Review of Cooling Effects from Roof Mitigation Strategies Against Urban Heat Island Effects

Abstract

The rapid increase in global human activities and urban surface modifications has exacerbated the urban heat island effect, prompting growing scholarly efforts to adopt various measures for mitigating heat islands worldwide. This paper reviews existing literature on rooftop mitigation of UHI, summarizes specific existing rooftop mitigation measures, and examines the comparative effectiveness of various rooftop mitigation strategies in reducing urban heat islands. Findings indicate that cool roofs are the most effective rooftop measure for mitigating UHI, followed by green roofs and photovoltaic roofs. Simultaneously, the cooling effectiveness of rooftop mitigation strategies is influenced by their inherent characteristics (reflectivity, coverage, orientation, etc.), geographical and climatic features (latitude, humidity levels, temperature extremes, diurnal temperature variation, etc.), and urban morphology (building density, height, shape index, etc.). The research status summarized herein provides valuable insights for policy formulation and guides future studies, thereby promoting more innovative designs for sustainable urban roofs to mitigate UHI.

1. Introduction

Rapid urbanization has led to the emergence of the urban heat island (UHI) effect and associated challenges [1,2,3]. The UHI is characterized by higher near-surface temperatures in urban areas compared to rural environments, resulting from the concentration of human activities and infrastructure following the transformation of natural landscapes [4,5]. The impact of this urban expansion on the thermal environment is deeply concerning. Growing evidence indicates that persistent urban warming reduces thermal comfort for residents and poses significant threats to sustainable urban development, including increased energy consumption [6], deteriorated air quality [7], and even elevated mortality rates [8]. Without intervention, many people could be exposed to dangerously high heat indices for most of the year by the end of this century [9]. Against this backdrop, mitigating the UHI has emerged as a prominent focus in contemporary urban environmental research.

The urban heat island effect, as a typical environmental issue in modern urbanization, has made the formulation and implementation of mitigation strategies a critical topic in global sustainable urban development. From the perspective of the physical mechanisms underlying heat island formation, the core driving factors include changes in the thermal properties of urban surface materials, reduced vegetation coverage, and the combined effects of anthropogenic waste heat emissions [10]. The development of green infrastructure is recognized as a core strategy for mitigating the urban heat island effect [11], encompassing urban green spaces [12,13], street trees [14,15], and vertical greening [16]. Marando F et al. analyzed the UHI impacts across 601 European metropolitan areas, finding that urban green infrastructure can reduce urban temperatures by an average of 1.07 °C to 2.90 °C [17]. However, given the current scarcity of urban land, significantly expanding green space is impractical. Therefore, adjusting the existing layout of green spaces and optimizing their structure to address current urban conditions is a key focus in the development of urban green infrastructure. Zhou H et al. employed genetic algorithms to optimize urban green space distribution, reducing the average summer urban heat island intensity index to 0.27 °C—a decrease of 20.5% [18]. Beyond green infrastructure, other researchers have explored lowering urban temperatures by altering pavement thermal properties [19]. Climate change has elevated outdoor surface temperatures, increasing pedestrians’ vulnerability to heat stress. Thermal comfort assessments are typically based on the human body’s thermal equilibrium and its interaction with the surrounding environment. Among these metrics, UTCI and PET are the most commonly used indicators for evaluating outdoor thermal comfort [20]. Ariane Middel and colleagues developed an open-source model called PanoMRT, which precisely evaluates human thermal exposure and comfort levels by quantifying the effects of longwave radiation and surface temperatures of the pedestrian layer on the human body [21]. The proportion of buildings and greenery significantly impacts pedestrian thermal comfort. By analyzing urban environmental changes in Berlin from 2000 to 2035, Fabrizio Ascione et al. found that increased building coverage and reduced green space coverage led to heightened thermal stress at the pedestrian level [22]. Siqi Jia et al. [23] similarly noted that during Hong Kong’s hottest weather, 74.8% of areas experienced severe to extreme heat stress. Increasing green coverage—including green facades, green roofs, ground vegetation, and street trees—could reduce the heat climate index by 1.5 °C to 4.4 °C [23]. In addition, these include employing high-albedo pavement surfaces to enhance reflectivity, utilizing water-retaining pavement materials to increase evaporative cooling, and converting road surface heat into sustainable electricity [17,24,25]. The physical expansion of urban built-up areas has led to an increasing proportion of rooftops within the city’s urban surface area. Consequently, among various measures to mitigate the urban heat island effect, rooftop retrofitting holds equal importance to green infrastructure and urban road systems.

Building roofs account for nearly 20–30% of total urban surface area, making rooftop spaces crucial for mitigating the urban heat island effect [26]. Utilizing this space for surface modification requires no additional land, offering substantial opportunities for low-cost implementation of mitigation techniques [27]. Roof mitigation strategies (RMS) for urban heat islands operate by altering rooftop thermal properties to reduce temperatures. Current mainstream RMS approaches primarily involve cool roofs (CR) [28,29] and green roofs (GR) [30,31]. However, with intensifying energy scarcity and widespread adoption of photovoltaic systems, photovoltaic roofs (PVR) [32,33] have garnered significant attention for their potential impact on urban heat islands. Despite extensive research, uncertainties regarding the structural integrity, sustainability, and thermal performance of RMSs remain inadequately addressed. Furthermore, current studies primarily evaluate the performance of individual strategies, yet significant variations exist across climate contexts, measurement metrics, and spatio-temporal scales. This makes direct comparisons between studies extremely challenging and often leads to conflicting conclusions. Simultaneously, research on the synergistic effects and trade-off mechanisms of composite strategies, such as photovoltaic green roofs, remains in its infancy, lacking systematic empirical summaries. Therefore, this paper presents recent advancements in RMSs. It summarizes studies conducted across different regions worldwide, along with their conclusions and recommendations, and analyzes the extent of urban surface temperature reduction. This facilitates the assessment of the feasibility and reliability of various RMS approaches for mitigating the urban heat island (UHI) effect. This information also provides guidance for future research in related fields concerning urban rooftops and UHI.

2. Review Methods

This literature review strictly adhered to the PRISMA protocol (Supplementary Materials) [34]. The protocol encompasses several key steps: (I) Literature search and study selection; (II) Screening—inclusion and exclusion criteria; (III) Data extraction and synthesis; (IV) Data reporting.

