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The technology of planted or vegetated roofs, also known as green roofs (GRs), is one of the most well-documented nature-based solutions for reducing building energy consumption, reducing UHI, and enhancing urban sustainability.
- green roofs
- energy benefits of green roofs
- green roof models
- thermal transmittance
- U-value
1. Introduction
Excessive urbanization and human activities in the urban environment are responsible for numerous climate change and environmental degradation-related consequences. These include global warming, the urban heat island (UHI) effect, acid rain, ozone depletion, polluted air and water, natural resource depletion, and biodiversity loss [1,2]. The building sector, in particular, is responsible for nearly 40% of the primary energy consumption, which has a significant impact on energy and the environment; therefore, a reduction in energy consumption is viewed as essential for achieving urban sustainability and climate neutrality [3,4]. As mitigation of the UHI is a key challenge in enhancing urban sustainability, many solutions inspired and supported by nature have been developed to reduce energy consumption while concurrently improving living conditions in the built environment [5].
The technology of planted or vegetated roofs, also known as green roofs (GRs), is one of the most well-documented nature-based solutions for reducing building energy consumption, reducing UHI, and enhancing urban sustainability [6,7]. GR technologies include roof gardens, vegetation roofs, ecological roofs, agricultural GRs, etc. [8]. GRs are engineered systems that include layers such as a roof barrier placed above a water-proofing layer, a drainage layer, a filler layer, the soil substrate, and vegetation [1,9].
GRs can be categorized into three groups regarding the substrate depth: (a) extensive [10], with shallow soil substrate between 15 and 20 cm, short plants, and insignificant needs of maintenance and irrigation; (b) semi-intensive [6], with medium needs of maintenance and irrigation; and (c) intensive [11], with increased soil depth up to one meter, requiring significant maintenance and irrigation.
Green roofing, also known as eco-friendly or vegetated roofing, entails installing a living, plant-based covering on the roof of a building.
2. Urban Sustainability and Green Roofs
The UHI is a global atmospheric phenomenon that is the most representative manifestation of climate change, consisting of the difference in air temperature between metropolitan cities, semi-urban or semi-rural areas, and rural areas [12]. UHI is caused by the low amount of evapotranspiration because of low vegetation, the absorption of solar radiation because of low albedo, air flow difficulties because of higher rugosity (a measure of rough and ridged surface morphology), and the large amount of anthropogenic heat release [13]. Every urban area exhibits UHI, which reflects the microclimate of densely populated areas [12,14,15,16,17].
UHI manifests itself primarily (but not exclusively) at night, particularly when the atmosphere is calm and clear. Depending on how the air is heated (solar radiation absorption, heat transfer from hot surfaces, and anthropogenic heat), two types of UHI are produced: surface and atmospheric [12]. The surface UHI is detectable in the morning, throughout the day, and at night. During the summer, when solar radiation descends vertically on Earth, its intensity ranges from 10 to 15 °C during the day and 5 to 10 °C at night. Typically, it is recorded with air sensors [12]. The atmospheric UHI is lower during the day and peaks at night, particularly before dawn. Its average intensity ranges from 1 to 3 °C during the day and 7 to 12 °C at night. It is measured directly by fixed or mobile meteorological stations and is frequently subdivided into the canopy layer and boundary layer UHI [18].
Green roofs (GRs) are structural elements of the roof of a building that are either partially or wholly covered with (green) vegetation. Although the term is occasionally used to refer to roofs that contain environmentally friendly (“green”) technologies, such as photovoltaics or wind turbines [19], the focus of this study is exclusively on planted roofs. These are areas where vegetation has been planted through technological intervention in an effort to improve living conditions and the urban environment. GRs are presented as a natural alternative to mitigate the negative effects of greenhouse gasses and traffic pollution in a sustainable manner, offering environmental, aesthetic, sociological, and economic benefits, particularly in urban areas [20].
