field and suggest that while research output increases, its long-term influence depends on key breakthrough studies.

The spatial distribution of reports (Figure 9) reveals an intense concentration of research efforts in Italy, with 25 identified reports, and Spain, with 16 identified reports. These nations account for a significant portion of the total studies, likely due to their urban sustainability initiatives and policy frameworks promoting green infrastructure. In contrast, research output in other Mediterranean countries appears notably lower, with only 2 records identified in France and Portugal, suggesting a more limited scientific focus on green roof technology in these regions. North African and Eastern Mediterranean countries, including Egypt (1 record), Greece (4 records), Cyprus (3 records), Israel (2 records), Jordan (1 record), and Lebanon (2 records), have a lower presence in the literature. This suggests a growing but still underrepresented research effort in these regions, potentially due to constraints such as limited funding, lower policy incentives, or reduced academic collaboration networks. The geographical disparities in research output highlight the need for increased cross-border collaboration and targeted policy support to strengthen the knowledge base on green roof technologies across the Mediterranean region.

The keyword analysis conducted using VOSviewer, such as in other review papers [141,142,143,144], offers insight into the primary research themes and their interconnections (Figure 10). The network visualization highlights dominant keywords such as “green roof”, “Mediterranean climate”, “urban heat island”, “sustainability”, climate change”, and “green infrastructure”. The clustering of keywords reveals distinct research directions. One cluster is centered around energy efficiency, thermal performance, and passive cooling strategies, reflecting the focus on the building energy performance of green roofs. Another cluster emphasizes urban heat island mitigation, climate change, and nature-based solutions, demonstrating the increasing role of green roofs in urban climate adaptation. Furthermore, an additional cluster focuses on the hydrologic performance of green roofs. Strong connections were observed between urban heat island and green roof keywords, as well as between green roof and energy savings keywords, underscoring the importance of these technologies in enhancing the urban environment. The presence of keywords related to experimental analysis, simulation techniques, and monitoring highlights the methodological diversity in the field, where both empirical and numerical approaches are widely employed. The use of co-occurrence network analysis in VOSviewer strengthens the reliability of these findings, as it systematically identifies thematic clusters based on citation relationships and research impact.

The bibliometric analysis demonstrates a growing scientific interest in green roof technologies in Mediterranean areas until 2020. While the field has seen notable citation peaks, the variability in annual outputs suggests opportunities for further research consolidation and interdisciplinary collaboration. The keyword analysis indicates an evolving research landscape, with increasing emphasis on climate adaptation strategies and energy efficiency, reinforcing the relevance of green roofs as a sustainable urban solution. Addressing regional research disparities and promoting cross-border collaborations could further enhance the impact and applicability of green roof technologies in Mediterranean climates.

3.3. Trend Analysis

The results from the analysis of the temporal evolution of research topics are shown in Figure 11. The analysis revealed a clear shift in research focus over time. In earlier studies (2016–2018), dominant keywords were “energy efficiency”, “green roof”, “energy consumption”, “passive cooling”, and “urban heat island mitigation”, reflecting an initial emphasis on the environmental and energy performance of green roofs. In more recent years (2020–2025), the prevalence of terms such as “climate change mitigation”, “sustainability”, “heat waves”, and “urban heat island” indicates an increasing focus on the broader sustainability aspects of green roofs, particularly in the context of climate resilience. Furthermore, there has been a recent growing association between “green wall”, “nature-based solutions”, and “urban green infrastructure”, suggesting an emerging research direction exploring the integration of green roofs with passive architectural strategies. Despite these advancements, keywords related to the synergy between green roofs and other passive solutions, intensive and semi-intensive green roofs, cost–benefit analysis, and long-term performance assessment remain underrepresented, highlighting critical research gaps that need further exploration.

Figure 12 depicts the outcomes of analyzing the field’s most active contributors and scientific collaboration. The study reveals the formation of distinct author clusters, with strong intra-group collaborations but limited cross-group interaction. The central cluster consists of researchers from Italy and Spain, reflecting regional concentration in scientific output. While these strong regional collaborations have facilitated advancements in green roof research, they may also lead to redundancy in topics and a lack of diversified perspectives. The limited international collaboration may hinder the integration of novel methodologies, comparative climate studies, and interdisciplinary approaches, which are crucial for ensuring the scalability and adaptability of green roof technologies across different Mediterranean sub-climates. Encouraging greater cross-border collaboration could enhance knowledge exchange, diversify methodological approaches, and foster innovation in green roof research.