2.1. Search Keywords

The urban heat island effect, though long recognized as a scientific issue, saw systematic mitigation strategies targeting the specific scale of “roofs” gradually emerge and flourish around the year 2000 with the rise in sustainable development concepts. This marks the starting point for exploring the entire process of this field’s development from inception to maturity. In the Web of Science database, the keywords “Roof mitigation strategy”, “Roof cooling effect”, and “Roof heat island mitigation” were selected. All documents from January 2000 to May 2025 were retrieved. The specific steps are as follows. First, the article type was filtered, limiting the search to research articles, yielding a total of 2452 articles from the Web of Science Core Collection. Second, duplicate papers with different keywords were removed for further analysis. This preliminary search yielded 1840 documents, which were subsequently optimized.

2.2. Screening—Inclusion and Exclusion Criteria

Inclusion criteria for this study were: (I) research articles related to the built environment; (II) articles published in English; (III) research articles with titles and keywords relevant to the search criteria. Exclusion criteria were: (I) articles containing relevant keywords or topics but not pertaining to built environment disciplines; (II) non-journal publications; (III) articles without full text availability. Finally, selected articles were further screened based on their abstracts to ensure relevance to the research objectives. After filtering titles/keywords/abstracts through the aforementioned criteria and excluding papers unrelated to the research theme, 412 high-quality relevant articles were ultimately obtained.

2.3. Data Analysis Methods and Tools

VOSviewer (v1.6.20.0) is a widely used software tool in bibliometric analysis. It enables the construction and visualization of various scientific network structures, thereby helping researchers identify the development context and emerging trends of specific disciplines. In this study, three main types of analyses were conducted using this software: (I) visual presentation and in-depth data mining of literature data; (II) systematic analysis of the co-occurrence relationships among keywords, journals, and countries to reveal the internal connections between research topics and academic cooperation; (III) literature co-citation analysis to identify the important knowledge bases and research evolution paths in the field. Additionally, Microsoft Excel (v2016) was used as an auxiliary tool in this study to screen and organize the initially retrieved literature, and generate data charts for supporting the analysis.

3. Result

3.1. Annual Publication Trend

Figure 1 shows the annual breakdown of the analyzed data. The temporal distribution of published literature can be divided into three phases: (I) The initial phase from 2004 to 2010. During this period, the global temperature index remained within a relatively stable fluctuation range. As the research field was in its infancy with limited understanding of the subject, publication volume was minimal, yielding only 1–3 papers annually. (II) The early development phase from 2011 to 2015. Annual publication volume increased significantly, averaging over 10 papers per year. In 2010, temperature indices consecutively surpassed previous records, sparking heightened global attention on climate change. The 2010 Cancun Climate Conference (COP16) formally included “urban adaptation” in its agenda for the first time. Additionally, the 2011 IPCC Special Report on Renewable Energy Sources and Climate Change Mitigation explicitly called for enhanced climate vulnerability assessments at the urban scale. These developments led to growing attention on urban heat island research starting in 2011. (III) Rapid Expansion Phase (2016–2025). 2015 ranked among the hottest years on record, with global temperature indices reaching unprecedented highs and the frequency and intensity of extreme weather events (such as heatwaves, floods, and droughts) increasing. This reality fueled public concern and generated strong demand for solutions, thereby stimulating further research and publications. Concurrently, software such as the Weather Research and Forecasting (WRF) model and urban microclimate models (e.g., ENVI met (v5)) evolved through multiple iterations, becoming more user-friendly and comprehensive. The signing of the Paris Climate Agreement that same year further propelled attention toward urban climate regulation, increased financial support, and directly spurred new research projects. The convergence of these factors led to a substantial increase in literature, reaching 305 papers within the decade. Against the backdrop of intensifying global climate change, rooftop mitigation of the urban heat island effect is increasingly recognized as a vital area of research for environmental and human well-being.

3.2. Distribution of Publishing Countries

Urbanization and climate change pose exceptionally severe challenges. Globally, international cooperation is crucial for advancing the mitigation of the urban heat island effect. In terms of country distribution, the selection criterion was set at a minimum of five articles per country/region. Based on these search criteria, only 20 out of 59 countries met the minimum requirement. Compared to the total number of countries worldwide (195), this level of representation is minimal, indicating that the majority of global literature on this topic is concentrated in a small number of nations. Figure 2 shows that China and the United States are the top publishers in the field of rooftop mitigation of urban heat islands, accounting for over one-third of the total publications. This geographical concentration likely stems from several factors: first, both China and the United States have invested substantial research funds in climate change and urban environment studies and established clear national strategies; second, these nations are undergoing rapid urbanization and confronting severe heatwave events, making urban heat islands an urgent research priority. Additionally, mature research communities and experimental platforms in these regions have fostered a virtuous cycle of sustained output. While China and the United States exhibit the highest level of collaboration, both also maintain strong cooperative ties with other nations, underscoring their representative status in these fields. Countries such as the United Kingdom, Italy, Germany, and Spain similarly demonstrate close collaboration, forming dense cooperative networks due to their geographical proximity.

3.3. Keyword Analysis

In this paper, co-occurrence analysis was performed using literature keywords. The minimum occurrence threshold was set to five instances per keyword to ensure the representativeness of the final results. Figure 3 presents the visual analysis of keywords.

Table 1. Top 20 high-frequency keywords.

Ranking	Keyword	Frequency	Ranking	Keyword	Frequency
1	Urban heat island	206	11	Vegetation	56
2	Influence	135	12	Energy	50
3	Green Roofs	122	13	System	50
4	Mitigation Strategies	81	14	Micro climate	48
5	Cool Roofs	80	15	Thermal performance	42
6	Performance	80	16	Benefits	41
7	Temperature	79	17	Comfort	41
8	Urban	67	18	Climate change	40
9	Mode	60	19	Albedo	39
10	Climate	59	20	Photovoltaic	39

lists the top 20 most frequent keywords, with “urban heat island”, “impact”, “green roof”, “mitigation strategy”, and “cool roof” being the five most frequently occurring keywords. In the keyword co-occurrence visualization, circle size corresponds to keyword frequency, lines indicate co-occurrence, and color represents publication year. The timeline reveals that terms like “photovoltaic”, “envi-met”, and “technologies” emerged in recent years within this research domain. This indicates growing attention to whether rooftop photovoltaic installations impact the urban thermal environment. Furthermore, with technological advancements, simulation methods have assumed greater prominence in related studies.

