Green roofs and facades: A comprehensive review

Renewable and Sustainable Energy Reviews 82 (2018) 915–939

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Renewable and Sustainable Energy Reviews journal homepage: www.elsevier.com/locate/rser

Green roofs and facades: A comprehensive review Ahmet B. Besir, Erdem Cuce

⁎

MARK

Department of Mechanical Engineering, Faculty of Engineering, University of Bayburt, Dede Korkut Campus, 69000 Bayburt, Turkey

A R T I C L E I N F O

A B S T R A C T

Keywords: Green roofs Urban heat island Global warming Energy saving Evapotranspiration

Based on United Nations Environment Program (UNEP), building sector accounts for 40% of total energy consumption. In European countries, 36% of total greenhouse gas emissions is attributed to buildings. In this respect, green roofs are considered to be one of the most appropriate sustainable solutions to resolve the urban heat island-related issues. Roofs account for nearly 20–25% of overall urban surface areas. Energy saving, thermal insulation, shading and evapotranspiration features highlight the key role of green roofs in overall thermal performance of buildings and microclimatic conditions of indoor environments. Within the scope of this research, the concept of green roofs and facades is comprehensively analysed in a holistic and thematic way. Following a historical overview of the technology, the research is split into various subfields such as energy saving in buildings through greenery systems, multifunctional thermal benefits including evapotranspiration, thermal insulation, shading and thermal comfort features, evaporative cooling for reducing cooling demand and minimising wind driven convection losses. The results achieved from the literature survey clearly indicate that green roofs and facades are key solutions to mitigate building-related energy consumptions and greenhouse gas emissions. According to the previous works, heat flow through the building roofs in summer can be reduced by approximately 80% via green roofs. The green roofs are reported to consume less energy in the range of 2.2–16.7% than traditional roofs during summer time. A similar tendency is observed for the winter season depending on regional and climatic conditions. The temperature difference between conventional and greens roofs in winter is found to be about 4 °C, which is remarkable. Energy demand of buildings in summer is highly dependent on the plant intensity as it is reported to be 23.6, 12.3 and 8.2 kWh/m2/year for extensive, semiintensive and intensive greenery surface, respectively. Greenery systems are also capable of providing thermally comfortable indoor and outdoor conditions. It is underlined that the annual average accumulation of CO2 reaches the level of 13.41–97.03 kg carbon/m2 for 98 m2 of vertical greenery system. The results of this research can be useful for dwellers, builders, architects, engineers and policy makers to have a good understanding about the potential of green roofs and facades to mitigate building-related energy consumptions and carbon emissions in a renewable, sustainable, energy-efficient and cost effective way.

1. Introduction Today, the majority of the world population lives in cities, and there is a growing tendency to urban life year after year. According to the recent report of United Nations, the population living in cities is expected to increase up to 67%, by 2050 [1]. There is a growing signifıcance of environmental issues at global scale, and urbanisation is of significant relevance. Several environmental problems such as urban heat island, greenhouse gas emissions and reductions of energy sources can be attributed to dense urbanisation. Susca et al. [2] monitor the urban heat island in four areas of New York City, and they observe an average of 2 °C temperature difference between the most and the least vegetated areas, which can be explained by the substitution of greenery areas with man-made building materials. For this reason, effective control of urbanisation and urban heat island through energy-effıcient ⁎

and eco-friendly buildings which use sustainable and recyclable sources is of vital importance. It is well-documented in literature that the total world energy consumption has drastically increased over the last four decades. It is clearly reported by International Energy Agency that the rise of global energy uses from 1971 to 2014 is about 93% [3]. The greatest part of this increment is related to buildings and according to United Nations Environment Program (UNEP), building sector accounts for 40% of total energy consumption [4]. In European countries, 36% of total greenhouse gas emissions is ascribed to the buildings. Further reports reveal that a signifıcant portion of energy is consumed by commercial buildings for heating, cooling and lightening purposes [5]. Improved comfort conditions of occupants as a consequence of technological developments also play a key role in total energy consumed in building sector. This output can be justified by the case of China as the main

Corresponding author.

http://dx.doi.org/10.1016/j.rser.2017.09.106 Received 26 July 2017; Received in revised form 11 September 2017; Accepted 26 September 2017 1364-0321/ © 2017 Elsevier Ltd. All rights reserved.

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including architectural approaches such as the design of greenery systems, material selection and site planning [10]. According to the results of a survey research conducted in the USA, green buildings consume approximately 30% less energy compared to traditional buildings [13]. Some standards associated with the concept of green buildings and efficient use of energy in building sector are LEED, Leadership in Energy and Environmental Design, (USA), BREEAM, Building Research Establishment Environmental Assessment Methodology, (UK) and Green Building Label (China) [14]. These standards cover several parameters such as energy, pollution, water, health and well-being, ecology, materials and waste [11]. Green buildings are considered as an ultimate and decisive solution to reduce energy consumed in building sector and to halt greenhouse gas emissions as a consequence of efficient use and control of energy transfer from building envelope [2]. While applying greenery systems on building surface, microclimatic conditions of buildings are controlled without excessive energy consumption [15]. The main goal of this paper is to review the greenery systems to meet the standards of green buildings in terms of thermal, environmental, social and economic aspects. The paper differs from the previous review works on green roofs and facades in terms of several aspects. First of all, on the contrary to the existing literature focusing on one specific topic only, the concept of green roofs and facades is investigated in a holistic and thematic way in this research. The review starts with a historical overview of the technology following by theoretical fundamentals and brief explanations. Then, the topic is comprehensively analysed by splitting the scope into various subtopics such as energy saving feature of greenery systems, thermal benefits covering thermal insulation, evapotranspiration, shading and thermal comfort features, evaporative cooling for minimising cooling demand and wind blockage impact. Moreover, green roofs and facades are evaluated in terms of cost and environmental benefits such as reduction in carbon emissions and enhancement in indoor and outdoor air quality. The impacts of greenery systems on human health are also considered within the scope of this study.

energy consuming sector is the building sector in the country [6]. In this respect, building sector is in the center of interest to mitigate the role of buildings in total world energy consumption and to minimize the building-related greenhouse gas emissions [7]. In recent years, many policy makers and governments take decisive measures to systematically reduce carbon emissions and energy use in buildings. Some of these measures are directly relevant to building energy regulations which are proposed and implemented by developed and developing countries such as the United States, the European Union countries and China [4,8]. In most cases, building energy regulations in new built applications cover required performance figures for both appliances and building envelope materials. Appliance and equipment standards are related to heating, cooling and water heating devices and lightning Systems. For instance, Energy Star label on an appliance is a sort of performance Standard which is used to address high-effıcient electrical devices. Building energy codes essentially deal with energy performance rates of buildings by considering regulations notably for buildings envelopes. Similar to the Energy Star label, Home Energy Rating System (HERS) score is used as building energy label [4]. Current energy consumption figures of the world clearly indicate that drastic measures need to be taken to minimize the role of building sector in total world energy use. As an example, the predictions reveal that energy consumed in buildings in China will reach up to 40% of total energy use in the near future if the government does not adopt appropriate policies with regard to building energy efficiency (BEE) measures on time. In this respect, the improvement in BEE is expected to play a crucial role in mitigating energy consumption rates, and thus in protecting the environment as well as achieving social and economic development. Some essential measures to enhance BEE can be split into five categories: a) improvement of energy codes for new buildings, b) energy labelling and evaluation of buildings, e) heat metering and energy-efficient retrofıts, d) increasing the use of renewable energy sources in buildings and e) energy efficiency supervision over large public buildings [6]. In European counties, there are some directives focusing on building energy consumption, such as the Energy Performance of Building Directives (EPBD) 2002/91/EC and 2010/31/EC. According to the said directives, the existing buildings in European countries are evaluated and certified by each member States. These attempts introduce the use of Nearly Zero Energy Buildings (NZEB) recommending low energy demand associated with the renewable energy use [9]. Recently, new regulations called net zero carbon buildings are adopted by the United Kingdom (UK) and other European Union (EU) countries to reduce the environmental impacts of energy use in homes [8]. UNEP addresses five main measures for urgent reduction of carbon emissions such as:

2. Greenery systems Rising population living in urban areas due to the welfare level of the big cities results in so many crucial problems such as pollution especially water, air and noise, global warming and urban heat island due to insufficient greenery areas. It can be easily asserted for such regions that the shortage of vegetation causes remarkable increases in surrounding temperatures which affect the thermal comfort conditions of indoor environments [16,17]. In this respect, the greenery systems are considered to be one of the most appropriate sustainable solutions to resolve the urban heat island-related issues. Greenery systems mainly comprised of green roofs and green walls are initially considered in the architectural style of the buildings. Widening environmental awareness leads to effective exploitation of these systems to enhance the building performance in terms of not only efficient energy use but also desirable indoor and outdoor environments [16,17]. From this point view, the integration of greenery systems to buildings in urban areas has a great potential to increase the quality of urban environment such as providing water and air quality, storm water management, dense vegetation in urban environments, a marked decline in temperature and carbon emissions as well as minimization of heat island effects [18–20]. Besides the profound effects on environment, the greenery systems provide additional benefits to the public such as social and economic aspects. The presence of greenery has a major psychological impact on urban dwellers as well as enhancing the visual aesthetics of the cities, and raising prices of real estate [17,21]. Moreover, greenery systems are capable of being devised as one of the passive design solutions which provide additional benefits such as insulating impact in winter and shading feature in summer. Therefore, it is emphasized that microclimatic conditions of existing buildings can be adjusted in a cost-effective and eco-friendly manner by utilizing