A detailed topic analysis was conducted on each record, categorizing studies based on key research themes. The heatmap analysis (Figure 13) quantifies the frequency of topics across studies and cross-references them with thermal and energy performance evaluations. The most extensively covered topics include extensive green roof typology, vegetation design impact on green roofs’ performance, and their mitigation effect on the urban heat island phenomenon. Notably, 47 studies addressed the thermal performance of extensive green roofs, while 27 focused on energy efficiency improvements. Furthermore, the impact of the vegetation layer on green roofs’ thermal performance was evaluated by 19 studies. Conversely, innovative green roofs, multilayer systems, and the synergy of green roofs with other passive solutions were covered in less than 10 studies, indicating that these areas remain significantly underexplored. Additionally, specific key themes, such as the impact of substrate design (13 records), water content (6 studies), and synergy with other passive solutions (3 articles) on green roof’s thermal and energy performance, are only marginally represented. Furthermore, fewer studies (less than 9) focused on the energy and thermal performance of intensive and semi-intensive green roofs. Addressing these gaps could provide a more holistic understanding of green roof functionality, particularly its material properties, long-term efficiency, and integration with urban planning strategies.

Table 5. Overview of all the reports used to create the heatmap, with specifications of all the analyzed parameters.

	EnPer	TherPer
ExtGR	[84,86,88,90,92,93,95,96,97,99,100,106,110,112,114,117,118,119,121,123,125,126,129,130,131,132,145]	[84,85,86,87,88,89,92,93,95,96,97,98,99,100,101,102,103,105,106,107,108,109,110,113,114,115,116,117,119,120,121,123,125,126,127,128,129,130,131,132,134,135,136,137,138,139,145]
IntGR	[100,104,114]	[100,104,114]
SemiGR	[114]	[114,137]
InnGR	[91,94,133]	[91,94,133]
MultiGR	[124]	[111,124]
SmScGR		[134]
Substr	[84,92,93,100,112,131]	[92,93,105,110,121,131,135]
Insul	[86,88,91,97,112,118,123,129]	[85,86,87,89,91,97,103,107,109,123,129,137]
Irrig	[88,90,95,100,119]	[85,103,116]
Veg	[88,93,96,100,118,145]	[85,87,93,96,102,104,106,107,109,113,116,120,121,127,134,135,136,139,145]
UHI		[92,101,102,103,104,111,119,120,133,135,137,138]
CompPS	[97,99,114,145]	[97,99,101,102,108,114,115,138,145]
SynPS		[101,115,120]
H2Ocont	[95,99,131]	[85,131,135]

details all the studies used to create the heatmap, specifically representing all the parameters analyzed in each work. The same codes used in the heatmap were used for the parameters reported in the table.

Overall, the trend analysis underscores the dynamic evolution of research priorities in green roof technology. While early studies predominantly focused on assessing extensive green roofs’ thermal and energy performance, recent research has expanded toward sustainability and climate adaptation strategies. However, gaps persist in investigating other green roof typologies, evaluating material-specific performance metrics, and integrating interdisciplinary approaches with passive design strategies. Immediate action is needed to strengthen international collaborations and encourage interdisciplinary research, ensuring that green roof technologies can evolve into a scalable and adaptable solution for urban sustainability in Mediterranean climates.

3.4. Qualitative Results: Thematic Analysis (Braun and Clarke, 2006 [140])

By implementing this systematic thematic analysis, the study achieved a comprehensive and structured synthesis of the existing literature, providing a nuanced understanding of green roof research in Mediterranean climates. This approach highlighted the key thematic areas shaping current research and identified critical knowledge gaps and methodological inconsistencies, offering a foundation for future investigations.

To enhance clarity and facilitate discussion,

Table 6. Overview of main identified themes from qualitative thematic analysis.