The emergence of “photovoltaic” as a keyword signifies the field’s shift from passive cooling strategies (such as cool roofs and green roofs) toward proactive integrated systems that combine “energy production with environmental regulation.” Meanwhile, the inclusion of ‘environment’ indicates that research perspectives have expanded beyond the singular metric of “cooling” to encompass comprehensive assessments of broader ecological and environmental benefits, including energy balance, carbon reduction, water resource management, and biodiversity. These high-frequency keywords reveal the current research focus on technical mitigation strategies—such as green roofs, cool roofs, and high-albedo materials—while also reflecting a gradual shift from examining the thermal performance of building envelopes themselves toward interdisciplinary studies. These encompass larger-scale urban planning policies, quantitative assessments of socioeconomic benefits, and the synergistic effects of multiple strategies.

3.4. RMS for Mitigating the UHI

The previous section provides a descriptive analysis of studies related to urban heat island mitigation by RMSs based on keywords, study locations, and relevant studies have confirmed its importance. The following section examines the specific principles, cooling effects, comparative strategies, and influencing factors of various roof measures to analyze the current research focus and trends in this field.

3.4.1. Cool Roofs

The most direct approach to altering the temperature environment of a building’s roof surface is to modify its thermophysical properties. The key to cool roofs lies in enhancing the albedo and thermal emittance of the roof surface [35,36]. By employing highly reflective roofing materials or coatings, the majority of solar radiation is reflected back into the atmosphere, significantly reducing the roof surface temperature [37]. Studies indicate that for every 0.1 increase in roof reflectivity, cool roofs can lower average urban ambient temperatures by 0.2 K [38]. Žuvela-Aloise M et al. found that increasing roof reflectivity from 0.45 to 0.7 can enhance the cooling effect from 0.25 °C to 0.5 °C [39]. The cooling effect exhibits significant spatiotemporal heterogeneity: cool roof coatings reduce temperatures by 13.5 °C at midday, with negligible effects during early morning and evening hours [40].

Past research has extensively employed field experiments and numerical simulations to investigate the cooling effects of cool materials [28]. However, due to high experimental costs, most full-coverage field studies have focused solely on the impact of cool roofs on individual buildings [29,36,40]. Compared to field experiments, numerical simulations are significantly less costly and enable quantitative research on cool coatings applied to diverse building surfaces across urban areas [41,42,43].

Table 2. Advantages and Disadvantages of Cool Roofs at Different Scale.

Scale	Position	Method	Albedo Value	Effect	Limit
Architectural scale	Chongqing, China [44]	Field Experiments and Computational Fluid Dynamics Simulations	High Reflectivity Coating (HRC), specific values not specified	The inner wall temperature is 6.3 °C lower than that of conventional roofs, with a cooling coverage area 65.1% larger than standard roofs. Energy-saving effects are more pronounced under intense solar radiation, and the optimized design offers significant summer energy-saving potential.	Simplified experiments may differ from real-world conditions, as they neglect the influence of the building’s surrounding environment on the thermal performance of the roof.
	Shanghai, China [40]	Field Experiment Coupled Moisture-Heat Transfer Model with THERB Software (v2.0)	Traditional Roof 0.2 Cool Roof 0.7	The surface temperature of the cool roof decreased by 3.3 °C during summer, achieving a 3.6% energy savings, but the thermal load increased by 10.4%.	Applicable only to the climate of Shanghai; suitability for other climate zones has not been verified.
	Melbourne, Australia [36]	Field Experiment	6 Types of Commercial Coatings	Six types of cool roof coatings exhibit varying cooling effects, with ThermaGuard HRC (Ensinger GmbH, Nufringen, Germany) demonstrating the highest cooling performance and Astec Energy Sta showing the lowest.	The experimental model is a single building and does not account for urban-scale effects.
	Nanjing, China [45]	Field Experiment	Cool Roof 0.85	Energy savings of 13.2% in summer, with energy consumption increasing by 2.8% in winter.	The experiment was conducted solely for short-term typical weather conditions and did not involve long-term performance monitoring.
	Lucknow, India [46]	Field Experiment Combined with EnergyPlus (v23) Simulation	Cool Roof: 0.7–0.94	Cool roofs can reduce heat gain by 33–71% and save 21–26% in energy consumption.	Insufficient year-round performance data under composite climate conditions, with inadequate consideration of urban environments.
	Seoul, Republic of Korea [29]	Field Experiment	Traditional Roof 0.2 Cool Roof > 0.7	Cool roofs reduce surface temperatures by 5.6 °C and indoor temperatures by 0.56 °C, but their effectiveness is weaker at night than during the day.	The experiment was conducted on a small scale (only 7 square meters) and has not been validated for large-scale application.
Urban scale	Ottawa-Montreal, Calgary, and Vancouver regions in Canada [42]	WRF model	0.1–1.0 (Cool Roof)	Vancouver exhibits the highest albedo cooling effectiveness (8.1–11.5 °C), followed by Calgary (4.5–7.6 °C), Montreal (5.2–5.5 °C), and Ottawa (4.0–4.7 °C).	Simulations and analyses based on historical extreme heat events provide relatively limited insights into evaluating urban thermal environments and the effectiveness of Nature-Based Solutions under future climate change scenarios.
	Seoul, Republic of Korea [47]	WRF model	Traditional Roof 0.2 Cool Roof 0.7	Lower daytime temperature by 1.0 °C at 2 m height, reduce wind speed by 0.5 m/s at 10 m height.	The model does not fully couple atmospheric chemical processes and has not been validated for long-term stability under extreme weather conditions.
	Kolkata, India [43]	WRF/SLUCM Model Simulation	Traditional Roof 0.15 Cool Roof 0.8	Cool roofs reduce net radiation by 251.9 W/m2, lower surface temperatures by 6.1 °C, and decrease the heat stress index by 1.8 °C.	Building height differences were not considered (uniformly set at 5–10 m).
	Dhaka, Bangladesh [48]	WRF model	Traditional Roof 0.2 Cool Roof 0.8	Cool roofs reduced afternoon temperatures by 0.57 °C, decreased urban heat island intensity by 0.38 °C, lowered 10 m wind speeds by 0.8 m/s, and increased CO concentrations by 52%.	Assuming the roof is uniformly covered with cool materials, disregarding actual variations in building density.
	Singapore [49]	WRF Model PUCM Model	Traditional Roof 0.2 Cool Roof 0.86	Cool roofs reduce daytime near-surface temperatures by 1.3 °C, cutting air conditioning energy consumption by 627 MW.	The LCZ classification does not fully reflect the characteristics of Singapore’s high-density buildings.
	Melbourne, Australia [50]	WRF Model PUCM Model	Traditional Roof 0.13 Cool Roof 0.7	Cool roofs significantly reduce daytime temperatures (maximum cooling effect of 0.5 °C).	Assuming the AC system operates 24 h a day, which deviates from actual energy consumption patterns Future impacts of urbanization and climate change are not factored in
	United States [41]	WRF Model	Cool Roof 0.7	Regional deployment has significantly cooled temperatures, but efficiency is negatively correlated with scale.	Lack of consideration for economic costs
	Vienna [39]	Climate Model MUKLIMO_3	Cool Roof 0.45–0.7	Reduce the average summer temperature in densely built environments by 0.25 °C to 0.5 °C.	The limitations of high-reflectivity roofing materials in practical applications within the Vienna region have not been considered.
	Greater Boston Area New England Region [51]	WRF Model	Traditional Roof 0.13 Cool Roof 0.88	It can reduce summer temperatures by 0.40 °C and heat-related mortality by 0.17–0.21%. However, it may increase cold-related mortality by 0.096% in winter.	Actual variations in indoor exposure were not considered. Interactions with urban form were not evaluated.