■ Increasing the energy efficiency of new and existing buildings ■ Increasing the energy efficiency of appliances ■ Encouraging energy and distribution companies to support emission reductions in buildings ■ Changing attitudes and behaviour toward energy consumption ■ Substituting fossil fuels with renewable energies [10]. As stated above, rising concentrations of greenhouse gas emissions and their negative impacts on environment are taken into consideration by many countries, and developed and developing countries invoke new solutions and designs for buildings to mitigate building-oriented energy consumptions. The concept of green structures is such an attempt that becomes widespread day after day. As reported by United Nations (UN), green buildings enable an opportunity of preventing negative effects of existing buildings on environment [11]. With green building design, eco-friendly structures are expected to engender less carbon emission and natural resource consumption by providing energy saving and increasing the use of recyclable materials [12,13]. Green buildings are multifunctional structures covering different ways of integrating renewable energy solutions into a building concept as well as 916

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2.2.1. Green roofs Roofs account for nearly 20–25% of overall urban surface areas. Therefore, greening the roofs has a great potential to affect the building and urban environment [1]. Green roofs, also known as eco-roofs, roof gardens and living roofs can be defined as the roofs coated with green vegetation and growing medium [18,28]. Green roofs are capable of providing several benefits to urban areas in terms of aesthetical and environmental aspects [29]. Some of these benefits can be illustrated as reducing greenhouse gas emissions, air pollution and urban heat island effects in highly populated areas, preventing the acid rain by escalating pH values, increasing the quality of city water, minimizing the risks of flooding by retaining the excessive water, providing better ecological habitat for urban life and wildlife, absorbing local noise pollution within urban areas, and improving the durability of internal membranes [29–33]. Another benefit of green roofs is the enhancement of architectural interest and biodiversity [34]. In addition to these, green roofs can improve the health of dwellers in urban district. Energy saving, thermal insulation, shading and evapotranspiration features highlight the key role of green roofs in overall thermal performance of buildings and microclimatic conditions of indoor environments [32]. In summer, heat flow through the building roof can be reduced by approximately 80% via green roofs [33]. Hence, the annual energy consumption is decreased owing to a small difference in temperature of indoor and outdoor air [30]. Green roof design consists of several components from top to bottom; vegetation (landscape materials), growing medium (substrate), filter, drainage material (moisture retention), root barrier, water proofing membrane, insulation layer and structural layer as shown in Fig. 1 [18,31]. Besides that, there are some additional components depending on the climatic conditions like irrigation systems [34]. Irrigation systems are required for hot and arid regions whereas they are useless for humid and temperate climates. Green roofs can be split into three categories (extensive, semi-intensive and intensive roofs) with respect to weight, substrate layer, maintenance, cost, plant community and irrigation as illustrated in Table 1 [28]. Intensive roofs are heavier and more expensive compared to other types. In addition, they require a higher level of maintenance. The extensive roofs do not have extra weight due to shallower growth substrate, and their maintenance cost is notably low [29]. Deeper growth substrates offer plenty of vegetation. Green roofs convert the impervious areas of a rooftop into multifunctional spaces using growing media and vegetation [33]. For this reason, green roofs are widely used for recreating space in urban areas [29,31]. By using local vegetation and growing medium, the requirement of irrigation and maintenance costs can be reduced as a consequence of local climatic conditions [30].

different types of greenery systems [15,17,22]. 2.1. Historical background The Hanging Gardens of Babylon dated back to 500 BC is admitted as the earliest examples of greenery systems [18]. Like Babylon, the Roman and Greek empires also employed these systems at their own eras. In the Mediterranean region, different plants notably vines were utilized to prevent the building envelope from excessive sunlight in summertime and to provide cooler and comfortable indoor conditions to occupants. The usage of the plants climbing the building greatly expanded in the UK and Central Europe during 17th and 18th centuries. In European and North American cities, ornamental elements were the center of attraction for the residents in urban areas during the 19th century so the woody climbers were the most commonly used greenery surfaces in buildings [17]. There was a general consensus among urban people that the living walls and green roofs are not compatible with modern architecture due to the difficulties in retrofitting. However, owing to the technological developments, rising comfort levels of occupants and social awareness on the environmental issues, the greenery systems have been in the center of interest year after year. Currently, it is welldocumented in literature that green roofs and facades have a great potential of providing energy saving in building sector and enabling thermally attractive and comfortable indoor and outdoor conditions [2]. Several attempts on living wall systems were inspired by botanical realms especially the areas with biodiversity [17,18]. Green facades played a key role in developing ecology of cities at the beginning of the 1980s. The majority of pioneer works conducted on green roofs was observed in Germany [17]. In this respect, the first comprehensive program was put into practice from the early 20th century by retrofitting the houses with greenery surfaces [18]. Between 1980s and the late of 1990s, approximately 246.000 m2 of green vegetation was integrated into the building facades located in Berlin [22]. Currently, annual increase in green roof coverage in Germany is reported to be about 13.5 million square meters, which is remarkable [23]. In other words, 10% of the houses in Germany can be considered as green buildings as of now [12,18]. In recent years, some developed countries such as USA, Canada, Australia, Singapore and Japan have invoked new standards for the cost-effective and energy-efficient retrofitting of existing buildings and new built applications with greenery systems. As a consequence of the new regulations, 15% of the roofs located in Switzerland have been covered by the greenery systems [24] yielding to 4 GW/year of energy savings. A similar regulation has been carried out by Canada in which the greenery systems have to cover between 20 and 60% of the roof area when the floor area of the building is greater than 2000 m2. In Japan, private and public buildings having floor areas which are greater than 1000 and 200 m2 respectively, are required to be covered by plants at least 20% [25]. Moreover, 70% of the building roofs in Portland, USA has to be retrofitted by green surfaces [26]. 2.2. Types of greenery systems For a proper understanding of the greenery systems and for an effective exploitation of such systems in buildings, first of all, common types of the greenery concept are clearly defined along with the features, distinctions, applications areas and potential benefits. Afterwards, the use of greenery systems in buildings is investigated in terms of several aspects such as energy saving, thermal comfort, environmental matters and architectural features. The most common utilization of greenery systems in buildings can be expressed as green roofs and green facades [27]. In addition to these, the green balcony is also considered as a part of the greenery systems as reported in literature [1].

Fig. 1. Schematics of different green roof components [18]. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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Table 1 Classification of green roofs according to type of usage, construction factors and maintenance requirements [1].

Maintenance Irrigation Plant communities Cost Weight Use System build- up height

Extensive green roof

Semi intensive green roof

Intensive green roof

Low No Moss-Sedum-Herbs and Grasses Low 60–150 kg/m2 Ecological protection layer 60–200 mm

Periodically Periodically Grass- Herbs and Shrubs Middle 120–200 kg/m2 Designed green roof 120–250 mm

High Regularly Lawn or Perennials, Shrubs and Trees High 180–500 kg/m2 Park like garden 150–400 mm underground garages ≥ 1000 mm

2.2.2. Vertical greenery systems Vertical greenery systems draw attention especially in densely populated areas to resolve energy and environment oriented matters of cities. The use of vertical greenery surfaces offers considerable opportunities associated with energy saving and ecological improvements for both buildings and (sub)urban areas. These multifunctional benefits can be basically explained by decreasing wall temperature depending on the influences of wind barrier, shading effect, less solar absorption, thermal insulation impact of vegetation and growth substrate. Vertical greenery systems also play a key role in providing microclimate and increasing the biodiversity and air quality [35]. In addition to thermal performance enhancement of buildings and indoor air quality, vertical greenery systems mitigate noise and air pollution, protect the building envelope from hazardous environmental effects such as excessive sunlight and acid rain. The vertical greenery also affects human psychology positively by having aesthetic appearance [36,37]. Compared to green roofs, green wall systems provide highly beneficial improvements to the environment since the green walls cover much more surface area than green roofs. In case of high-rise buildings, the difference between the surface area of green walls and green roofs can reach roughly 20 times [27]. Vertical greenery system is simply defined as greening vertical layer (facades, walls, blind walls and partition walls) and the main intention is to grow the plant on the wall of buildings. The vertical greenery system is also named as vertical garden, green wall, vertical green, vertical landscaping and bio walls [15,17]. Green wall consists of two different systems called green facade and living wall. The difference between green facades and living walls can be expressed that vegetation grows over the building envelope naturally and growing substrate on the ground as well in green facades. Yet, living walls are comprised of pre-vegetated plants and cladding structures offering plenty of plants for coating the building facade uniformly as shown in Table 2 [15,17,38]. In comparison to green facades, living walls require some essential materials such as supporting elements growing substrate and irrigation system to maintain various plants. Therefore, maintenance costs are notably high for living wall systems [39]. But, living walls usually perform better performance compared to green facades owing to pre-cultivated plants and transferability. Besides, in case of having unexpected problems regarding the plants, it is easy to renew pre-cultivated plants [1]. Classification of green walls according to their constructional features is illustrated in Fig. 2. Green facades can be split into two main parts and three sub-categories which include traditional green facade, double-skin green facade

Table 2 Dichotomy of vertical greenery systems [15]. Green facade

Living wall

In green facade, plants are rooted on the ground in soil and climb on facade and covers elevation

Living walls are pre-vegetated sheets that are attached to a structural wall or frame

(DSGF) and modular trellis. In traditional green facades, the plants use the envelope as supporter material and growing media stays on the ground. DSGF based on double-skin framework along the entire surface has air cavity between the wall and a vertical support structure. Modular trellis has perimeter flower pots for vegetation roofs [17,36]. Types of green facades are shown in Fig. 3. The main types of living walls are described as continuous and modular systems in which the main difference is the growing media. In continuous systems, growing media is not a requirement because of a geotextile membrane. This material can be utilized instead of soil. Plants in continuous systems grow through irrigation using hydroponic techniques. As reported by Charoenkit and Yiemwattana [40], modular living walls can be designed as including planters, pocket-shape planters and panels as shown in Table 3. There are two types of panels utilized in the applications which are single and grid panel. Each of the components used to supply growing substrate is fixed to supporting materials. As previously mentioned, living walls offer a great variety of plants and easy to replace the damaged plant with the fresh one [40]. As it is addressed by Manso and Castro-Gomez [17], modular living walls can be comprised of trays, vessels, planter tiles and flexible bags. Trays are usually made of rigid containers and contain the soil. Vessels are placed vertically to each other. Planter tiles can be utilized in the interior or exterior of building. Lightweight materials are used in flexible bags. Flexible bags can be implemented on surfaces with different forms such as curved or sloped [17].