Theme	Description
Layering and materials selection	The selection of geometry, materials, and vegetation for each layer is crucial in defining the thermal and energy performance of green roofs. In particular, the design of the substrate and vegetation layer plays a key role in indoor comfort regulation, UHI mitigation, and seasonal thermal efficiency. The inclusion of additional insulating layers significantly deteriorates green roof performance.
Local Climate	The type of green roof and the technical characteristics of each layer must be carefully designed in response to local meteorological conditions to ensure optimal performance and durability.
Irrigation Strategies	Implementing effective irrigation strategies is essential for optimizing the thermal and energy performance of green roofs. Water content regulation within the substrate regulates insulation in winter and cooling efficiency in summer. Irrigation during winter reduces insulation effectiveness, while in summer, it is fundamental for cooling performance.
Synergy with Other Passive Solutions	Combining green roof technology with other passive strategies can significantly enhance a building’s thermal and energy performance, leading to greater efficiency in heat regulation and overall sustainability.
Comparison with Other Passive Solutions	Green roofs generally exhibit lower cooling effects compared to living walls and green façades. However, they outperform cool roofs in terms of energy efficiency. Advanced and innovative green roof designs can achieve superior thermal and energy performance compared to conventional green roof systems.
Advanced models	Artificial neural network models provide reliable prediction for Mediterranean green roofs

presents a structured overview of the identified themes and their corresponding descriptions, enabling a clear interpretation of recurring research patterns. Additionally, the thematic map illustrated in Figure 14 visually represents the interconnections and hierarchical relationships among themes, offering a valuable framework for understanding how different aspects of green roof performance are conceptually correlated in the literature.

The thematic map provides a structured representation of key themes and their interconnections within the analyzed literature. It offers a systematic overview of dominant research areas, emerging trends, and critical knowledge gaps in green roof performance for Mediterranean climates. Visually demonstrating the relationships between layering and material selection, local climate conditions, irrigation strategies, and synergy with other passive solutions enhances the understanding of established topics while identifying underexplored areas that warrant further investigation.

Beyond its role as a conceptual framework, the thematic map increases the usability of this review by guiding future research directions, ensuring that forthcoming studies build upon existing knowledge while addressing current gaps. It also serves as a practical reference for policymakers, architects, and urban planners, supporting evidence-based decision-making for optimizing green roof design and integrating it into sustainable urban strategies. The thematic map reinforces academic discussion and practical applications by synthesizing qualitative insights into a structured and actionable research tool, contributing to advancing climate-responsive urban solutions.

3.5. Quantitative Results: Thematic Analysis

Building upon the qualitative thematic analysis, a quantitative thematic analysis was performed to assess the prevalence, distribution, and impact of key themes identified across the reviewed literature. This approach systematically quantifies green roof performance in Mediterranean climates, offering a data-driven perspective on thermal and energy efficiency. Each study was coded based on its thematic focus, enabling assessment of the dominant research trends and their associated performance outcomes. The performance of green roofs was assessed by comparing this technology with traditional roof configurations, providing insights into their effectiveness under different climatic conditions.

Local climate considerations were discussed in almost all the studies, underlining the necessity for climate-responsive design strategies. A geographical analysis revealed that research conducted in Southern Europe Mediterranean zones represented 84% of the dataset, while studies in North African regions accounted for 14%. Furthermore, records in the Middle East and Western Asia regions accounted for 2% of the dataset. Quantitative assessments demonstrated that local meteorological conditions significantly influenced green roof performance, with observed ambient temperature reductions spanning 0.03 °C to 7.4 °C and annual energy savings ranging from 10% to 34.7%, contingent on regional climate variability. Green roofs provide substantial yearly cooling and heating demand reductions depending on location. Cooling energy needs were reduced by 2.2% to 100% while heating energy efficiency was enhanced by 6.3% to 66%.

Among the analyzed themes, layering and material selection emerged as critical determinants of green roof performance, and were explicitly addressed in about 60% of the reviewed studies. Substrate composition, vegetation type, and overall thermal behavior were strongly correlated. These layers proved crucial in indoor comfort regulation and UHI mitigation. Reported thermal benefits included indoor temperature reductions ranging from 0.2 °C to 2.3 °C, with peak indoor surface temperature decreasing by up to 12.1 °C. Green roofs allowed for thermal stabilization benefits, reducing daily fluctuation by 68%. Furthermore, these technologies documented a significant UHI mitigation effect, leading to an ambient temperature reduction of up to 7.4 °C in summer and 5.7 °C in winter. The optimal design of the vegetation layer ensured a reduction in the external roof’s surface temperature by up to 86%, contributing to beneficial effects on urban overheating. Particular attention must be paid to choosing optimal insulation design strategies. Accurate layering design can enhance winter thermal retention from 0.3 °C to 5 °C. Although no single layer has been universally recognized as the most effective, the greatest design flexibility is found in the substrate, vegetative layer, and, when included, the insulation layer. The depth of the substrate and the selection of vegetation are critical determinants of both energy efficiency and thermal performance, actively contributing to temperature regulation throughout the year. Conversely, while the insulation layer can enhance heat retention, improper design or unnecessary inclusion may compromise the overall performance of the green roof, potentially reducing the building’s annual energy-saving potential by up to 9%.