summarizes comparisons between perspectives at different scales for buildings and cities. It can be seen that the building-level approach is suitable for obtaining more realistic experimental data, but may suffer from model simplification errors and lack consideration of the overall urban environment. From an urban perspective, simulations can be conducted over larger areas based on city-level climate data, but results may be biased due to potentially overly idealized data.

Through large-scale deployment of cool roofs, cooling effects across different climate zones can be further analyzed at the urban scale, thereby exploring relationships with climate distribution. Multiple studies have utilized numerical simulation models (such as the WRF model) to evaluate cool roofs in various climate regions, including tropical climates like Singapore [49], Bangladesh’s Dhaka [48], and India’s Kolkata [43]; Melbourne in subtropical climates [50]; and Beijing, China [52], and Seoul, Republic of Korea [47] in temperate climates. These studies indicate that enhancing roof reflectivity generally reduces urban canopy air temperatures. However, cooling effects vary across climate zones, with data indicating cooling performance follows the order: temperate > tropical > dry-hot > composite climates [53]. Zhao S et al. found latitude also influences cool roof effectiveness: as latitude decreases and solar radiation increases, cool roofs progressively enhance heat island mitigation [54].

In addition to directly lowering urban air temperatures, cool roofs can indirectly mitigate the urban heat island effect by reducing heat transfer into buildings through lower roof temperatures. This decreases energy consumption of air conditioning systems and reduces waste heat from outdoor units, with particularly pronounced effects during summer heat island intensification [44,46]. However, the high reflectivity of cool roofs may have adverse effects in winter. Increased solar radiation reflection during colder months heightens indoor heating demands [45], and the temperature reduction induced by cool roofs may even contribute to a 0.096% increase in cold-related mortality rates [51]. CR do increase heating energy consumption by reducing solar radiation absorption; however, studies by Mirata Hosseini et al. indicate that this penalty has actually been overestimated. In cold regions, roof surfaces are often covered with snow in winter (with an albedo of up to 0.6–0.9), which weakens the high reflectivity advantage of cool roofs. Consequently, the actual penalty is significantly lower than the theoretical value [55].

The adoption of cool roofs (CR) in different climate zones leads to varying degrees of heating penalty. Salem Algarni found that in tropical climates, the use of cool roofs can reduce the annual energy consumption required for building cooling by approximately 52.5 kWh/m²; in contrast, the maximum annual increase in energy consumption caused by winter heating is about 3.1 kWh/m², and its heating penalty is almost negligible (<1%) [56]. Keivan Bamdad [57] also found in his research that cool roofs (CR) can reduce the annual energy load by up to 14% and 22% in tropical and subtropical climates, respectively. However, in relatively cool and mild temperate climates, CRs increase the total energy demand of buildings [57]. To address this heating penalty, numerous researchers are exploring temperature-adaptive cool roof materials that enhance reflectivity in summer while reducing it in winter.

3.4.2. Green Roofs

Urban green spaces have been proven to significantly mitigate the urban heat island (UHI) effect. Enhancing urban greenery is currently a key strategy for cooling cities, making green roofs (GR) one of the primary solutions for alleviating UHI [58,59,60]. Green roofs effectively mitigate the urban heat island effect and improve urban environmental quality through evaporative cooling [61,62], thermal insulation [63], and increased urban green coverage [64].

Research indicates that green roofs can reduce rooftop temperatures and contribute to overall urban cooling, with their effectiveness well-documented [30,65]. Furthermore, decreases in urban air temperatures and near-surface temperatures show a clear positive correlation with increased green roof coverage. Sharma A found that in the Chicago metropolitan area, increasing green roof coverage from 25% to 100% could enhance rooftop surface temperature reductions by over 2 °C [31]. Imran H M observed that in Melbourne, increasing green roof coverage from 30% to 90% enhanced the heat island mitigation effect on rooftop surfaces from 1.15 °C to 3.8 °C [66].

The cooling effect of green roofs exhibits significant sensitivity to varying climatic conditions. At the small-scale level, Adilkhanova I found that under 90% green roof coverage, the cooling effect on clear days was markedly higher than on rainy, high-humidity days [67]. Dong X et al. further analyzed overcast days, post-rain clear days, cloudy days, and clear days, revealing that GR’s thermal mitigation potential under sunny conditions is markedly stronger than on overcast days [68]. From a broader regional perspective, UHI mitigation effects may vary across different regional climates. Jamei E noted that cities in arid-hot climates achieve greater temperature reductions from green roofs compared to humid-hot climates, while temperate zones exhibit the lowest reductions [69]. Similarly, Gao M concluded that UHI mitigation strategies yield better outcomes in Xi’an’s semi-humid climate than in Wuhan’s humid climate [70].