There are three types of GRs [1,21]: extensive [10,22], intensive [23], and semi-intensive [24]. Extensive GRs are most common, with a soil depth of less than 20 cm, which makes them less expensive and simpler to install. The vegetation comprised grass, herbs, mosses, and short grass. Extensive GRs do not require additional maintenance because soil water is retained, making the plants resistant to high temperatures and droughts [10,22,25]. In intensive GRs, the average soil depth is up to one meter. These GRs are considered to be more specialized, as they can accommodate a wider plant variety and achieve a more aesthetically pleasing and realistic outcome. Their structure is, however, more demanding and intricate, requiring additional care and retention-promoting measures. Consequently, they are more expensive and require more construction and maintenance [23]. Finally, semi-intensive GRs represent a compromise between the two alternatives. Typically, small plants, low vegetation, and grass are used in such situations, while installation, maintenance, and watering needs are minimal [24].
GRs represent a contemporary variant of the conventional roof garden, as they consist of soil, vegetation, and plant species. GRs can be installed on any building material, including concrete and wood, similar to roof gardens. GRs are engineered to improve the microclimate, reduce the energy load of buildings, and achieve superior aesthetic results [26,27]. Plant selection is a fundamental aspect of GRs. Due to structural restrictions on the total weight of a building’s roof, there are constraints on the size and weight of the plants. Additionally, the choice of vegetation affects air and runoff quality as well as energy conservation. Root barriers built in the GR serve as an insulating layer and prevent any structural damage caused by plant roots. Excess water is diverted to rainwater drainage, while a filtering layer prevents clogging by ensuring that the growing medium has access to the drainage layer [28].
The layers typically found in GRs include [1,9,20] vegetation, i.e., plants that improve the air and runoff quality, act as a moisture barrier [29], and contribute to energy conservation [26,30]; soil, which serves as the growing substrate [31]; filtering, which separates the soil layer from drainage material [32]; drainage material, which enhances the thermal properties of GRs and maintains a balance between air and water [30]; root barrier, which prevents damage to the structure [31]; and the waterproofing layer, which is extremely important for protecting the building structure [29]. Important parameters that must be considered in GRs include [33] climatic conditions, such as meteorological parameters, geographic conditions of the wider urban area, and the in-house temperatures during the different seasons; the static strength of the building as well as the building type, e.g., residence, offices, laboratories; and the GR type, taking into account thickness and irrigation, as well as the overall design of the GR, plant selection, construction materials, etc.
GRs are living ecosystems, and plant selection is important because it can enhance the functionality, appearance, and overall environment of buildings [34,35,36]. The type of plant can vary depending on the GR type (extensive, semi-intensive, or intensive). Plants may be native or non-native, may exhibit a variety of functional characteristics (annual or perennial, succulent or not, shrubby or herbaceous), and may possess particular structural characteristics, such as root growth, which may affect the stability of the roots. The literature indicates that intensive GRs may host a broader plant selection but require frequent maintenance, whereas extensive GRs typically rely on a limited range of plant species. For instance, it is recommended to avoid woody plants (like Phanerophytes), because their well-developed roots could damage the roof’s insulation layers over time. It is also recommended to avoid annual plants (like Therophytes) and biennial Hemicryptophytes due to their shorter lifespans and inability to provide continuous green cover. The use of native plants has received significant attention lately, due to their superior adaptation to local environmental conditions, greater ecological benefits, and greater aesthetic appeal compared to non-native species [37,38].
The scientific literature does not offer specific data on the variation in surface temperature of different types of GRs throughout the day. Regarding indoor and outdoor air temperatures at the level of a GR, some experimental and simulation studies have been conducted. An experimental study with a roof lawn garden conducted in Osaka, Japan, showed a significant indoor air temperature reduction underneath the planted roof of up to 30 °C during the summer months [39]. Two test cells, one with a simple concrete roof and the other with a GR, were compared experimentally [40]. The concrete roof’s outdoor air temperature ranged from 14 to 38 °C, while the indoor air temperature varied from 16 to 38 °C. For the GR, the outdoor air temperature ranged from 22 to 27 °C, while the indoor air temperature fluctuated between 23 and 28 °C. In an experimental study conducted in Shangai, China, measurements were taken in two experimental rooms, one covered by a conventional roof and one by a GR system [10]. The results demonstrated that the GR contributed to a 32.5 °C decrease in the outer surface temperature amplitude variation, while the roof temperature difference between the green and conventional roofs increased by up to 5 °C. Simulations in different climatic conditions showed that the air temperature at the roof level decreased by an average of 12.8 °C and up to a maximum of 26 °C [41].