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Fig. 2. Classification of green walls according to their construction characteristic [17].

Fig. 3. Types of green facades [36]. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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Table 3 Types of living walls [40]. Type

Sub-category

Continuous LWs

–

Modular LWs

System components ➢ ➢ ➢ ➢

Planter Pocket-typed planter Vertical panel Grid panel

➢ ➢ ➢ ➢ ➢ ➢ ➢

Characteristic

Fabric layer or porous screen Waterproof membrane Structural framework (optional) Mesh lattice at the outer layer to hold a fabric layer against wind (optional) Irrigation system Modular components made of plastic, metal, or ceramic Substrate Drainage system Waterproof membrane Structural framework made of wood or metal Irrigation system

3. Energy saving in buildings

➢ ➢ ➢ ➢

No requirement for soil Use of hydroponic system to provide water and nutrients to plant roots Lightweight vegetated wall with small planting depth

➢ Use of soil or light-weight growing medium with greater depth allowing plants to be rooted in ➢ Easy replacement of individual unhealthy plants by removing specific planters

be considered as heating to mitigate building-oriented energy demand. Although the energy demand for cooling is also growing steadily, it is still incomparable with the heating demand [41]. Among the potential solutions to minimise heating demand of buildings through the cost-effective and eco-friendly retrofits on building envelope, greenery systems stand as one of the most appropriate measures. Potential energy savings in buildings via greenery systems are studied both experimentally and numerically by many researchers in literature. According to the results, the reductions of heat loss from the roofs in summer and winter are about to 70–90% and 10–30% respectively by using green roofs. The research carried out by Liu and Baskaran [13] reveals that green roofs can reduce heat gain and heat loss by 95% and 26% during 22 months of observation period. In another work, vertical greenery systems are found to reduce the peak cooling load transferred through the walls by 28% on a clear sunny day [55]. It is also reported that greenery surfaces absorb about 70% of incoming solar radiation. In this section, the impacts of greenery systems on energy savings and especially thermal benefits in buildings such as shading-effect, thermal insulation, evapotranspiration and wind effect are comprehensively investigated. The analyses on green roofs and greenery vertical systems are carried out separately for an easier understanding and tracking the results.

Building envelope, which is also named as building fabric or shell, separates the indoor of a building from outdoor. Energy performance of a building in terms of building envelope can be described as minimising the energy requirement for heating and cooling owing to the structural properties of the envelope. The envelope components consist of external walls, floors, roofs, ceilings, windows and doors as shown in Fig. 4. Energy loss as a crucial issue for energy performance is entirely dependent on building age and type, climate, the materials of a building envelope, dweller behaviour and geographical location [41]. The effect of building envelope on energy consumption is undeniable. The amount of energy used in building sector accounts for onethird of global energy consumption for space heating and cooling. In addition, energy consumption in cold climates has an increasing trend approaching 50% [42]. In buildings, heat loss takes place at walls, roofs and floors due to comprising external areas of buildings. By providing insulation against heat loss, either in cold regions or hot climates, the energy consumption can be decreased remarkably as illustrated for different building envelope materials in literature [43–53]. Fig. 5 shows the overall heat transfer coefficients (U-values) of insulation levels with respect to the building codes of some countries [54]. With using current codes (in given countries), it is observed that no more excessive energy consumption occurs for heating and cooling in cold and hot days. Comfortable indoor conditions are provided to occupant in buildings as desired. Building energy efficiency can be enhanced by achieving a considerable reduction in urban heat island (UHI) effects. In this respect, low-cost, energy-efficient and eco-friendly retrofits on building envelope are highly required to minimise the said effects. As it is clearly shown in Fig. 6, there are significant differences in ambient temperature values (from 2 to 4 °C) between urban and rural regions [41]. In 2010, residential sector and service buildings in OECD (Organisation for Economic Co-operation and Development) and nonOECD countries are found to be responsible for a significant amount of energy consumption as depicted in Fig. 7. As mentioned previously, the largest consumption belongs to residential heating so the key issue can

3.1. Thermal benefits of green roofs In building sector, green roofs attract attention day after day owing to their considerable role in mitigating energy demand by enhancing thermal performance of buildings [18,34]. A research conducted in south Italy reveals that green roofs (without any insulation material) are cooler by about 12 °C in comparison with conventional roofs according to the average surface temperature measurements in summer. Even in winter season, the temperature difference between conventional and greens roofs is remarkable by almost 4 °C. From this point of view, the buildings with green roofs consume less energy in comparison with conventional roofs [30]. The multifunctional benefits of green roofs in terms of thermal performance enhancement in buildings can be Fig. 4. Building envelope components [41].

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Fig. 5. Insulation levels vary greatly, from old buildings to buildings meeting stringest current codes [54].

illustrated as thermal insulation, evapotranspiration and shading [1,12,18]. 3.1.1. Thermal insulation Through several studies in literature, it can be easily asserted that the thickness of growing media notably affects the thermal insulation feature of green roofs. the study conducted by Permpituck and Namprakai [56] investigate the thermal insulation feature of two different green roofs (thicknesses of substrates 10 and 20 cm) in comparison with a conventional roof. With respect to results, the reduction in heat transfer and energy consumption is remarkable. Heat transfer is found to decrease by 59 and 96% while energy consumption by 31 and 37%, respectively for 10 and 20 cm thick green roofs compared to the case of bare roof [56]. A similar attempt is done by Lui and Minor [34]. They evaluate the heat transfer rate between 75 and 100 mm thick green roofs in comparison with a reference bare roof. As a result of the study, it is reported that the thermal performance of green roofs depends on the thickness of greenery surface [34]. In another work, the impacts of soil thickness on green roof performance are analysed in terms of thermal insulation and energy saving for cooling and heating seasons [57,58]. A mathematical model is developed to calculate heat transfer for cooling seasons in Greece. According to the results, adding air pockets in less thick substrate yields better thermal performance in green roofs. Another finding from the research is that the decrease in soil moisture leads to reduce the heat flux across the roofs as shown in Table 4. Thermal resistance of soil is observed to increase by 0.4 m2 K/ W with using 100 mm thickness of dry growing media. The results presented in Table 4 are also verified by the study carried out by Wong et al. [59]. Moreover, owing to the extra insulation soil layer, it is found

Fig. 7. Buildings in OECD and non-OECD countries, end-use energy by sector [41].

Table 4 Effect of soil moisture content on green roof U-values [34]. Soil

Green roof U-value (W/ m2 K)

Green roof with water storage Uvalue (W/m2 K)

0% moisture 20% moisture 80% moisture

0.42 0.46 0.53

0.38 0.41 0.48

Fig. 6. Urban heat islands increase energy consumption [41].

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that the heat tends to be stored inside the building [60]. Theoretical and numerical works are also carried out to analyse the thermal insulation feature of green roofs. For instance, the theoretical model proposed by Tsang and Jim [61] aims to determine the thermal performance of buildings with green roofs utilised in the tropical region of Hong Kong. Through the results they achieved, it is understood that three main parameters are of vital importance to consume less energy in green roof applications; albedo of vegetation, heat convection between air and canopy and the water within the growth medium. It is reported that the bare roof albedo of 0.15, in comparison with 0.30 of green roof, renders 75% higher heat storage. Moreover, an increment of the heat convection rate can lead to occur evapotranspiration within growing media and vegetation. Thus latent heat can be removed conveniently with increasing air velocity. The reduction in heat storage could be by 45%. Lastly, the change in water content of the soil between 30 and 60% alleviates heat storage at a rate of 24% [61]. The water content of substrate has different impacts on thermal performance of green roofs depending on climatic conditions. As an example, the growing media with water content has an advantage due to the evapotranspiration to enhance heat dissipation, hence to minimise the cooling demand during summer period [1,34]. This output is justified by Lazzarin et al. [62] that the wet green roofs provide 40% better performance compared to conventional insulated roofing. According to the results, it is recommended that the effective way is to use wet substrate in summer and dry soil in winter. This is because, dry soil is utilised to increase heat storage and thermal insulation feature in winter. On the other hand, the wet substrate is implemented to have enhanced convection and conduction effects in summer period [63].

Table 5 Comparison of the energetic exchange of the dry or wet green roof with traditional roof during cooling season [67]. Energy related parameters

Incident solar radiation Solar reflectivity Solar absorption Outside adduction Evapotranspiration Thermal accumulation Inside adduction

Type of roofs Dry roof green

Wet roof garden

Traditional roof

100 23 39 24 12 1.3 1.8

100 23 39 13 25 0.6 0.4

100 10 0 86 0 0 4.4

In line with the role of evapotranspiration in reducing heat penetration from the green roofs to the buildings, several experimental attempts are done to evaluate the range of heat fluxes in green roofs compared to the conventional roofing. As an example, a hospital located in Northeast Italy is experimentally investigated, in which wet and dry green roofs are evaluated in terms of heat flux and the results are compared with the traditional roof as given in Table 5. The heat loss from wet green roof is found to be two times greater than that of the dry green roof due to evapotranspiration. Compared to the traditional roof, the heat flow into the building through dry green roof is found to be about 60% less, which is remarkable [34,67]. In addition, it is observed that the wet green roof has additional evapotranspiration benefits compared to the dry roof. As a consequence of evapotranspiration, heat penetration into the building in summer, thus the cooling demand of buildings is considerably reduced via passive cooling strategy [12]. Briefly, evapotranspiration and its key role in minimising solar radiation penetration into the building are main reasons of cooling effects of green roofs. Table 6 illustrates the cooling effects of green roofs and compares the temperatures between outer surfaces of green roofs and indoor air for the climatic conditions of different cities [68–83]. It is understood from the results that the change in the temperature between outer and inner surfaces of green roof is notable [63]. If green roofs are deployed widely, it is reported in a recent study that the average temperature of the related area can be reduced by 0.3–3.0 °C, which is promising [18,65]. On the other hand, it needs to be emphasized that the evapotranspiration rate is a function of the types of green roof. There are some dependable factors for evapotranspiration such as solar radiation, water content of the growing media, density and height of canopy. The findings reveal that the evapotranspiration for intensive green roofs is higher due to the thickness of soil and dense vegetation. Fig. 8 provides details about the evapotranspiration of intensive roofs compared to semi-intensive and extensive green roofs. During cooling periods, especially the increase in solar radiation affects the evapotranspiration amounts of intensive green roofs as it is clear from the data [84].