However, under identical climatic conditions, the cooling effect of green roofs is influenced by building layout and urban spatial characteristics [71,72]. Luo T et al. [73] classified 36 industrial districts in Shanghai into microclimate zones and found that green roofs significantly outperformed low-rise, loosely built areas in cooling compact mid-rise plots. They noted that cooling effectiveness negatively correlated with sky view factor and average building area, while positively correlating with average building shape index [73]. Zuo J et al. categorized urban spatial forms into nine types and found that the cooling effect of rooftop greening exhibits significant variations across building densities, with compact building clusters demonstrating optimal cooling performance [74]. Furthermore, the cooling effect of rooftop greening on the pedestrian layer diminishes significantly as the vertical distance between the roof and ground increases, reaching a threshold at approximately 50 m in height [75].

While implementing green roof technology, it is also essential to comprehensively consider the subsequent maintenance costs of green roofs, with specific expenses varying depending on the choice of plants [76]. Watering is essential during the early stages of plant growth or during periods of drought [76], the most critical standard in subsequent GR maintenance is drainage [77], and next is waterproofing [78]. Inadequate follow-up maintenance may lead to leakage risks. Drainage layers can become clogged by debris accumulation (such as fallen leaves and soil), preventing effective rainwater runoff. This causes water to pool on the roof surface, ultimately triggering leaks. It is worth noting that due to its high structural load capacity, GR may not be suitable for green roof retrofits in older buildings. A precise suitability assessment must be conducted prior to any retrofitting [79]. If the anticipated benefits are stormwater management and rainwater harvesting and retention, the preset GR type must be fully considered during the design phase [80]. Although the initial construction costs of green roofs are typically higher than those of conventional roofing systems, the energy savings and environmental benefits achievable during operation and maintenance can offset or even exceed the initial investment to some extent, thereby achieving cost-effectiveness over the entire lifecycle [81].

3.4.3. Photovoltaic Roofs

In recent years, the global push for renewable energy has highlighted the importance of photovoltaic (PV) roofs. While generating clean energy, the impact of PV roofs on urban thermal environments has been extensively discussed in recent studies [82,83]. However, current research presents two opposing viewpoints: mitigation and exacerbation of the urban heat island effect.

On one hand, scholars who support the view that PVR can mitigate the urban heat island effect argue that photovoltaics not only reduce fossil fuel consumption by generating clean energy but also achieve cooling through photothermal conversion principles, energy substitution effects, and shading. Zhong, Y. et al. utilized the WRF model to find that PVR can reduce surface sensible heat flux by approximately 3.02 W/m² in autumn [84]. Similarly, Tan H et al. simulated with WRF that PVR installations could lower near-surface temperatures by 0.6 °C during summer [32]. Salamanca F et al. simulated the extreme heat event of July 2009 using WRF and found that rooftop photovoltaic panels (η = 14%) could reduce Phoenix’s peak temperatures by up to 0.7 °C while decreasing daily cooling energy demand by 8–11% [85]. Using numerical simulation methods, these studies have identified varying degrees of cooling effects from PVRs under different climatic conditions. The cooling impact of PVRs with different urban forms on the urban surface varies: open spatial configurations exhibit the most pronounced warming effect in spring and autumn, while mid-rise open-space blocks in summer and large low-rise buildings in winter demonstrate the strongest cooling effects [86]. Additionally, the installation configuration of photovoltaic arrays also influences rooftop cooling performance. Chiteka, K. et al. found that optimizing orientation and tilt angles can significantly reduce photovoltaic array heating while maximizing energy production [87]. Liu S et al. observed that installation height affects cooling performance, with elevated mounting structures demonstrating markedly superior cooling effects compared to low-mounted structures due to shadowing effects [88].

On the other hand, scholars who argue that PV roofs exacerbate the urban heat island effect contend that their impact is not entirely positive. To efficiently convert solar radiation into electricity, PV surfaces typically feature low albedo. Due to their low reflectivity and heat diffusion properties, they may intensify the UHI effect [89]. Extensive research indicates that large-scale deployment of PV roofs could amplify the urban heat island phenomenon [33,90]. Houchmand, L.J. et al. simulated convective heat flux for different roof types and found that PV installations lead to varying degrees of UHI enhancement across all roof types [91]. Houchmand, L.J. et al. found that on sunny days, temperatures above photovoltaic roofs exceeded those above bare roofs by more than 4 °C, but at night they produced a slight cooling effect (2.72 °C per hour and 0.46 °C per month) [92]. Fassbender et al. found through metrological analysis that photovoltaic systems increase daytime ambient air temperatures by +1.35 K while lowering nighttime temperatures by −1.19 K, with the daily daytime heating effect exceeding the nighttime cooling effect [93]. Although PV installations may exacerbate urban heat islands, research by Ghenai, C. et al. indicates that integrating cool roofs with PV systems can significantly enhance roof reflectivity (from 0.2 to 0.8) while increasing PV power generation by 15% [94].

3.4.4. Composite Roofs

To maximize the potential of rooftop spaces, people have begun integrating PV, CR, and GR systems to achieve synergistic benefits. Examples include photovoltaic-green roofs (PV-GR) and photovoltaic-cool roofs (PV-CR). However, the integration of cool roofs and green roofs is rarely studied due to conflicting approaches to solar radiation (green roofs absorb solar radiation while cool roofs reflect it) and potential spatial conflicts in layout.

PV-GR involves installing distributed photovoltaic panels above vegetation, enabling them to leverage both the cooling effect of plant transpiration and the energy benefits of solar power. The photovoltaic panels, cooled by the vegetation, experience a significant increase in conversion efficiency [95]. Simultaneously, the synergistic effect of both components substantially enhances the rooftop’s ability to mitigate the urban heat island effect. Chen B et al. [96] compared cooling effects under GR, PVR, and PV-GR conditions, finding all three mitigated urban heat islands to varying degrees. However, PVR and PV-GR demonstrated superior performance, with PV-GR achieving the most effective cooling compared to the other two standalone approaches [96]. In a subsequent study, Chen B et al. further concluded that PV-GR (0.23 °C) significantly outperformed GR (0.13 °C) in cooling efficacy [97]. Green roofs enhance photovoltaic panel performance by lowering their temperature. Kaewpraek et al. conducted a year-long field study on the roof of a building named Feuangfa in Songkhla Province, Thailand, finding that planting vegetation beneath photovoltaic panels increased power generation efficiency by approximately 2% [98]. Similarly, Arenandan, V. et al. compared PV system efficiency with and without greenery, finding that green roofs could increase solar power generation efficiency by an average of 1.6% [99]. PV-GR performance also depends on the height relationship between the two [100]. Zluwa, I. et al. found that optimal cooling effects occur within 75–100 cm above the GR surface [101]. Similarly, Osma-Pinto, G. et al. [102] noted that when PV panels are installed at heights below 25 cm, vegetation reduces PV efficiency despite aiding cooling. Conversely, heights above 50 cm increased PV efficiency by 0.4%, indicating an optimal PV-GR height range of 30–70 cm [102].