Green roofs and cool materials can help mitigate the UHI and improve the urban climate alongside large-scale carbon sinks, such as urban parks or forests near cities, and negative emission technologies, such as carbon capture and storage [39,40,41,42]. GRs in particular have been associated with many benefits, and scholars have produced substantial quantitative findings, establishing GRs as a highly favored nature-based approach for improving urban climate, mitigating the urban heat island effect, and ameliorating living conditions in urban settings [38].
Several factors influence the thermal (heating/cooling) and energy performance of GRs. Of these, the GR type (extensive, intensive, and semi-intensive); structural design elements, such as thermal insulation [42]; growing medium (soil) parameters, e.g., composition, thickness, thermal conductivity, moisture content; plant types and characteristics (e.g., Leaf Area Index, LAI), height, leaf reflectivity, leaf emissivity, and stomatal resistance [31,43,44,45,46] play a crucial role [47,48,49,50]. The effectiveness of GRs is also affected by the urban structure in terms of urban block density, height, and arrangement [51,52] as well as building design characteristics, e.g., building height, materials, orientation, and insulation [42,53,54,55]. The potential combination of GRs with green walls and facades, other greenery systems, and blue infrastructure can improve their overall performance [47,50].
However, the thermal performance and, consequently, the energy savings potential of GRs are strongly dependent on the prevailing climatic conditions, which drive both the physical (heat and mass transfer) and the biological (transpiration and photosynthesis) processes operating through a GR system. The key meteorological and climatic parameters that control energy transfer processes and mechanisms (such as conduction, convection, evapotranspiration, evaporative cooling, and thermal storage) in GRs are incident solar radiation (a major component of the surface energy balance), ambient air temperature, atmospheric humidity, wind speed, and soil moisture content.
Local climate and prevailing meteorological conditions govern the function of the structural elements of a GR system. For instance, soil water content affects soil thermal behavior by increasing thermal conductivity and heat capacity, whereas soil moisture determines the water availability for evapotranspiration [56]. Soil moisture content is regulated mostly by the precipitation regime (amount, intensity, and timing), mean air temperature, and relative humidity. In some climatic zones where precipitation is insufficient, such as arid and semi-arid climates or climates with distinct dry periods, supplementary irrigation is applied. The presence of a snow layer during the winter has a negative impact on the thermal performance of GRs in cold climates [42]. The majority of the cooling effect of GRs is attributed to evapotranspiration, which (with the exception of soil moisture) is determined by solar radiation, wind speed, relative humidity, air temperature, and sky conditions [57]. Moreover, evapotranspiration depends on vegetation characteristics, such as density, LAI, height, and stomatal resistance [57]. The shading provided by the plant canopy also contributes to the cooling effect of GRs. The dominant climate conditions influence the growth and health of plants, whereas plant behavior varies with the seasons. Thus, the proper selection of plants should take into account both structural characteristics and ecological factors (for instance, native plants are better adapted to local climatic conditions) [42,58]. Lastly, the seasonality of certain climates modulates the thermal performance of GRs accordingly, with better energy performance during the warm (cooling) season rather than in the cold (heating) season [42,47,48,50].
Given the central role of local climate in the energy performance and economic and environmental benefits of GRs, many research works have investigated and established the relationship between climatic background and the energy performance of GRs. Several review papers have attempted to compile and summarize the available findings for different climate zones and climatic conditions. For instance, Jamei et al. [49] evaluated the profit in building energy demand from GRs in three climate zones: temperate, hot-humid, and hot-dry. It was found that the reduction in cooling load is greatest in temperate climates (mean of 50.2%), whereas the corresponding values for hot-humid and hot-dry climates were 10 and 14.8%, respectively, highlighting the strong influence of structural elements such as thermal insulation, growing media, irrigation, and plant selection.