In 5% of the reviewed studies, irrigation strategies were identified as a crucial performance-enhancing factor. Within this subset, 66% reported that controlled irrigation optimized thermal efficiency, particularly by enhancing cooling performance during summer months. Conversely, 33% of the subset studies indicated that winter irrigation compromised insulation effectiveness, leading to heat loss. Empirical measurements showed that optimized irrigation strategies improved summer cooling efficiency by up to 500%, depending on irrigation frequency and substrate water retention capacity. These findings highlight the importance of dynamic irrigation management, where adaptive scheduling based on seasonal and climatic variations could enhance energy savings while preventing heat loss.

Additionally, 4% of the studies explored the synergy between green roofs and other passive solutions, demonstrating the potential of integrating green roofs with additional passive cooling and insulation techniques. Hybrid strategies combining green roofs with photovoltaic (PV) panels were assessed for thermal and energy-related benefits. The findings revealed that photovoltaic–green roof systems reduced soil temperature by 17.8% to 26.1%, depending on vegetation type. Furthermore, green roofs improved PV efficiency by up to 3.33%, primarily due to the cooling effect induced by vegetation. The presence of a vegetative layer reduces rooftop surface temperatures through evapotranspiration, limiting thermal stress on PV panels and optimizing their electrical efficiency. This synergy underscores the potential for optimizing urban rooftop configurations, making green roofs integral to multi-functional sustainable design strategies.

In contrast, 21% of the studies focused on comparing green roofs with other passive solutions. The outcomes revealed that innovative green roof solutions provided substantial thermal and energy benefits. These systems reduced external surface temperatures by 25 °C compared to traditional roofs, ensuring enhanced outdoor thermal comfort. Additionally, innovative systems led to indoor air temperature reductions of up to 2 °C, with improvements in heating and cooling performance reaching 31% and 50%, respectively. Comparisons with cool roof systems indicated that while cool roofs provided superior UHI mitigation, green roofs, when irrigation is provided, outperformed cool roofs in overall energy performance, achieving approximately 10% better total energy savings with similar cooling benefits and superior heating efficiency.

Multilayer green roofs demonstrated high thermal performance, ensuring external surface temperature reductions of up to 40% and indoor temperature reductions of up to 3.7 °C. Comparisons with rooftop greenhouse systems indicated that rooftop greenhouses were more effective in cooling energy reduction, achieving 50% energy savings compared to 7% for green roofs. Other studies compared green roofs with alternative green solutions, such as living walls and green facades. The results showed that living walls and green facades provided the most effective benefit in mitigating UHI, with enhanced cooling effects at the pedestrian level. Green facades ensured temperature reductions of 1.2 °C compared to 0.18 °C for standalone green roofs. Green roofs were also compared with high-albedo pavements and wetted roofs, with high-albedo pavements proving to be the most effective in mitigating UHI and wetted roofs outperforming green roofs.

Table 7. Overview of key findings and insights related to different aspects analyzed in the literature.