The emergence of PV-CR stems from new technologies in bifacial photovoltaic panels [103]. Due to CR’s high reflectivity, bifacial panels increase the radiation received on the rear side, significantly enhancing the efficiency of photovoltaic systems. Ghenai, C. et al. analyzed photovoltaic performance under varying reflectivity levels, finding that increasing albedo from 0.2 to 0.5 and from 0.2 to 0.8 boosted annual power generation of bifacial PV by 7.75% and 14.96%, respectively [94]. Rahmani F. utilized cool roofs to reduce solar cell thermal voltage and roof heat flux, thereby increasing solar power generation efficiency by 10.4% [104]. In terms of energy efficiency, CR has a positive effect on photovoltaics. However, studies indicate that the integration of photovoltaics and CR does not yield the same benefits for urban cooling. The barrier effect of photovoltaic panels between the roof and the atmosphere, the additional convective heat flux from the panels, the retention of reflected flux beneath the panels, and the heat emitted by the panels all contribute to higher surface temperatures on CR-PV roofs compared to CR alone [91].

4. Comparison of Different Roof Measures

Urban rooftop spaces hold immense development potential, yet remain finite in nature. Therefore, choices must be made between PVR, CR, and GR. All options require comprehensive consideration of costs, effectiveness, benefits, and sustainability to maximize returns at minimal expense.

4.1. Cooling Effect

Past research has thoroughly demonstrated that the cooling effect of an RMS exhibits a significant correlation with urban structural characteristics [86], while also being influenced by geographical environment and local climate [69,70]. Jia S et al. conducted a comparative analysis of 43 megacities worldwide and found that, in terms of global cooling effects, gray roofs (GR) outperformed cool roofs (CR) (0.1 °C for GR vs. 0.02 °C for CR). However, New Delhi achieved the highest cooling effect with GR at 0.80 °C, while Beijing demonstrated the best cooling effect with CR at 0.23 °C [105]. Therefore, for different RMSs, analysis and discussion should be conducted for identical or similar regions and local climates to achieve more objective comparisons. In the Mediterranean climate of Barcelona, Spain, Houchmand L J et al. conducted field measurements on three types of RMS and found that CR had the lowest surface temperature, PV-CR was next, and PV had the highest surface temperature. The surface cooling effectiveness ranked as follows: CR > PV-CR > PV-GR > GR > PVR [91]. Not only that, Lu H et al. utilized the WRF-BEP+BEM model to compare CR and GR effects by altering only roof albedo and vegetation coverage. They found that cool roofs were more effective in arid, high-radiation areas (Calgary), while vegetation proved superior in humid, highly urbanized regions (Vancouver). Combined measures enabled synergistic cooling throughout the day. The study also indicated that CR consistently achieved greater cooling effects than GR, even across different local climate zones [42]. Tan H et al. conducted WRF simulations for the Chicago area, analyzing that CR could reduce near-surface temperatures by 1.5 °C, followed by GR at 1.2 °C, and finally PVR at 0.6 °C [32]. By combining analyses of different literature sources for the same region, as shown in

Table 3. Comparison of Different Roof Mitigation Strategies.

Climate Zone	Region	Roof Strategy	Effect		Conclusion
			Roof Surface Temperature	Near-Surface Air Temperature
BWh	Phoenix [86]	CR + PVR	——	Daytime CR: 0.4–0.8 °C PVR: 0.2–0.4 °C Nighttime PVR: 0.4–0.8 °C CR: 0.1–0.4 °C	Daytime CR > PVR Nighttime PVR > CR
BSk	Calgary [42]	CR + GR	——	CR: 4.5–7.6 °C GR: 1.3–2.9 °C	CR > GR
Cfa	Guangzhou [97,106]	CR + GR + PVR + PV-GR	——	PV + PV-GR:0.3–0.7 K GR: 0.1 K; CR: 0.6 K	PV-GR > PVR > CR > GR
				GR: 0.12–0.18 K CR: 0.16 K
Cfb	Melbourne [66]	CR + GR	GR: 3.8 °C; CR: 5.2 °C	GR: 1.15 °C; CR: 1.5 °C	CR > GR
	Vancouver [42]	CR + GR	——	CR: 8.1–11.5 °C GR: 4.7–5.5 °C	CR > GR
Csa	Barcelona [91]	CR + PV-CR + PV-GR + GR + PVR	Taking advantage of the negative sensible heat flux on the surface of a cool roof, heat is absorbed from the near-surface air, thereby indirectly achieving near-surface cooling.		CR > PV-CR > PV-GR > GR > PVR
	Beijing [75,107,108]	CR + GR	GR: 3.19–3.62 °C	GR: 0.05–0.41 °C CR: 0.8–1.5 °C	CR > GR
Cwb	Seoul [47,109,110]	CR + PVR	——	CR: 0.8–1 °C GR: −0.481–1.057 °C	CR > GR
Dfa	Chicago [32]	CR + GR + PVR + PV-GR	——	CR:1.5 °C, GR:1.2 °C, PVR:0.6 °C	CR > GR > PVR
Dfb	Ottawa [42]	CR + GR	——	CR: 4.0–4.7 °C GR: 3.3–5.3 °C	CR > GR
	Montreal [42]	CR + GR	——	CR: 5.2–5.5 °C GR: 4.0–5.9 °C	CR > GR

, the specific comparative effects of various RMSs become clearly evident. Generally speaking, climatic zone differences influence the effectiveness of RMSs but do not completely reverse the relative merits of measures. Across most regions, CR demonstrates the highest cooling capacity, followed by GR, while PVR yields the poorest results.