Similar to the findings of Susca [48], the energy savings for cooling purposes due to the installation of GRs on non-insulated rooftops ranged from 9 to 20% for hot-arid and continental desert climates, respectively, to 67% for temperate zones, and up to 75% for tropical climates with dry winters, whereas the decrease in heating energy demand ranged from 20 to 63% in warm climates (such as equatorial savanna and warm temperate climates with warm and humid summers). Energy savings are negligible in hot-arid desert climates, whereas a 30% increase may be observed in warm temperate, fully humid, and hot summer climates. The performance of insulated GRs degrades as the reduction in cooling building energy demand varies between 5 and 9% in temperate climates, and 10 to 13% in arid climates. The reduction in heating energy demand can be about 30% in the Mediterranean climate but is negligible in warm temperate, fully humid with hot summers, and hot arid steppe climates. The effectiveness of GRs in mitigating the UHI was also evaluated, revealing a decrease in nocturnal air temperature between 0 and 20 °C at rooftop level, depending on the climate region, while the effect at pedestrian level was negligible in all considered climates.
In general, the implementation of GR systems has proved to be more effective for cooling energy savings during the summer than heating energy savings during the winter [42]. According to a literature survey by He et al. [50], the energy reduction potential of GRs for building cooling ranges from 3% (in fully humid cold climates with hot summers) to 90% (in warm temperate climates with dry winters), while the potential reduction in heating energy demand ranges from 0.58% (in a temperate Mediterranean climate) to 60% (in a humid temperate climate with hot summers) depending on the climatic zone and the characteristics of the GR type. A higher cooling energy reduction was reported in humid temperate climates with a hot summer (57.6%), in humid temperate climates with a warm summer (57%), and in Mediterranean climates (50%), whereas in the hot arid and equatorial savanna climate zone, a decrease of about 45% was observed. Jamei et al. [49] estimated a 50.25% mean reduction in energy consumption for cooling and 20.2% for heating, respectively. Note, however, that in hot arid and hot semi-arid climates, certain GR types may cause a 5.9 to 25% increase in heating energy demand [50]. Even in Mediterranean climates, two extensive GR systems resulted in a 6 and 11% increase in energy consumption during the heating period, compared to conventionally insulated roofs [59]. GRs have demonstrated greater energy efficiency in the summer (cooling) in temperate warm climates, whereas greater energy loss has been observed in the winter in colder climates [42,48,50].
In conclusion, any effort to optimize the energy performance of a GR system by modifying and/or calibrating structural parameters (e.g., insulation, growing media, irrigation level, and plant varieties) should be designed in accordance with the local climate. Next, the following section discusses the energy and environmental benefits of GRs [60,61].
2.1. Energy Benefits of Green Roofs
Considering that about 20% of the urban surface is covered by roofs, GRs play a significant role in the energy efficiency of the building envelope [25,62,63] and the thermal comfort of inhabitants [64,65,66]. GRs contribute to a substantial reduction in the cooling load and annual energy savings [27], with the cooling load reduced by up to 70%, leading to energy savings of 10 to 60% annually [66]. The energy efficacy of GRs depends on the type of GR (intensive, extensive, etc.), vegetation, prevailing climatic conditions, and the architectural shape and characteristics of the building [67,68].
The positive impact of GRs on indoor temperature is evident when comparing white and green roofs [69]. GRs protect building envelopes from temperature increases above ambient levels and reduce air temperature through evapotranspiration [70]. GRs can reduce indoor air temperature by up to 15 °C [71] or even more if there is shading by adjacent trees, with the benefits primarily confined to the upper floors of buildings with installed GRs. Combining GR with green walls can reduce indoor temperatures and improve thermal comfort. GRs can also reduce the outside temperature, with the degree of temperature reduction contingent on the GR’s surface area and the vertical distance from the pedestrian level.