Aspect Analyzed	Key Findings	References
Ambient temperature reduction	0.03–7.4 °C	[102,105]
Annual energy savings	10–34.7%	[86,123]
Cooling energy demand reduction	2.2–100%	[88,93]
Heating energy efficiency increase	6.3–66%	[86,110]
Indoor temperature reduction	0.2–2.3 °C	[84,88]
Peak indoor surface temperature decrease	Up to 12.1 °C	[139]
Daily thermal fluctuation reduction	From 68%	[89]
UHI mitigation effect (summer)	Up to 7.4 °C	[105]
UHI mitigation effect (winter)	Up to 5.7 °C	[105]
External surface temperature reduction	Up to 86%	[111]
PV efficiency improvement (with green roof)	Up to 3.33%	[141]
Irrigation-based cooling efficiency increase	Up to 500%	[100]
Innovative green roof external surface temperature reduction	Up to 25 °C	[123]
Innovative green roof indoor temperature reduction	Up to 2 °C	[91]
Innovative green roof heating performance increase	Up to 31%	[94]
Innovative green roof cooling performance increase	Up to 50%	[94]
Multilayer green roof external surface temperature reduction	Up to 40%	[124]
Multilayer green roof indoor temperature reduction	Up to 3.7 °C	[124]

summarizes the findings on green roofs’ thermal and energy performance. Additionally,

Table 8. Overview of key findings obtained when comparing green roof technology with other passive solutions.

Comparison	Key Findings
Green Roof vs. Cool Roof [97,101,108,117,123,128]	Cool roofs provide superior UHI mitigation. However, irrigated green roofs outperform cool roofs in overall energy savings, achieving +10% higher efficiency.
Green Roof vs. Living Wall [102,138]	Living walls demonstrate greater effectiveness in UHI mitigation, particularly at pedestrian level, by ensuring better microclimate regulation.
Green Roof vs. Green Façade [102,108,115,138]	Green facades outperform green roofs in UHI mitigation, achieving 1.2 °C higher ambient temperature reduction.
Green Roof vs. High-Albedo Pavement [104,108]	High-albedo pavements provide the best UHI mitigation effect among passive strategies, due to their high solar reflectance.
Green Roof vs. Wetted Roof [103]	Wetted roofs show superior UHI mitigation compared to green roofs, particularly in extreme heat conditions.
Green Roof vs. Rooftop Greenhouses [99]	Rooftop greenhouses achieve 43% greater cooling energy savings than green roofs, making them a more effective solution for cooling demand reduction.

presents the key results comparing green roofs with other passive solutions.

This quantitative thematic analysis provides a structured synthesis of the empirical evidence on green roof efficiency in Mediterranean climates. The findings reinforce the importance of site-specific design, adaptive irrigation management, and hybrid passive strategies to maximize performance. Moreover, the results highlight the necessity of integrating green roofs into broader urban sustainability frameworks, emphasizing their potential for enhancing energy efficiency, mitigating urban overheating, and improving outdoor thermal comfort. Finally, the data highlight research gaps, particularly in underexplored climatic subregions and innovative roof configurations, indicating opportunities for future investigations to refine best practices for sustainable urban development. Strengthening interdisciplinary collaboration and advancing technological innovations in green roof configurations will be essential to unlocking their full potential in climate-responsive urban planning.

4. Structured Response to Research Questions

A comprehensive analysis of the reviewed literature highlights the significant role of green roofs in enhancing thermal and energy performance in Mediterranean climates. While numerous studies provide valuable insights, disparities in geographical coverage, methodological approaches, and long-term assessments indicate the need for further research. This section offers a detailed response to the identified research questions:

• Have the thermal and energy performance of green roofs in Mediterranean climates been sufficiently analyzed in the existing literature?

The outcomes presented in Table 7 confirm that green roofs significantly improve thermal comfort and energy efficiency in Mediterranean climates. Their widespread adoption in densely populated urban areas could be crucial for improving urban sustainability.

However, research efforts remain geographically imbalanced and methodologically heterogeneous. A substantial 71% of the reviewed studies focus on Italy and Spain, while the remaining 29% of study locations span other South European regions, Middle Eastern climate areas, and North Africa.

Table 9. Geographical information from each selected study.

Study Location	References
Cyprus	[118,119,139]
Egypt	[117]
France	[103,137]
Greece	[104,115,125,136]
Israel	[113]
Italy	[86,88,89,91,92,94,95,96,97,99,102,106,109,111,117,121,123,124,126,128,129,132,133,138]
Jordan	[84]
Lebanon	[105,135]
Portugal	[100,114]
Spain	[81,83,86,89,97,103,104,106,108,112,113,116,123,126,127,141]

provides the geographical information of each selected report. This geographical disparity raises concerns about the generalizability of findings across the Mediterranean basin, particularly in arid and semi-arid subregions where green roof performance may differ significantly. Further research is needed to evaluate their efficiency in these less-studied areas.