3.1.2. Evapotranspiration Thermal performance of buildings depends on structural details of building envelope and to improve the thermal performance, climatic conditions need to be taken into consideration. Not only for winter and more freezing conditions but also for tropical and temperate regions, the main challenge is to provide thermal regulation of building envelope. For these reasons, many studies in literature focus on comfortable indoor conditions to maintain ideal room temperature, relative humidity and less energy consumption. Green roofs can mitigate energy consumption, and enhance air quality by the effect of evapotranspiration [64]. The evapotranspiration is composed of evaporation and transpiration. The water content of the soil is transferred from soil into the air during evaporation. Unlike the evaporation, transpiration takes place in leaves and body of the plants such as releasing the water vapour from stomata on leaves and pore skin of canopy [1]. Leaves remove heat from the medium by means of radiative heat loss of long wavelengths (400–700 nm). Sensible and latent heat are also dissipated through convection and evaporation, respectively [65]. Humidity plays a key role to maintain thermal regulation in both humid and dry environments. When the amount water content of air increases by vegetation, heat is dissipated by evapotranspiration. The increment of moisture in the air might have both positive and negative effects on indoor conditions with regard to climatic features [66]. Therefore, it is of vital significance to provide adequate water evaporation from canopy and substrate in green roof applications [63]. According to the results of a mathematical model conducted for the summer climatic conditions of China, 58% of heat from a green roof is lost through evapotranspiration while about 31% long wave radiative exchange and 1.2% stored or transferred heat into the building from the roof [34]. The way of heat dissipation can be split into of three main parts such as evapotranspiration, long wave radiation heat loss and photosynthesis. The evapotranspiration heat accounts for 51.5%, whereas long wave radiation heat loss and photosynthesis covers 40% and 8.5%, respectively. The heat transferred into the building is less than 0.5% in most cases. The outer roof temperature is notably affected by solar radiation whereas the effect of the water content of air is almost insignificant in comparison with the solar radiation [63].

3.1.3. Heat flux, shading effect and energy saving Green roof should be deployed on building envelopes to boost thermal comfort and shading-effects on roof surfaces. Shading-effect on roof surfaces is an important solution to sustain thermoregulation in buildings. Therefore, thermal comfort occurs in the buildings due to the evapotranspiration, photosynthesis and shading effects of vegetation on the roof surfaces. Green roofs provide decline in the urban heat island effects and the enormous growth in thermal benefits of the buildings [12]. Moreover, green roofs contribute to less energy consumption and minimize the money to expenditure for thermal comfort [85]. Saadatian et al. [12] reports a study with related to leaf area index (LAI) offering more shading effects on green roof surface areas. LAI is of vital important factor to provide energy saving through evapotranspiration and solar shading [1,57]. Coverage ratio and leaf thickness lead to regulate on indoor thermal comfort in cooling period [1]. 922

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Table 6 Cooling effect for green roof [63,64]. Author(s)

Location

Cooling effect Ambient temperature

Onmura et al. [68] Niachou et al. [69] Wong [70]

Takebayashi [72] Wu et al. [73] Alar et al. [74] Pompeii II [75] Zhang et al. [76] Susca et al. [77] Wu [78]

Osaka, Japan Loutraki, Greece Singapore Tokyo, Japan USA Central Florida Toronto Kobe, Japan Shenzen, China Tartu, Estonia Chicago, USA Shanghai, China NY, USA XingTai, China

Li [79]

Chongqing, China

Feng et al. [80]

Chongqing, China

Bass et al. [81] Alexandri & Jones [82]

Toronto Riyadh, Saudi Arabia Mumbai, India Moscow, Russia London, UK La Rochelle, France

Kravitz [71] Qiu [64]

Salah et al. [83]

Roof surface temperature 30–60

2 4 0.8

0.2–1.8

32–43 7–22 ≥ 1.6 10 0.8–7.9 3.4 4–21 3.29

2 Cloudy:2.25 °C (max), 1.32 °C (avg) Clear:2.62 °C (max), 1.72 °C (avg) Clear and raining: 0.3 (avg) Clear days: 0.2 (avg) Garden style 4.54 (avg) Simple style 2.81 (avg) 1–2 12.8 26.1 9.1 19.3 30

Depending on the results, the max temperature of bare soil is determined by 42 °C and the surface covering with vegetation is not excess of 36 °C. This experimental study carried out during between March and April months. In addition to this, the change in surface temperature depends on characteristic of the vegetation and max daily variation is about 26 °C [12]. In another study conducted in Midwestern USA, the extensive green roof with lower thickness of substrate and no irrigation system is deployed on insulated existing building. As a result of findings, the variation of temperature with related to gravel roof is higher in comparison to green roof in summer day and the green roof is roughly 20 °C cooler than gravel roof [86]. According to an experimental study, the bare soil is 42 °C, the uninsulated roof is 57 °C and the roof with under a dense vegetation layer is nearly 26.5 °C [1]. Likewise, temperature of black roof decreases from 80 °C to 27 °C thanks to install green roofs on buildings [12,34]. An experimental research conducted in Estonia with 100 mm thickness of growing media shows that temperature variation in cooling period is decreased tremendously by installing roof garden. Furthermore, thermal insulation becomes effective in winter because of substrate layer [74]. Another study conducted by Wong et al. [59] focuses on a thermal resistance of green roof with different type of plants. Also Fig. 9 shows that the thermal resistance of green roof increases with insulation and U-value depends on characteristics of vegetation

Fig. 9. Comparison of U-values for different types of roofs on commercial building in Singapore [34,59].

such as turf, shrubs and trees. While looking into previous studies, the heat flux decreases with installing the green roof. The reduction in the heat flux is given in Table 7 [86–89]. These studies indicate that the heat flux can be decreased by depending on the vegetation and substrate layer. In addition to these studies, the effect of different drainage system on green roof is experimentally investigated by Coma et al. Fig. 8. Annual evapotranspiration for extensive, semi-intensive and intensive green roofs [84]. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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in winter conditions. The rates of heat losses are changing in the range of 8% and 14% compared to the reference roof [90]. Based on similar study, the growth media consist of 4–8% organic material of substrate. Comparing the results, if increased the growing media, thermal resistance also increases and the value is nearly 3.1 m2 K/W. Both green roof and traditional roof after accumulated snow layer show similar heat flux rate. In winter, the green roof reduces 23% of heat flux in comparison with reference roof without snow layer but after accumulated snow the rate decrease to 5%. During winter time, the snow layer provides extra insulation to the roof [91]. Depending on the literature, the green roof is highly effective to reduce energy consumption and maintain the thermal comfort of indoor environment also affect the UHI. The reasons of these benefits are shading effect of vegetation, evapotranspiration including not only plants but also growing media and finally the effect of thermal insulation especially in winter. Moreover, the energy consumption decreases with the increase in growing media and insulation layer. Even, the different types of green roof show remarkable results particularly on cooling season.

Table 7 The effect of green roof on heat flux. Author

Getter et al. [86] Liu and Minor [87] Morau et al. [88] Oliveri et al. [89] Bevilacqua et al. [30]

Heat flux Heating period

Cooling period

13% reduction 10–30% reduction

67% reduction 70–90% reduction 51–63% reduction 60% reduction

30–37% reduction

carried out [29]. Also the green roof with two distinct drainage system was compared to insulated reference roof without greening system. Figs. 10 and 11 give information about the experimental studies especially construction of green and reference roof also drainage layer. The energy consumption between reference and green roof are compared to each other in heating and cooling season. The results indicate that the extensive green roof consumes less energy (the range of 16.7% and 2.2%) than reference roof during summer time, but less energy is consumed by reference roof with insulation (6.1% and 11.1% much more energy consumption by the extensive green roof), vice versa, for the winter in Fig. 12. On the contrary, another experimental study carried out by Silva et al. [84] highlights the performance of three different type of green roof such as extensive, intensive and semi-intensive. Additionally, traditional roof with different insulation properties is compared to the green roof [84]. Fig. 13 illustrate that energy needs in winter are roughly same for both green roof types but for summer, the demands are really noticeable since the energy demand of the extensive is rather higher compared the rest of the green types. Energy demands are 23.6, 12.3 and 8.2 kWh/m2/year extensive, semiintensive and intensive respectively. With respect to thickness of insulation layer, energy needs for heating and cooling decrease with the increase in insulation width for both green roof and traditional roof types in Fig. 14. Compared to black roof, semi and intensive green roof have more energy saving but the extensive roof consumes more energy because of more solar absorption and less evapotranspiration than the rest of semi and intensive in summer. In turn white roof, the green roof is not effective to reduce energy consumption compared to white roof due to having highly reflective properties as depicted in Fig. 15. On the other hand, the study conducted by Arkar et al. [90] shows that heat loss of green roofs is lower than the conventional roofs in heating period but without snow. To obtain the results, the authors designed lightweight mineral wool growing media used in green roofs

3.2. Vertical greenery systems Many researchers claim that the ambient temperature can be reduced by increasing the greened urban, however the horizontal space is considered insufficient to be planted in most cases. For this reason, the concept of living wall is a key factor to enlarge the greened area in urban environment by applying to building facades. Vertical greenery systems are capable of reducing the temperature through the building facade by shadowing, thermal insulation and transpiration cooling [22]. The use of greenery systems can mitigate waste heat thus greenhouse gas emissions, and stabilise notably hot ambient temperature arising from urban heat island effect. In addition to these, greenery systems provide better human health and new habitants for species as well as reducing air pollution and enabling sound insulation [92]. Overall, vertical greenery systems contribute to energy saving of the buildings through several mechanisms such as shading, evapotranspiration, thermal insulation and wind blockage effect [93]. 3.2.1. Shading effects and thermal insulation Shading effect is one of the most prominent benefits of greenery systems for energy saving as the vegetation contributes to solar radiation interception. The density of foliage and the coverage notably affects the shading performance on building facade. Green facade plays a

Fig. 10. Construction section of (a) reference cubicle (b) green roofs cubicle [29]. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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Fig. 11. Experimental cubicles (a), stratigraphy of the substrate and drainage layer (b) (rubber crumbs on the left and pozzolana on the right) [29].