4.2. Spatial Applicability

Urban environments are complex and dynamic, making the selection of appropriate rooftop strategies crucial for enhancing effectiveness. CR typically improves roof reflectivity by replacing coatings or components, such as applying different types of cool roof coatings [36] or tiles [111] on flat roofs, and using high-reflectivity new tiles [112] on traditional pitched roofs. These solutions impose low structural load requirements and are suitable for various roof types. Green roof systems feature complex construction, with loads ranging from 15 kg/m² to 350 kg/m² depending on substrate depth [113]. Additionally, appropriate vegetation should be selected based on climate zones to enhance ecological sustainability [114,115]. Photovoltaic panels are lighter than vegetation structures and have lower climate requirements. Their flexible layout allows for different installation methods tailored to various roof types. However, to achieve higher photovoltaic production efficiency, the optimal PV orientation for different regions must be considered [116].

People also exhibit varying degrees of visual acceptance toward different strategies. Highly reflective coatings or components may cause visual conflicts, necessitating the development of non-intrusive cool coatings and components that do not compromise the visual appearance of buildings and other urban surfaces [112]. Photovoltaics primarily take the form of distributed PV and building-integrated photovoltaics (BIPV). Distributed PV, covering building surfaces, is widely perceived as disrupting the original architectural image [117], whereas BIPV integrates PV as part of the building envelope, making it more visually acceptable [118]. Green roofs introduce natural vegetation to urban vertical surfaces, significantly increasing urban greening rates. This approach, balancing aesthetics and benefits, has led to extremely high consumer acceptance of green roofs [119].

4.3. Economic and Ecological Value

The preceding analysis has compared the effectiveness of three rooftop strategies in mitigating the urban heat island effect. As the three primary technical approaches, they demonstrate distinct value beyond core cooling functions, particularly in terms of economic and ecological benefits.

Economic costs are a key factor limiting the adoption of RMS systems. Research indicates that among the three types of RMS, PVR achieves the highest cost savings. Lee E et al. compared the installation and energy consumption costs of conventional roofs versus green roofs, finding that green roofs save 17.4 kW/m² annually in heating energy and 21.2 kW/m² in cooling energy, reducing costs by $94/m² and $114/m² respectively [120]. Nie, X. et al. similarly found that GR could save 6.64 kW/m² of cooling energy annually [121]. PVR, leveraging its own power generation capacity, demonstrates significant economic advantages over both GR and PVR. Li H et al. simulated that PVR’s annual power generation capacity equates to energy savings exceeding 135 kW/m² [122]. In terms of cost, PVR > GR > CR [123]. Due to its low investment cost (4.47–5.87 USD) and maintenance expenses, CR can recoup its investment within two years, delivering economic benefits in the short term [124]. Green roofs exhibit a longer payback period ranging from 13 to 18 years [79,125], whereas photovoltaic rooftops typically achieve payback within 6 to 12 years [126,127,128]. However, studies indicate that despite higher costs and extended payback periods, green roofs can significantly extend the service life of rooftops [129].

Beyond economic costs, sustainable development remains a key focus in urban construction, with ecological service values and carbon emission reductions being critical considerations. While all three approaches achieve some reduction in the urban heat island (UHI) effect, green roofs (GR) and plant-based vertical roofs (PVR) demonstrate superior performance in carbon mitigation [130,131,132,133,134]. Zheng Y et al. found that annual CO₂ reductions from installing green roofs on different building types in Shanghai ranged from 47.68 to 64.34 kt [135]. Rayegan S et al. [136] assessed the potential for urban photovoltaic installations using the Urban Building Energy Model (UBEM). They found that besides generating substantial electricity annually and achieving 40% energy savings, local power generation could reduce CO₂ equivalent emissions by over 0.2 megatons [136]. Additionally, maintaining biodiversity is a key feature of green roofs [137].

4.4. Influencing Factors

The influencing factors for various RMSs are summarized in

Table 4. Factors influencing rooftop mitigation strategies.

Measures	Cool Roof	Green Roof	Photovoltaic Roof	Hybrid Roof
Albedo	Positive correlation	Positive correlation	——	Positive correlation
Coverage	Positive correlation	Positive correlation	Positive correlation	Positive correlation
Plant Selection	——	Crassulaceae plants most effective	——	Crassulaceae plants most effective
Orientation	——	——	Positively correlated with optimized azimuth angle	Positively correlated with optimized azimuth angle
Equipment height	——	——	Negative correlation	Positively correlated with height difference
Climate	Temperate > Tropical > Hot-dry > Composite climate zones Effectiveness summer > winter	Positively correlated with temperature and humidity	Effectiveness night > day	——
Latitude	Negative correlation	Negative correlation	——	——
Building height	Negative correlation	Negative correlation	Negative correlation	Negative correlation
Building density	Negative correlation	Positive correlation	Negative correlation	——
Building shape index	——	Positive correlation	——	——
Load	Low	High	Medium	Medium–High
Cost	Low	High	Medium	Medium–High
Visual acceptability	Medium	High	Low	Medium–Low

. Regarding the intrinsic attributes of each strategy, these factors include reflectance, coverage, plant selection, orientation, and equipment height. Among these, increased reflectance positively impacts all strategies except photovoltaic roofs, while photovoltaic components exhibit lower correlations due to their low reflectance and high absorption rates. All RMSs exhibit a positive correlation with coverage. Among plant selections, Crassulaceae species demonstrate the most effective cooling performance. Regarding orientation and height, photovoltaic implementation requires consideration of optimal azimuth angles and photovoltaic height clearances to achieve both peak photovoltaic efficiency and maximum cooling effect. In terms of climate, RMS high-temperature cooling often demonstrates superior performance in hot climate zones and low-latitude regions. Regarding urban form, building height exhibits a negative correlation with various RMS cooling effects, while building density shows differing responses depending on the specific RMS. Beyond the aforementioned factors influencing RMS cooling effectiveness, comprehensive consideration must also be given to load capacity, cost, and visual acceptability. An appropriate rooftop strategy should be adopted by holistically evaluating the building’s physical properties, climatic characteristics, urban form, and spatial applicability.