Furthermore, long-term performance assessments are insufficient. Only 11% of studies considered an extended monitoring time frame [86,87,88,107,110,130]. The other 89% of records concentrate on seasonal or, at most, annual analyses, which restrict insights into the durability and enduring efficiency of green roofs over long periods. Understanding their long-term thermal behavior and potential degradation is crucial for optimizing future implementations.

Additionally, the role of water content in summer thermal performance remains underexplored. While empirical findings indicate that irrigation can enhance cooling efficiency by up to 500% [100], parametric analyses on optimal irrigation levels for maximizing cooling effects are still insufficient. Investigating these factors further could help refine the best green roof irrigation management practices in Mediterranean climates.

• Do green roofs significantly improve thermal and energy performance compared to conventional roofing systems in Mediterranean climates?

Empirical evidence unequivocally demonstrates that green roofs significantly enhance thermal comfort and energy efficiency compared to conventional roofing systems. During summer, green roofs effectively lower indoor and outdoor ambient temperatures, substantially improving urban microclimate conditions. Specifically, they mitigate the UHI effect, achieving outdoor temperature reductions of up to 7.4 °C during peak summer periods [105]. Additionally, they enhance indoor thermal comfort by reducing air temperatures within buildings by 0.2 °C to 2.3 °C [84,88], thereby minimizing reliance on active cooling systems and improving occupant well-being. In winter, green roofs reduce outdoor temperatures by up to 5.7 °C [105]. This underlines their capacity to regulate seasonal temperature fluctuations effectively.

Furthermore, green roofs provide substantial annual energy efficiency. Empirical assessments indicate annual energy savings ranging from 10% [123] to 34.7% [86], depending on climatic conditions, roof design, and vegetation characteristics. Additionally, they significantly reduce peak indoor surface temperatures by up to 12.1 °C [139], thereby decreasing cooling loads and mitigating extreme indoor heat accumulation.

Comparisons with traditional roofing systems highlight that green roofs provide superior thermal stabilization, reducing daily temperature fluctuations from 68% [89]. Moreover, their impact on cooling energy demand reduction (2.2% [93] to 100% [88]) and heating energy efficiency improvement (6.3% [86] to 66% [110]) underscores their role in optimizing indoor thermal comfort while reducing energy consumption.

Despite these advantages, performance variability remains a concern, as climate, irrigation, vegetation type, and substrate composition influence results. While green roofs consistently outperform conventional roofs, optimization of their configuration is necessary to maximize benefits. Multilayer green roofs demonstrate higher thermal efficiency than traditional roofs, reducing external surface temperatures by up to 40 °C [124] and indoor temperatures by 3.7 °C [124]. Furthermore, innovative green roof configurations have demonstrated up to 2 °C indoor temperature reduction [91], 31% improvement in cooling performance [94], and 50% enhancement in heating efficiency [94], further reinforcing their potential in Mediterranean climates.

• How do green roofs compare to other passive technologies in Mediterranean climates?

Green roofs have been compared to various passive cooling strategies, with mixed outcomes depending on the evaluation criteria. Cool roofs generally outperform green roofs in UHI mitigation due to their high solar reflectance. However, when irrigation is implemented, green roofs provide better overall energy savings (+10% [123]). Living walls and green facades show greater effectiveness in cooling at the pedestrian level, with green facades achieving a 1.2 °C more significant ambient temperature reduction than green roofs [138].

Rooftop greenhouses, on the other hand, reduce cooling energy more effectively, achieving 50% energy savings compared to 7% for green roofs [99]. In contrast, high-albedo pavements and wet roofs outperform green roofs in UHI mitigation, making them more suitable for large-scale urban cooling. These findings indicate that while green roofs offer significant benefits, their integration with complementary passive strategies can enhance performance in Mediterranean climates.

While these comparisons provide valuable insights, further research is needed to assess how hybrid solutions—such as integrating green roofs with high-albedo materials or evaporative cooling techniques—can maximize overall energy performance [104,108]. The effectiveness of each strategy is also highly dependent on climatic conditions, requiring localized adaptation strategies to optimize benefits.

• What are the primary factors influencing green roofs’ thermal and energy performance in Mediterranean climates?