Fig. 12. Electrical energy consumption of heat pumps for cooling (a) in July and heating (b) in January. Controlled temperature at 24 °C [29].

Fig. 13. (a) Energy needs during heating and cooling periods for three types of green roofs, (b)heating and cooling energy needs and annual energy use (bubble size) for three types of green roofs [84]. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 14. Heating (a) and cooling (b) energy demands [84].

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Fig. 15. Green roofs energy saving when compared to black and white roof solutions [84]. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Table 8 Traditional green facades. Previous works on vertical greenery systems as passive tool for energy savings [25,96]. Authors

Publication year

Type of study

Location

Köppen classification

Period of study

Orientation

Foliage thickness (cm)

External wall surface temperature reduction (°C)

Hoyano [2,96] Di and Wang [55] Köhler [97]

1988 1999 2008

Case study Case study Case study

Japan China Germany

Cfa Dwa Cfb

West West -

– 10 –

13 16 3 Summer; 3 Winter

Eumorfopoulou and Kontoleon [98] Stenberg et al. [99] Perini et al. [22] Susurova et al. [100] Cameron et al. [101] Bolton et al. [102] Haggag et al. [103] Yin et al. [94]

2009

Case study

Greece

Cfb

Summer Summer Summer/ winter Summer

East

25

5.7

2011 2011 2013 2014 2014 2014 2017

Case study Case study Simulation Experimental Experimental Experimental Case study

England Netherlands USA UK UK UAE China

Cfb Cfb Dfa Cfb Cfb Bwh Cfa

All year Autumn Summer Summer Winter Summer Summer

West-south North-west South North-south North East South

10 to 45 20 – – – – 4

1.7–9.5 (summer) 1.2 7.9 7–7.3 + 0.5 (winter) 6 Max:4.67 Average:2.56

facade to 120 cm distance for a foliage thickness of 20 cm. As shown in Fig. 17, the temperature difference is determined to be 1.2 °C compared to the bare wall for autumn season. In another study conducted by Susorova et al. [100], external and internal wall temperatures are compared to the bare facade temperature. According to the measured results, the mean temperature difference between external wall and bare facade is found to be in the range of 7.9–5.7 °C. On the other hand, the internal temperature difference between green and bare facade is measured to be 0.9 °C. Overall, it is understood that the vegetation coverage plays a crucial role in shading effects to minimise energy demand in cooling season. The impact of coverage on cooling demand is lower in night time compared to daytime because of absence of evapotranspiration and solar radiation [94]. The effect of insulation is strongly relevant to the properties of the layers used in the greenery systems such as soil (thickness and material), the air cavity and notably vegetation. The layer of vegetation on the wall serves as a thermal insulation material. Double-skin green facade has also some extra thermal insulation feature due to air cavity between vertical greenery system and facade. It is underlined in previous literature that the energy transfer through the wall can be reduced by about 0.24 kWh/m2 through such systems. Furthermore, the amount of energy consumed by an air conditioner is decreased by 20% by deploying a double-skin green facade depending on the properties of vegetation. It is also reported that a reduction of 0.5 ℃ in room temperature can reduce the electricity energy consumption by 8% [36]. According to the results of an experimental research carried out in Tokyo, vertical greenery system can decrease the wall temperature from 5 to 8 ℃. In another experimental research, it is observed that the maximum reduction is 8.4℃ in summer time with hot and humid air in

key role in coolıng performance of buildings as it is well-documented in literature [94]. Through the previous works, it is clear that the vegetation can be utilised to provide thermal comfort in dwellings. The temperature between exterior and interior walls can be reduced and regulated by shadowing effects of green facades [95]. Due to ventilation blind effects taken place by leaves of climbing plants, the warm air is dissipated from top and superseded by cool air from exterior [36]. According to the research carried out by Di and Wang [55], the average surface temperature under green facade is measured to be about 8 °C. Maximum temperature reduction is observed to be 16 °C compared to the front of the green facade. In the same study, it is found that the west-facing wall receives 189 W/m2 solar radiation. The leaves reflect 28 W/m2 and a part of 133 W/m2 is absorbed by leaves whereas the rest of solar radiation passes through the leaf layer. The heat fluxes concerning with average transpiration, thermal convection and long-wave radiation to the building facade are reported to be 42, 40 and 18%, respectively. The comparative results obtained from the preliminary analyses of traditional green facades are shown in Table 8 [25,96]. The table covers findings regarding the foliage thickness, temperature reduction, orientation and climate classification [97–103]. The classification presents annual and monthly temperature and precipitation, and also five major climate regions and subcategories based on the precipitation and temperature given in Fig. 16 [104]. By implementing traditional green facades, the surface temperature reduction is achieved to be 2 to 16 °C for cooling season. Based on the research carried out by Bolton et al. [102], the difference is observed to be 0.5 °C for the winter season. The research conducted by Perini et al. [105] is related to analysing the temperature differences compared to bare wall. In the case study, the temperature measurements are performed from green 926

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Fig. 16. World map of Koppen-Geiger climate classification [104].

The case study conducted by Perez et al. [27] takes place in Spain under Mediterranean climate zone. According to the results, the temperature reduction in double-skin green facade is measured to be 5.5 °C during April. Maximum temperature reduction is reported to be 15.2 °C. The air cavity in double-skin green facade enables the adjustment of environmental conditions for winter and summer. In this respect, higher temperature and lower relative humidity in winter, and lower temperature and higher relative humidity in summer are aimed to be provided by double-skin green facade. This feature can be attributed to the wind barrier and evapotranspiration effects of vertical greenery systems. In another research carried out by Perez [109], the temperature of indoor environment is found to be 3.8 °C higher in winter. On the other hand, 1.4 °C reduction is achieved in summer compared to conventional wall. A case study conducted by Perini et al. [105] which is related to double-skin green facade takes place in Autumn. The results of temperature reduction in double-skin green facade are presented in Fig. 18. The temperature difference between bare wall and double-skin green facade is roughly 2.7 °C. The output can be explained by the operation time since the ambient temperature in Autumn does not exceed 18 °C for the said region. Through another experimental research performed by Suklje et al. [110], the temperature of green facade is reported to be 4 °C lower than the black layer. In the study belonging to Koyama et al. [111], the correlation between foliage thickness and temperature reductions are identified and while increasing the percentage of foliage between 13 and 54%, the external surface temperature is observed to reduce in the range of 3.7–11.3 °C. The results clearly prove the effectiveness of greenery surfaces on thermal regulation of building envelope. Another type of vertical greenery systems as a building application is living wall design. Table 10 presents some earlier studies on living walls [113–121]. It can be seen from the data in Table 10 that several parameters are evaluated by researchers for thermal performance analysis of living walls such as external building wall temperature reduction, foliage thickness, air

Fig. 17. Direct green facade, temperature profile [105]. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Hong Kong [82]. The impacts of green facade on the energy demand of buildings are investigated by Kontoleon and Eumorfopolou [106]. Based on the green wall analysed, the vertical green system with the foliage thickness of 25 cm and 100% coverage rate is found to reduce the energy demand by 18.17% in cooling season and the energy saving related to the south face is lower than the building facing east orientation. Another research conducted by Stec et al. [107] compares the impacts of blind and bio-shade within double-skin green facade on energy demand and surface temperature. The temperature belonging to the case of bio-shade is observed to be 20 °C lower than that of the blind. On the other hand, the reduction in cooling demand is reported to be 20%, which is remarkable. By taking double-skin green facade into consideration it can be easily asserted that many researchers investigate this vertical greenery system comparatively as illustrated in Table 9 [108–112] in terms of different parameters such as region, orientation, foliage and air layer thickness [27,96]. Depending on the data, it is understood that the external wall temperature reduction is greater than 16 °C, which is notable. Temperature reduction feature of green facades is usually analysed as a function of foliage thickness, air layer and orientation. 927

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Table 9 Pervious study on the VGS as passive energy savings double-skin green facades [27,96]. Authors

Publication year

Location

Type of study

Köppen classification

Period of study

Orientation

Foliage thickness (cm)/coverage(%)

Air layer

External building wall temperature reduction (℃)

Hoyana [96] Wong et al. [108] Perini et al. [105] Perez et al. [22] Perez et al. [109]

1988 2010

Japan Singapore

Experiment Experiment

Cfa Af

Summer Winter

South-West –

55% –

– –

1–3 4.36

2011

Netherlands

Case study

Cfb

Autumn

–

10

20

2.7

2011

Spain

Case study

Csa

Summer

South-East

20

50–70

15.18

2011

Spain

Case study

Csa

All season

–

–

Max 15.18 in summer Average 5.55 in all season

2013

Slovenia

Experiment

Summer

South-west South-east North-west –

–

–

4

2013

Japan

Experiment

Cfa

Summer

South

54-52-29–52-13%

–

4.1–11.3-7.9-6.6-3.7

2015

China

Case study

Cwa

Summer

East,south

–

–

West, North

Csa

a) sunny b) cloudy c) rainy Summer

a)5 b)1to2 c)1to2

LAI in the range of 1.1 and 3.5

(a) 15

15 16.4 16

Suklje et al. [110] Koyama et al. [111] Jim et al. [112]