Numerous theoretical and practical studies have been conducted to evaluate the energy conservation attained by GR systems [13,26,27,41,72,73,74,75]. GR energy benefits include a significant reduction in the roof’s thermal transmittance (U-value), leading to improved roof insulation and a reduction in the cooling and heating load; a decrease in indoor air temperature during the summer, mainly caused by evapotranspiration, but also by shading provided by roof plants; a reduction in surface temperature in the summer, influencing heat transfer processes; and improved indoor air conditions and thermal comfort [13,26,76,77]. Building characteristics and heat transfer mechanisms, which are predominantly determined by building components and roof U-values, have a substantial impact on energy savings, which are reflected in the reduction in cooling and heating loads. In addition, the characteristics of the plant canopy as reflected by the Leaf Area Index (LAI), which influence shade, evapotranspiration, and latent and convective heat changes, have a significant impact on system energy behavior [73,78,79,80].
2.2. Environmental Benefits of Green Roofs
GRs are characterized by important environmental benefits. Vegetated GR surfaces and substrates enhance environmental services in urban areas, help regulate building temperatures, improve storm-water management, and mitigate UHI effects [13]. The improvement in outdoor air quality is directly related to lower carbon emissions by at least 30%, which are a result of both the reduced energy consumption through evapotranspiration and photosynthesis as well as the deposition of pollutants in planted areas [81,82]. Water quality is improved through the control of runoff and the filtration of water pollutants [21].
By providing shading from direct solar radiation, GRs affect the microclimate and energy consumption of buildings [19,40,83]. GRs improve outdoor air quality because plants act as sinks for air pollutants [84,85], eliminating emissions [86,87,88], decreasing surface temperature (through evapotranspiration) by 3 °C, reducing photochemical reactions, contributing to energy savings by over 35% [87], and playing a significant role in carbon sequestration (via photosynthesis) [89]. Additionally, GRs improve indoor air quality and contribute positively to indoor and outdoor thermal comfort. Outdoor comfort pertains to the condition of having lower surface and ambient temperatures by 5 °C, especially during the summer, thereby reducing the cooling load [90,91,92,93]. GRs further improve air quality by enhancing the deposition of air pollutants onto vegetated areas, resulting in a 40% reduction in pollutant concentrations near infrastructure, and a 40% improvement in air purification [81,94]. GRs also contribute to the intensification of carbon dioxide (CO2) concentrations through essential plant functions such as evapotranspiration and photosynthesis [95].
GRs contribute to a considerable control effect on overflow volume and to the mitigation of flooding caused by heavy rainfall. The ability of GRs to control the overflow of urban rainfall has been demonstrated by research studies [6,11,21,89,96]. Retention of rainwater, which typically accounts for 40 to 60% of total precipitation, depends on several factors, including GR type, humidity, vegetation, plant size, as well as rainfall duration and intensity [97]. In addition, nearly 5% of the water eventually returns to the atmosphere via evapotranspiration, thereby reducing the burden on the city’s sewage networks. Furthermore, GRs contribute to the preservation and propagation of local flora and fauna, even in the urban built environment.
In the middle of an urban area, GRs provide a peaceful and quiet place, with noise and pollution reduced by 10%, which can improve mental, physical, and overall health [98]. GRs reduce noise in the urban environment outside the building as well as in the living, working, and leisure spaces inside the building. Ground traffic and other street canyon sounds can travel to the rooftop of a building. GRs enhance sound absorption relative to a conventional (non-green) roof and decrease transmission losses relative to a conventional roof. Planted rooftops are also one of the few remaining means of restoring vegetation to urban space. The combination of plant material, planting type, and design proposals offers a variety of landscape architecture possibilities for designing extensive low herbaceous plantings, accessible gardens with seating areas, dining areas, ponds, promenades, etc. Moreover, the planted buildings offer a solution for the establishment of additional recreational space Skinner 2006 [99]. Converting the roof of a building into an accessible garden has a social dimension in addition to its economic and aesthetic benefits. A space is created that enables its occupants to engage in a variety of activities, such as play, recreation, or the development of interpersonal relationships and sociability.