A combination of climatic conditions, construction design, and operational factors influence green roofs’ thermal and energy performance in Mediterranean climates. One of the most critical variables is climate sensitivity, as regional meteorological conditions dictate how green roofs can regulate indoor temperatures and reduce energy demand. Given the diversity of Mediterranean climates (from humid coastal regions to arid inland areas), tailoring green roof configurations to site-specific climatic conditions is essential to maximizing their effectiveness.

Other key determinant are layering, materials’ choice, and insulation design. The composition and depth of the substrate, as well as the insulation layers, directly affect thermal efficiency. While increased insulation can enhance winter heat retention, excessive insulation may diminish summer cooling benefits, reducing overall annual energy savings by up to 9% [86]. The challenge is to optimize layering strategies that balance seasonal thermal requirements without compromising energy savings.

Vegetation selection also fundamentally influences green roof performance. Plant species exhibit varying evapotranspiration rates, directly impacting their cooling potential. Studies suggest drought-resistant vegetation is particularly suitable for arid Mediterranean regions, as it ensures thermal benefits with minimal irrigation requirements.

The role of irrigation management remains crucial yet underexplored. Empirical findings indicate that optimized irrigation can enhance summer cooling efficiency, yet excessive winter irrigation may lead to unwanted heat loss. Despite its importance, systematic research on the optimal irrigation regimes for different Mediterranean subregions remains limited. Future studies should focus on parametric analyses to determine the most effective watering strategies for maintaining high thermal performance while minimizing water consumption.

Finally, hybrid integration with other passive technologies offers significant potential for improving green roof efficiency. Combining green roofs with photovoltaic (PV) panels has been shown to enhance PV performance by up to 3.33% by lowering panel temperatures [141]. Integrating green roofs with high-albedo pavements or evaporative cooling strategies could also amplify urban heat island mitigation and energy savings. However, the synergistic effects of these hybrid solutions remain primarily unexplored in Mediterranean climates and warrant further investigation.

A holistic understanding of these factors is essential for optimizing green roof technology and ensuring adaptability across diverse Mediterranean environments. Future research should emphasize experimental and simulation-based studies to refine best practices, improve energy performance metrics, and develop climate-specific guidelines for green roof implementation.

• What are the main research gaps and methodological limitations in current studies, and how can future research address these gaps?

Despite notable advancements, significant research gaps hinder a comprehensive understanding of green roof performance in Mediterranean climates. Studies have focused primarily on Southern Europe, particularly Italy and Spain, leaving North African and Middle Eastern regions underrepresented. Given the climatic variability across the Mediterranean basin, expanding research into these areas is imperative to developing a more universally applicable knowledge base.

Another critical shortcoming is the lack of long-term performance assessments. Current research primarily relies on short-term experimental frameworks, which fail to account for aging effects, seasonal variations, and vegetation dynamics over extended periods. Future studies must adopt longitudinal approaches that monitor green roofs over multiple years and incorporate advanced modeling techniques to predict long-term efficiency trends.

Methodological inconsistencies further complicate cross-study comparisons. Variability in simulation models, performance indicators, and experimental setups undermines result reproducibility. Establishing standardized methodologies and uniform data reporting frameworks is essential to enhance comparability and foster meta-analytical assessments.

Additionally, research on irrigation strategies and vegetation selection remains fragmented. While empirical evidence supports the role of irrigation in enhancing cooling performance, parametric studies quantifying optimal irrigation thresholds are scarce. Future investigations should prioritize controlled experiments assessing the interplay between irrigation frequency, substrate properties, and vegetation types under varying climatic conditions.

Hybrid passive strategies also require greater exploration. Integrating green roofs with photovoltaic panels, reflective materials, and evaporative cooling systems has shown promising synergies, yet empirical validations remain limited. Addressing these gaps through interdisciplinary research incorporating remote sensing, AI-driven modeling, and real-world monitoring networks will be instrumental in optimizing green roof applications for Mediterranean urban sustainability.

5. Conclusions

This review establishes a foundation for advancing green roof research and implementation in Mediterranean cities, contributing to global climate change mitigation efforts in urban areas. It addresses a critical gap in the literature by providing a thorough and systematic investigation into the role of green roofs in Mediterranean urban sustainability, particularly focusing on thermal and energy performance.