Perez et al. [93]

2017

Spain

Experimental

East West South

(b) 30 (c) 50

research, two different systems and two different building facades are used to evaluate the heat resistances which are in the range of 0.31 and 0.68 m2 K/W. On the other hand, the comparison of energy saving through double-skin green facade and green wall during cooling and heating period is evaluated by Coma et al. [96]. Their results reveal that the energy saving based on green wall is more notable than the doubleskin facade in both winter and summer. The reductions in energy consumption in summer for green wall and facade are determined to be 23.4 and 19.4%, respectively compared to the bare wall. The energy consumption also reduces by 4.2% through green wall implementation. Considering the double-skin green facade, there is no change in energy consumption in comparison with the bare wall. The research carried out by Perini et al. [119] indicates that the average energy saving is approximately 26.5% compared to reference wall during cooling period. Besides, the researchers evaluate the air temperatures for the cases of with and without green wall. 10 °C reduction in air temperature is achieved in the case of green wall. In another research, five different layers in terms of solar transmittance (the range from 0.43 to 0.14) are evaluated for potential reduction in solar radiation. The results show that from 40 to 80% of incoming solar radiation can be prevented through the said layers. 5–30% of the incoming radiation passes through the green wall to the interior section of the building [114,122]. In terms of wall surface temperatures, it is reported that the temperature differences between vertical greenery systems and bare wall are in the range of 11.6–20 °C [20,114]. The energy demand for air conditioning can be notably reduced by means of shading effects of vegetation. The reduction in energy consumption is reported to be about 19% [123]. The case study conducted by Hoelscher et al. [123] compares the performance of three different types of vegetation applied on different buildings. The results of the case study are presented in Fig. 20. Fig. 20(a) compares the temperature of leaves and exterior wall and Fig. 20(b) illustrates that the temperature differences between wall behind the vegetation and the bare wall. In the light of the results, it is concluded that not only the characteristics of plants but also the orientation of buildings are key factors for thermal regulation of building envelope. Maximum temperature reduction is observed to be about 14 °C between leaves and bare wall as shown in Fig. 20(a). If the wall behind the vertical greenery system is considered, maximum temperature difference is nearly 15 °C as depicted in Fig. 20(b). Fig. 21 presents

Fig. 18. Double-skin green facade temperature profile [105]. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

cavity, orientation, wind speed and heat flux reductions. The temperature difference between living wall and bare wall is found to be in the range of 1–31.9 °C. The range of the heat flux reduction is observed to be 30–70 W/m2 during daytime and 1.5 W/m2 during night. Eight different vertical greenery systems deployed in Singapore are investigated in terms of overall thermal performance of buildings and potential reductions in external surface and ambient temperature as shown in Fig. 19. Comparative results reveal that the highest thermal performance relating to the reduction of external wall and substrate temperature is provided by vertical greenery systems 3 and 4 [114]. Table 11 identifies the features of VGS 3 and 4. In another work, three main different experiments carried out by Chen et al. [117] demonstrate the comparative interior, external surface and interior space temperatures as well as relative humidity on bare walls and living walls. Based on experiments, it is observed that the external surface temperature reduction decreases by increasing the air layer. The maximum reduction in external surface temperature is approximately 21 °C whereas the interior temperature difference is measured to be 7.7 °C. The interior space temperature is reduced by 1.1 °C. The energy saving of the living wall is reported to be 0.4 kWh in comparison with the bare wall. The research conducted by Tudiwer and Kornejic [121] focuses on the heat resistance in winter season for a specific green wall. In this 928

2013 2014 2017

2017

2017

2017

Mazzali et al. [38] Oliveri et al. [118] Coma et al. [96]

Perini et al. [119]

De Jesus et al. [120]

Tudiwer and Korjenic [121]

2013

Chen et al. [117]

2013

2010 2011 2011

Cheng et al. [115] Jim et al. [116] Perini et al. [105]

Mazzali et al. [38]

2010

Wong et al. [114]

2013

2009

Wong et al. [113]

Nori et al. [96]

Publication Year

Authors

929

Experimental

Experimental

Case study

Case study

Case study

Case study

Experiment

Experiment

Experiment (thermal) Experiment Simulation Case study

Simulation

Type of study

Table 10 Pervious researches related to green walls [27,96].

Cfb

Csa

Cfb

Csb Csa Csa

Cfa

Cfa

Cfa

Cwa Cwa Cfb

Af

Af

Köppen classification

–

–

Summer Autum Winter

Summer

Autumn Summer Winter

Summer

Summer

Summer

(a) open 5 (b) close 3 Open 5 None 15 30 50 – 0.5–5.0 –

– – –

– – –

East South East West South South

South

South

(a) south-west (b) south-west

–

Adjustable 3–60

– –

– 15 4

–

–

Air layer

64% – 10

–

% variable

Foliage thickness (cm)/ coverage(%)

–

South

West

West South West

–

–

Late summer – Autumn

Orientation

Period of study

20 daytime 2 night time Summer: 2.7 Autumn: 2.3 0.44–3.52

Day:12/ night :3 15.1–31.9 4.5 6.5 16.5

(a) sunny day:24.6 (max) (b) cloudy day:3.1 (min) Day: (a) 12–20/ (b) 16 Night: (a) 2–3/ (b) 6

20.8

Day: 1 to 10.94 Night: 2 to 9 16 8.83 5

–

External building wall temperature (℃)

The humidity in Summer less than in Autumn Heat resistance 0.31 and 0.68 m2K/W

Average energy saving 26.5%

30 W/m2 heat flux reduction – Wind speed reductions of 0.46 m/s in the air layer 2.5 W/m2 heat flux reductions from building wall to air layer (a) indoor max 1.4 ℃ (b) indoor min 1 ℃ Heat flux reduction (a) 70 W/m2 (b) 1.5 W/m2 – – Average energy savings 2.96–4.20%

10–31% energy cooling load reductions –

Others

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Fig. 19. Control wall and the 8 VGS in Singapore [114].

the comparisons of temperature of the bare wall (exterior and interior) and wall behind the vegetation (exterior and interior). At daytime and night, interior greened wall is cooler than the interior bare wall whereas the change in temperature for exterior and interior wall for night and daytime is quite different. A remarkable temperature difference between the greened and the bare walls in a hot summer period is clearly illustrated in Fig. 21(c).

Table 11 Description of vertical greenery systems in Singapore [114]. VGS

System typology

Description

Plant size

3

Living wall – Grid and modular, vertical interface, mixed substrate Living wall – Modular panel, vertical interface, inorganic substrate

Plant panels embedded within stainless steel mesh panels inserted into fitting frames.

Small

Employed the Parabienta system with a patented growing medium (composite peat moss) as a planting media inlay. The peat moss panel encased in a stainless steel cage is hung onto supports lined with integrated irrigation.

Small

4

3.2.2. Evaporative cooling Green facades lead to cooling effects that occur through water evaporation taking place in plants and substrate. Evapotranspiration enables notable reductions in surface temperatures of building walls owing to green facades through the evaporation and transpiration of plant by absorbed heat energy. During the process, the air layer over

Fig. 20. (a) Mean temperatures of the greening and bare exterior building walls & (b) mean surface temperatures of the greened and bare exterior building walls on three investigated facades a) south south-west exposed facade greened with Parthencissus tricuspidata, b) east exposed facade greened with Hedera helix, c) west exposed facade greened with Fallopia baldschuanica [123].

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a similar study, Susorova et al. [100] underline that solar radiation, building orientation and air temperature have remarkable impacts on thermal behaviour of buildings by comparison with air relative humidity and wind speed. The research performed by Chen et al. [117] investigates the effects of open and close air layer on relative humidity by using ventilation systems. The relative humidity rates of sealed, open air layer and ambient are 88.2, 74.7 and 75.6% respectively as shown in Fig. 23. The authors stress that the evaporative cooling is enhanced by deploying sealed air layer. The excessive humidity is not suitable in living conditions. Therefore, natural ventilation provided by open air layer is essential to supply thermally comfortable indoor environments to the dwellers. 3.2.3. Blockage of the wind The blockage effect is related to vertical greenery systems including structure and vegetation. It is clearly underlined in previous literature that the wind effect on the building envelope is of vital importance in terms of thermal insulation performance of building facade. Wind speed becomes considerable with increasing building height as the wind speed reaches 100 km/h at 65 m above the ground. The combination of strong winds and high air temperature cause notable decreases in water vapour pressure. In this case, evapotranspiration effects become noticeable. The evaporation from the leaves is controlled by the vegetation according to a threshold value. If the extreme conditions are sustained, the vegetation stops evaporation by closing of stoma [92]. Evapotranspiration increases with wind speed by improving transfer of water vapour from plant to dry ambient air. However, vertical greenery systems can affect the wind speed by the side of the exterior wall and remarkably reduce temperature of the exterior wall because of the convection effects [22,127]. The cooling demand of building can be reduced by 25%, but for warmer winter season such as Hong Kong, the reduction in heating demand is not taken into consideration [36]. In a computer simulation research conducted by McPherson et al. [128], the effects of wind speed and solar radiation on the energy performance of dwellings located in different climate zones of the US are investigated. For cold climate region, the wind reduction is found to be beneficial for energy saving. The results also indicate that heating costs are reduced by 8% but annual cooling costs rise about 11% [27]. The impacts of vertical greenery on wind speed are analysed by Perini et al. [105]. According to the results achieved, the wind speed within the foliage decreases nearly 0.43 m/s in comparison to 10 cm distance from bare wall and the wind speed inside vegetation is assumed to be zero. For double skin facade, although the reduction in the wind speed inside foliage is measured to be 0.55 m/s, the increment of the wind speed in the air cavity is determined to be 0.3 m/s. Fig. 24 depicts that the change of the wind in living walls from 10 cm in front of the green facade to air cavity is 0.45 m/s. Through the heating period, cold wind results in the decline of temperature within the interior space. In this respect, blockage the cold wind is important to reduce energy consumption of the buildings. Greenery systems can be described as wind blocking structures as they are capable of diminishing the effects of wind on buildings. The feature of wind barrier depends on the density and permeability of the foliage, orientation of the facade and wind speed [13,22]. On the other hand, the wind speed has negative impact on thermal resistance of the building. By changing wind direction and speed with vertical greenery system, stagnant layer of air forms due to the leaves and foliage of the vegetation [98]. Owing to the said feature, energy demand of building is reduced by means of the decline of the wind strength [13]. The research carried out by Dinsdale et al. [129] reports that the reduction in energy demand of building is roughly 25% when integrated with greenery systems. It is understood from the aforesaid works that there are three prominent aspects of vertical greenery systems which are shading and insulation, evapotranspiration and wind blockage. The results clearly reveal that vertical greenery systems can be used as passive energy saving tool in buildings. Based on the previous