5. Discussion

The applicability and effectiveness of RMSs are highly dependent on urban form parameters (such as building density, height, layout type, and surface coverage). The efficacy of rooftop mitigation strategies increases as building height decreases and floor area ratio increases [138]. Urban form significantly influences the selection of rooftop mitigation strategies, with the core mechanism being that different spatial structure parameters determine the implementation effectiveness and applicability of specific rooftop measures by altering local thermal environments and energy flow patterns [139]. In high-density compact urban areas (e.g., central business districts), low surface permeability and reduced sky view factor increase longwave radiation retention, intensifying the urban heat island (UHI) effect [140]. Here, CR [141] emerges as the preferred choice due to its high reflectivity. Simultaneously, as the height-to-width ratio (H/W) of the street canyon increases, wind speed significantly attenuates in narrow, tall streets. This necessitates the adoption of passive evapotranspiration strategies (such as living green walls) to enhance humidity and cooling, thereby granting GR a distinct advantage [142]. Conversely, low-density sprawl areas with reduced natural surface cover require a combined strategy of rainwater harvesting green roofs and solar panels to alleviate thermal stress while supplementing energy [143]. Multi-objective algorithms grounded in urban form data can quantify the synergistic benefits of rooftop interventions. Parametric modeling validates that in areas with high building height variability, optimizing solar panel tilt angles increases power generation efficiency by over 15% [144], while vertical greening systems reduce building cooling loads by 10–20% through shading [145]. RMSs must be rooted in spatial heterogeneity analysis of form data and implemented through city-level integrated simulation platforms for dynamic optimization design [146].

Existing literature exhibits significant geographical concentration, primarily focusing on China, the United States, and certain European countries. This implies that the current knowledge base on RMSs is largely grounded in research conducted within specific climatic zones (temperate, continental, and Mediterranean climates), inevitably limiting the universality of research conclusions. While the core technology possesses physical universality, its optimal implementation and net benefits are strongly dependent on local conditions. In regions with similar climatic conditions, a universal RMS potential assessment model should be established to advance climate-adaptive design.

Photovoltaic rooftops are crucial for urban sustainable energy transitions, yet their role in mitigating urban heat islands remains controversial. Current research on the extent of photovoltaic impacts on urban heat islands primarily focuses on the balance between environmental warming caused by high-reflectivity panels and the energy benefits generated. Nevertheless, studies analyzing the advantages and disadvantages of implementing PVR under different climatic conditions remain scarce. Future research should therefore focus on the energy balance between PVR systems and their surroundings to establish a systematic framework for evaluating the net effect of photovoltaic roofs (PVR) on urban heat islands (UHI). This involves identifying key determinants of PVR (panel albedo, installation height, solar shading, climate conditions), quantifying them through climate and physical data fusion, dynamically determining optimal threshold ranges for different climate zones, and analyzing synergistic trade-offs to determine under which conditions PVRs may mitigate or exacerbate UHIs.

6. Conclusions

Global warming further exacerbates the persistent effects of the urban heat island (UHI), intensifying the heat stress endured by countless urban residents, particularly those vulnerable during periods of extreme heat. Roofs serve as a crucial means to mitigate urban heat and enhance human comfort. This paper reviews findings from 412 studies examining the role of reflective roofing systems (RMS) in mitigating urban heat islands across diverse geographical, climatic, and national contexts through screening and filtering methodologies. To summarize the core findings of this review, the following conclusions are drawn:

1.

In terms of the cooling effects of various strategies:

Research indicates that RMSs employ cool roofs, green roofs, photovoltaic roofs, and composite roofs as mainstream strategies for combating urban heat islands. Extensive studies demonstrate that these approaches can mitigate the heat island effect to varying degrees, with cool roofs and green roofs proving more effective than photovoltaic roofs in reducing urban heat islands. Furthermore, integrating photovoltaic systems with the former two can further enhance their cooling effects.

The effectiveness of cool roofs in mitigating the urban heat island effect is positively correlated with the reflectivity of cool coatings or cool components, and negatively correlated with geographical latitude. Concurrently, a certain degree of spatiotemporal heterogeneity exists: the effect is pronounced at midday but less significant in the early morning and evening, and it is effective in summer while potentially having negative effects in winter.

The cooling effect of green roofs is influenced by climatic factors. Compared to humid and hot climates, cities with dry and hot climates can better leverage the thermal mitigation benefits of green roofs.

Photovoltaic rooftops offer exceptional energy efficiency, yet their impact on urban heat islands remains a subject of debate. Some argue that they mitigate urban heat islands to a certain extent while reducing energy consumption, while others contend that their low reflectivity causes rooftops to absorb more heat, thereby exacerbating the urban heat island effect.

2.

In terms of comparing various strategies:

The three types of roofs exhibit varying effectiveness in mitigating the urban heat island effect. Practical implementation requires comprehensive consideration of additional factors, including roof load-bearing capacity (green roofs > photovoltaic roofs > cool roofs), orientation, and the visual acceptability of each measure to the public. Integrating photovoltaic systems with vegetation or cool-paint coatings can further enhance their cooling effect, delivering combined benefits of energy savings and temperature reduction.

All three types of rooftop systems offer distinct advantages: Cool roofs demonstrate superior applicability and deliver the highest economic benefits; green roofs are more visually appealing to people and contribute to maintaining ecological diversity; and photovoltaic roofs can integrate seamlessly with building structures, enhancing architectural aesthetics while generating greater economic returns from solar energy production.

RMSs serve as a key strategy for addressing the urban heat island effect, with extensive existing research confirming its significance. Future studies should integrate climate adaptation, technological innovation, and policy coordination to achieve innovation. Current research heavily relies on regional climate characteristics. Future studies should further develop climate-strategy matching models, employing distinct evaluation frameworks and mitigation strategies for different climate zones—such as optimizing green roofs in humid tropical regions while enhancing cool roofs in arid areas. Additionally, resilience designs addressing extreme weather events caused by climate warming, including heatwaves and torrential rains, should be strengthened. Current single strategies struggle to address the complexity of the urban heat island (UHI) phenomenon. Further optimization of material components—such as adaptive reflective materials and combinations of green vegetation types—is needed to enhance research on composite rooftop strategies. Concurrently, integrating rooftop strategies with urban solutions (ventilation corridors, vertical forests) can amplify environmental benefits. Developing multi-scale quantitative models will enable the quantification of interactions from perspectives including urban morphology and local microclimates. Future development should also advance toward intelligent solutions, such as integrating machine learning and digital twin technologies for optimization, to enhance comprehensive socio-eco-economic benefit assessments and sustainability analyses.