Conducted under the PRISMA framework, this review systematically evaluates the potential of green roofs to enhance urban resilience by identifying targeted research questions using the PICO(C) structure. The findings reveal that green roofs significantly improve urban thermal conditions and energy efficiency. Empirical studies demonstrate that they reduce outdoor ambient temperatures by up to 7.4 °C during summer, mitigating urban heat island effects. Additionally, they enhance indoor thermal comfort by lowering air temperatures within buildings by 0.2 °C to 2.3 °C, and their impact on energy efficiency is substantial, contributing to annual energy savings ranging from 10% to 34.7%, depending on climatic conditions and design parameters.

Green roofs also play a crucial role in building heat regulation, reducing peak indoor surface temperatures by up to 12.1 °C and decreasing daily thermal fluctuations. These cooling benefits extend to external surfaces, where green roofs can lower rooftop temperatures by up to 86%, improving the built environment’s overall thermal stability. Advanced green roof configurations, including multilayer and innovative green roof designs, demonstrate even greater thermal advantages. Multilayer green roofs reduce external surface temperatures by up to 40 °C and indoor temperatures by 3.7 °C, while innovative green roofs lower external surface temperatures by up to 25 °C and improve indoor thermal conditions by reducing air temperatures by 2 °C. Furthermore, innovative designs enhance energy performance, increasing heating efficiency by up to 31% and cooling efficiency by up to 50%.

These findings underscore the critical role of green roofs as an effective nature-based solution for climate adaptation in Mediterranean cities. However, despite their well-documented benefits, key research gaps must be addressed to optimize their design, implementation, and long-term effectiveness. These gaps can be summarized as follows:

• Geographical disparity: existing studies disproportionately focus on Southern Europe, raising concerns about the generalizability of findings across the Mediterranean basin. Future research should target arid and semi-arid subregions, where climate conditions may significantly influence green roof performance.
• Long-term performance monitoring: a deeper understanding of how green roofs perform over extended periods is required. Studies should assess substrate degradation, vegetation adaptation, and changes in thermal performance over time.
• Optimization of design variables: green roof performance varies significantly depending on climate, irrigation, vegetation type, and substrate composition. Establishing standardized guidelines for optimal configurations is essential for maximizing their benefits.
• Hybrid integration with other passive solutions: to enhance overall energy efficiency, further research is needed to evaluate how green roofs can be combined with high-albedo materials, evaporative cooling techniques, and smart climate-responsive facades.
• Bridging experimental and computational approaches: future studies should leverage advanced simulation tools, remote sensing, and AI-based modeling to refine predictive assessments of green roof performance across different urban contexts.
• Implementation pathways for urban planning and policy: to maximize impact, research must extend beyond academic discourse and provide actionable recommendations for policymakers, urban designers, and stakeholders. Incentive programs, regulatory frameworks, and cost–benefit analyses should be developed to support the large-scale adoption of green roofs in Mediterranean cities.

Beyond their thermal and energy benefits, green roofs provide acoustic insulation, reducing environmental noise and enhancing urban soundscapes. Previous studies demonstrate that green roofs can effectively absorb and diffuse sound waves, mitigating urban noise pollution. This additional function strengthens their role as a multifunctional urban sustainability solution, contributing to improved living conditions in dense metropolitan areas.

Additionally, technological advancements have accelerated green roof innovation in recent years. Several innovative green roof systems now incorporate enhanced water retention materials, sensor-based adaptive irrigation mechanisms, and hybrid integrations with renewable energy technologies such as photovoltaic panels. These innovations highlight the increasing synergy between scientific research and practical implementation, reinforcing the viability of green roofs as a scalable and adaptable solution for climate-resilient urban design. Future studies should further explore these technological advancements, assessing their effectiveness and integration potential within the Mediterranean context.

To ensure that green roof applications reach their full potential, it is essential to promote interdisciplinary collaboration among engineers, architects, urban planners, and policymakers. Leveraging emerging technologies such as remote sensing, AI-driven predictive modeling, and real-world monitoring networks will provide more accurate assessments of green roof efficiency and guide their implementation at a city-wide scale.

Ultimately, advancing research in this field will contribute to urban sustainability and strengthen climate-adaptive policies, reinforcing the role of green infrastructure as a cornerstone of resilient and energy-efficient cities.