Fig. 21. Mean surface temperature of the vertical green system and bare (a) exterior building wall, (b) interior building wall as well as (c) temperature differences between the greened and the bare walls in a hot summer period. The results are related to west exposed facade building [123].

the plant is not only humidified but also cooled. In the evapotranspiration, 680 kWh energy is consumed for each cubic meter of water evaporation. As a result of this, vertical greenery systems can be considered as a passive air conditioner. By implementing vertical greenery systems in buildings, remarkable reduction in facade temperature can be achieved [36]. Depending on the plant characteristics for evapotranspiration, different results can be reached. For instance, while raising surface area of evapotranspiration, thermal comfort can be grown drastically by exploiting smaller and denser leaves of plants [124]. There are four main factors affecting the evapotranspiration rate such as (a) soil moisture, (b) plant type, (c) stage of plant development and (d) local climate consisting of solar radiation, wind speed, humidity and temperature [125]. Likewise, 60% of heat within the leaf can be removed by evaporation. This ratio is depending on especially wind speed and moisture content [107]. Larsen et al. [126] address three types of ratio; a) 60–75% having high speed and high moisture, b) 45–65% for lower wind speed and less moisture and c) 23–40% belonging to dry climates with both low wind speed and humidity. An experimental research conducted by Stec et al. [107] refers to the shading effect with ivy layer for double-skin green façade. It is understood from their results that the reduction in temperature inside air cavity is in the range of 20–35% in comparison with behind blind layers. After measuring the absolute humidity, the increment is reported to be between 0.5 and 1.8 g/kg. By taking into account the increase in the temperature inside the air cavity, any change is not observed in relative humidity. Similarly, in a case study, the average evapotranspiration of the building facing south direction is found to be in the range of 5.4 and 11.3 mm per day. 157 kWh/day of cooling energy is provided by the evapotranspiration rate [27]. In addition to these, the experimental double-skin green facade has a window in the office building and the interior temperature and humidity are measured in reference to without green facade. The reductions in indoor temperature are 5.6 and 3.5 °C during summer daytime and night respectively. By compared to non-double-skin green facade with office window, it is reported that the relative humidity is 4.7 and 13.7% higher for July and October [122]. In the case study carried out by Cameron et al. [101], the cooling effects of one climbing plant are analysed. The plant reduces the surface wall temperature by about 7 °C, and 40% reduction in cooling demand is provided by evaporative cooling. In the case study conducted by Hoelscher et al. [123], the cooling effects of the greenery systems are associated with the shading and evaporative cooling. The share of the cooling effects is illustrated in Fig. 22. It is clear from the data that the shading effects are much more dominant than those of transpiration except cloudy days, and the maximum shading effects are seen in drought stress. On the other hand, both orientation and plant type influence the share of cooling effects. In 931

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Fig. 22. Total cooling effect (transpiration & shading) as well as share of transpiration and shading on this cooling effect, each in relation to the max. cooling effect of the day, for a) a south southwest exposed facade greened with Parthenocissus tricuspidata on clear summer day b) a west exposed façade greened with Fallopia baldschuanica on a clear late summer day c) a cloudy day (same plant with b) d) plants under drought-stress (same plant with c) [123].

52.84 GJ for scenario 1. The energy savings are found to be 5.4% (green roofs) and 8.4% (green wall) for scenario 2 and 3, respectively. The energy demand for heating is notably greater than the cooling demand. The heating demand for January is seven times higher than annual cooling demand of the building. The energy savings in cooling are reported to be 3.2 and 7.3% for the case of green roof and green wall, respectively. Fig. 26 illustrates the heat transfer rates from green wall and green roof [13]. The heat gain through the walls is measured to be 0.916 GJ for both scenario 1 and 2, whereas the covered wall with vegetation is found to be 0.32 GJ during typical summer time. Heat loss from the wall is 0.803 GJ for scenario 1 and 2. While it is approximately 1.45 GJ for scenario 3. On the other hand, the heat gains through the roof of scenario 1 and 3 show a similar performance with 0.422 GJ. On the contrary, the heat gain of the green roof in scenario 2 is 24% lower than scenario 1 and 3. Heat gain in scenario 2 (0.141 GJ) is roughly one-third of the heat gain with respect to scenario 1 and 3. The heat loss through the roof without green vegetation is determined to be 0.388 GJ for scenario 1 and 3. It is clearly seen that the heat loss from the green roof is notably greater than the building having no green vegetation on the roof. The heat transfer through the walls and roof during heating season can be viewed in Fig. 27 [13]. The heat loss from the green wall (7.64 GJ) is reduced by almost 20% compared to the other scenarios. The minimum heat loss from the roof takes place in scenario 2 with 2.63 GJ.

Fig. 23. Comparison between living wall with and without ventilation [117].

literature, it can be concluded that a significant amount of energy consumed in dwellings can be mitigated in a cost-effective manner by installing vertical greenery systems on building facade. 3.3. Comparison of the greenery systems in terms of energy demand The simulation research performed by Feng and Hewage [13] evaluates the impacts of greenery systems on building energy demands in Canada. Three different scenarios are applied to the same building. In the first scenario no green roof or green wall is used. In the second scenario, only green roof is installed on the building roof whereas the third scenario also includes a vertical greenery system. Fig. 25 illustrates the energy consumption for heating and cooling during January and December. In December and January, the energy savings are reported to be 0.6 and 2.1%, respectively in terms of heating. In terms of cooling demand, the month of July reaches the peak consumption of

4. Cost Francis and Lorimer [130] emphasize that the main challenge of the green roofs and facades is their maintenance and investment costs as 932

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Fig. 25. Energy consumption of the studied green building [13]. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 24. Vertical greenery system, wind speed profile [105]. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

well as installation issues. Therefore, the importance of cost-effective greenery systems is addressed by many researchers like Feng and Hewage [13]. It is reported that the installation of the green roof may not be economically viable except the goal of energy saving. By using the full green roof, thermal performance of a building increases yielding to an energy saving of about $215/year, which is promising. However, this value might not be seen attractive by some people due to the long payback periods. Consequently, the greenery systems are not reasonable to be installed into the buildings in cold climates because of low heating energy saving [13]. Despite the negative aspects, the greenery systems can notably contribute to the economy, environment and social life of the city by decreasing air pollution, heat islands effects and noise pollution [23,131]. Moreover, foliage prevents the high angle sunlight in summer and leads low angle light to enter the building in heating seasons. Furthermore, deterioration and thermal stress accused of solar radiation on the external surfaces such as panting and cladding can be ceased by employing greenery systems. In this respect, the costs for maintenance of buildings with greenery systems can be ignored [132].

Fig. 26. Heat transfer through the walls/roof of the studied green building [13]. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 27. Heat transfer from the wall/ roof of the studied green building during heating season [13]. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

933

934

0.86 0.41

2.15 2.60 30.12 N/A 7.03–7.11 4.73–4.81 4.77–4.85 0.98 0.14 0.95 0.99 Mediterranean, Italy N/A Vertical greenery system Marchi et al. [37]

Modelling carbon flux of the entire system

Green roofs Luo et al.[136]

30 cm

Green roofs Whittinghill et al. [135]

Measured from the above, below-ground biomass, and substrate

Subtropical, China

Mixed sedum species Mixed prairie species Mixed perennial plant species Mixed herb species L. vicaryi (Perennial plant) N. auriculata (Flowering plant) L. spicata (Flowering plant) 1. Zoysia matrella (grass) 2. Sedum spurium (Succulent plant) 3. Salvia nemorosa (Herbaceous plant) 4. Rosmarinus officinalis (Herbaceous plant) 5. Geranium sanguineum (Flowering plant) 6. Carex brunnea (Herbaceous plant) 7. Fatsia japonica (flowering plant) Temperate, USA 20.4 cm

0.375 4 Sedum species Temperate, USA Green roofs

Method

Getter et al. [133]

There is an unequivocal impact of the indoor environment on human health and psychology. By improving the quality of indoor

Type of greenery systems

5.2. Indoor environment quality

Author(s)

Table 12 Reviews of researches about Carbon sequestration of greenery systems [40].