2.3. Sociopolitical Aspects of Green Roofs
UHIs are a growing concern for policymakers and city planners due to their adverse impacts on natural ecosystems, public health, and economies. Existing reviews typically investigate UHI policies and technological solutions in isolation, lacking a synthesis of integrated interventions. Degirmenci et al. [100] identified four key areas that are underrepresented in the literature: coordination effects of policy and technology responses, synergistic effects of a combined approach of policy and technology interventions, aggravating effects of inadequate policy and technology responses, and moderating effects of policy and technology responses on the relationship between anthropogenic heat and incremental environmental burden. The urgency of the UHI problem underscores the significance of integrated policies, such as green roof initiatives, for mitigating the adverse environmental and health effects of urban heat and fostering sustainable urban development.
Countries worldwide are adopting policies to promote green roofs due to their many environmental, social, and economic benefits, often offering financial incentives or reductions in water and property fees to encourage their implementation [6]. Examples include Tokyo’s law requiring certain buildings to have GRs [101]; financial incentives in German cities such as Darmstadt [101]; drainage fee reductions in Cologne, Mannheim, and Bonn; and reimbursement in Basal, Switzerland [101]. Toronto has mandated green roofs covering 50 to 70% of a building; Quebec offers compensations per square meter; and various US states have their own policies promoting GRs, such as the Floor Area Ratio Bonus in Portland and sewer charge reductions in Nashville [102,103]. Singapore also offers financial benefits through its Gross Floor Incentive Scheme, while countries like China, Hong Kong, Malaysia, and South Korea are actively promoting green roofs and considering putting in place incentive policies [104].
Liberalesso et al. [105] analyzed 143 policies in 113 cities to determine the prevalence of green infrastructure incentive policies in Europe and North America. These incentives include tax reductions, financing mechanisms, permits, certifications, legal obligations, and streamlined procedures, with financial subsidies and legal mandates being the most prevalent global strategies. South America prioritizes property tax reductions, whereas North America takes a balanced approach. The study emphasized policy effectiveness and suggested tailoring homeowner participation incentives to local contexts. It also promotes knowledge-sharing networks to accelerate the adoption of effective green policies and promote sustainability in urban areas worldwide.
The European Union aligns with nature-based solutions via policies inspired by the H2020 Expert Group report on “Nature-Based Solutions and Re-Naturing Cities” [106] and the EU research policy [107], focusing on innovative business models, long-term financing, and legal frameworks for nature-based solutions. In accordance with the United Nations 2030 Agenda for Sustainable Development Goals [108], the recently adopted Biodiversity Strategy by the European Commission aims to protect nature and combat ecosystem degradation by 2030 [109]. Moreover, certain European cities have implemented policies to promote GRs, such as mandatory regulations in Copenhagen for roofs with slopes below 30 degrees; financial incentives in Vienna ranging from 8 to 50 euros per square meter; and Hamburg’s comprehensive strategy, with incentives covering 40% of construction expenses, along with subsidies for educational institutions [110].
In New York, Chicago, and Philadelphia, GR policies may be classified into two categories [111]: mandatory regulations and incentive programs, each of which has its own advantages and disadvantages. These policies seek primarily to mitigate the effects of climate change, such as UHIs and stormwater management. Worldwide, common incentives include financing and obligations, with notable initiatives like Philadelphia’s Density Bonus, New York’s Green Infrastructure Grant, and Chicago’s Green Permit Program addressing environmental justice concerns. However, prospective conflicts may arise between private property owners and the public regarding costs, necessitating additional research in a variety of contexts. These cities are in the early stages of GR implementation, underscoring the need for expansion to ensure more equitable policies that incorporate all facets of environmental justice through community education and knowledge-sharing initiatives at the local level.
However, a review of the literature on GRs in ASEAN countries [112] reveals uneven development, with challenges such as regulatory gaps, limited expertise, and high installation costs. Government-backed regulations and the development of region-specific GR technology to reduce installation costs are required to address these issues.
This entry is adapted from the peer-reviewed paper 10.3390/su152215976
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