Substrate depth

Climate and location

Vegetation type

Carbon emissions are crucial for the world today since they dramatically affect not only human beings but also living beings. Due to the hazardous environmental effects of carbon emissions, some developed countries and non-profit organizations concentrate on this issue for urgent minimisation of carbon emissions. From this point of view, mitigating global energy consumption and expanding greenery areas are considered as a solution by many authorities. The greenery systems have profound impacts on the reduction in carbon emissions owing to less energy consumption and photosynthesises process using carbon in the atmosphere. A research indicates that the energy saving with using vertical greenery systems is about 2.65 MW h × 106 per year and at the same time, a significant amount of CO2 emission can be reduced because of the less energy consumption. Based on a report from China Light and Power Group in 2009, roughly 0.83 kg CO2 is emitted into atmosphere for per kWh electricity generation. By deploying doubleskin green facade, 2.2 × 103 kg of CO2 emissions can be reduced per year. According to US Department of Agriculture, a medium size of mature tree can reduce approximately 133 kg of CO2 per year [36]. There are two ways used to describe the carbon reduction which are called as above-ground and below-ground. the differences between above and below ground are to form leaves and stems, and plant roots and growing medium respectively. The amount of CO2 used by plants increases from the morning and hit the peak in midday. Toward the sunset, CO2 amount decreases rapidly and the maximum increase of CO2 in the atmosphere arise at nigh time. In terms of the stabilisation of carbon emissions, greenery systems have beneficial effects to assimilate carbon in the environment. The plants can be defined as carbon sink instead of carbon source. Throughout growing time of the plants (May to August for Europe), the absorption of carbon from the atmosphere reaches the highest level [40]. The study in US with related to green roofs demonstrate that excessive temperature and water shortage prevent the carbon sequestration from environment by above-ground. However, in the first growing time, the below-ground accumulates a significant amount of carbon [133]. After accumulating carbon through photosynthesis, the plant converts the carbon to biomass. the converted biomass gathers different parts of the vegetation such as stem 47%, leaf 45% and foliage 41.5%. Different types of vegetation, also, capture various amounts of carbon contents. For instance, shrubs and trees include highest level of carbon nearly 50%, carbon contents of herbaceous perennials are roughly 42.69% and lastly 45% of carbon is contained by grass [134]. The growing medium also accumulates carbon that is required to grow the vegetation. “The amounts of CO2 within soil depends on decomposition of plant residues, the generation of microbial biomass, the mortality of microbial biomass and the soil respiration” [37]. Table 12 illustrates the amount of carbon sequestrations (kg carbon/ m2/year) based on the greenery systems [135,136]. Through the literature, it can be easily asserted that the performance of green roofs on carbon capture is much more remarkable than that of vertical greenery systems as shown in Table 12. Annual carbon accumulation of green roofs is in the range of 0.375–30.12 kg carbon/m2, whereas this value for vertical greenery systems is between 0.99–0.14 kg carbon/m2. Based on the study focused on CO2 sequestration by Marchi et al. [37]. It is reported that the captured CO2 by the plants is between 0.44–3.18 kg carbon/m2 for vertical gardens. It is underlined that the annual average accumulation of CO2 reaches the level of 13.41–97.03 kg carbon/m2 for 98 m2 of vertical greenery system.

6 cm

5.1. Reduction of carbon emissions

Measured from the above, below-ground biomass, and substrate Measured from the above, below-ground biomass, and substrate

Carbon sequestration (kg C/m2 year)

5. Environmental benefits

0.32

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to minimise the excessive sound causing some health problems. Insulation materials and also vegetation comprised of greenery systems in both indoor and outdoor of the buildings can be considered as a key solution in this respect [148]. Some researches interestingly reveal that while increasing the interaction between humankinds and vegetation, and other living species, the mood of human beings also changes positively [149,150]. Grinde et al. [151] point out that by using greenery in the building, the indoor environment quality can be enhanced through reducing air pollution, noise and synthetic materials used in furniture. The greenery utilised outside the buildings affects the human activity and mood positively, as well. Overall, it can be concluded that the greenery applied in interior or exterior improves the performance, well-fare and human health.

environment, the conditions of the occupants in terms of psychological and physiological well-being can be maintained at desired levels [137,138]. Furthermore, the indoor quality influences the productivity of employees and the life qualities of dwellers lived in urban areas [138]. Physical environmental conditions and psychological components notably affect the indoor environment quality. The components of physical environment are classified as physical conditions (temperature, light, noise and air quality), space (plan and privacy), and aesthetic aspects (colour and quality). In view of all that has been mentioned so far, the comfort of the occupants living in the buildings depends on human health and mood as well as environmental factors [138,139]. Indoor air quality is defined as the degree of air quality of the building associated with interior space. Air quality might influence the occupants both positively and negatively. The positive effect of the air quality is to improve the work performance of occupants in office buildings especially dealing with the tasks of reading, typing and mathematical analyses [140,141]. Regarding the negative effect, there are some health problems based on poor air quality to reduce the productivity in the offices. Common health problems can be exampled as sick building syndrome symptoms, allergies and asthma symptoms [138]. The measurement of air quality is really complicated as the process highly depends on several physical and chemical parameters. These parameters are relative humidity, temperature and level of air contaminants. It can be easily asserted that the exterior conditions of the buildings have notable impacts on these parameters. Indoor air quality is improved by using heating, ventilation and air-conditioning (HVAC) systems to regulate relative humidity and filtrate air contamination [142]. On the other hand, indoor active living wall also provides same advantages through passive living wall but both the installation and maintenance are complex and expensive because of working together with air-conditioner. Evaporation and transpiration from irrigation and vegetation respectively bring about the cooling effect of indoor living wall on interior space areas. There is no need to air filtration for room due to the production Oxygen and bio-filtration [1,143]. The emissivity and reflectance also influence the thermal behaviour of facade in terms of energy demand in buildings. During cooling season, the reflectance should be higher to reduce the energy consumption and throughout the winter season, the emissivity is increased to maintain the warm interior temperature [144]. The “unwanted sound” is prescribed as noise by World Human Organization. While designing the building, the acoustic performance parameters are required to be analysed notably for office buildings since the noise or the poor acoustic performance reduce the productivity of the employees [145]. The source of noise is split into two parts that are exterior and interior sounds. The exterior sections consist of land and aviation traffic, public and machinery. The conversation between employees and the sound of electric equipment such as PC, telephone and so on constitute the interior noise for office buildings [26,146]. Similar to the offices, the residences are also exposed to the same noise pollution, but the equipment causing the interior sound is different from the office equipment such as household appliances. The changes in acoustic and temperature cause the same effects on the occupants. For instance, the increment of 1 °C has the equal impact on the productivity with the change in noise of 2.6 dB [147]. The excessive sound might cause a stressful environment for occupants of the buildings. As mentioned in pervious researches, the occupants, exposed to enormous noise from traffic or transport, have serious health problems such as high blood pressure and stress hormones [138]. Similarly, office and household appliances consisting of refrigerator, air conditioner, telephone, PC, mobile phone and so on, have similar level of stress causing reduction in the productivity of the occupants [145]. As can be understood, the noise occurring both indoor and outdoor triggers long-lasting health problems due to stress and anxiety. To maintain desirable comfort conditions in terms of acoustic of the building, there are some materials that can be applied into the building

6. Health Plants contribute to thermal performance, energy saving and reduction of carbon emissions in urban areas. Through the beneficial features of vegetation on human health and psychology, dwellers living in urban environment can reach psychological, mental and social wellbeing, and expanding the greenery areas can reduce heat island impacts due to energy consumption. The installation of greenery systems to urban dwellings offers biodiversity and long-running ecosystem to form urban greenery areas [65]. Human health is noticeably influenced by excessive heat stress. Based on researches, mortality rate increases among elder people (≥ 65) due to the excessive temperature such as the increment of daily minimum air temperature from 20 °C to above 30 °C. The excessive temperature has also an effect in reducing sleep quality of dwellers [123]. Fernandez-Canero et al. [152] give information about the cost-effective methods to cool indoor environment. It is reported that the indoor temperature can be reduced by 6 °C while increasing the indoor relative humidity by 15%. 7. Testing procedure of green roofs and facades There are different techniques in literature for experimental performance assessment of green roofs and facades. However, the most common testing procedure is co-heating test to evaluate the thermal behaviour of the buildings at pre and post-retrofit when a greenery surface is considered on building envelope or roof. Co-heating tests are conducted in heating season [153]. Within the scope of the method, the enclosure, which corresponds to the internal volume of air in a building, is kept at a constant temperature in the range of 20–25 °C through a temperature-controlled heating system. Heat transfer occurs from the building elements to the outside since the external air temperature is lower than the temperature of internal air [154]. The rate of heat transfer from the building elements is measured by sensitive heat flux sensors (q, W/m2) as well as the temperature difference (ΔT, K) across the said elements. Then, the overall heat transfer coefficient (U, W/m2 K) is achieved by the ratio of heat flux from the building element to the temperature difference [155]. Since the thermal resistance (R, m2 K/W) is the inverse overall heat transfer coefficient, the lower Uvalue the greater thermal resistance across the building element. In this respect, accurate and reliable determination of U-value for the building envelope with and without greenery system is of vital importance to evaluate the energy saving potential of green roofs and facades. For optical and thermal comfort performance assessment of buildings retrofitted by green roofs and facades, light meters, UV meters and IR meters are commonly utilised as well as appropriate CO2 concentration ratio, relative humidity and temperature measurements of internal air. At pre and post-retrofit, light transmission, UV and IR penetration through the building elements are experimentally determined through sensors to have an idea about the UV and IR blockage rate of greenery surfaces as well as transmitted light intensity in living spaces. Internal air, relative humidity and CO2 concentration ratio measurements reveal the level of thermal comfort achieved for indoor 935

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Fig. 28. The characteristic turf houses of Iceland as a pioneer application of green buildings [156]. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

renders 75% higher heat storage.

environment after the building retrofitted by green roofs and facades. The data is usually gathered time-dependently through sensitive datalogging systems.

• External surface temperature is observed to reduce in the range of

8. Conclusions

•

In this study, green roofs and facades are comprehensively analysed in terms of different aspects. The review is presented in a thematic and holistic manner for an easier understanding of the findings from experimental and numerical works in literature. The main goal of the research can be expressed as evaluating the role of greenery systems to mitigate building-related energy consumptions and carbon emissions. Within the scope of this purpose, the research is split into several subtopics for an elaborative analysis. Greenery systems as energy saving tool in buildings, multifunctional benefits of green roofs and facades such as evapotranspiration, thermal insulation, shading and thermal comfort features, wind blockage ability and evaporative cooling effect of greenery surfaces to reduce cooling demand in buildings are investigated in detail. Some characteristic findings achieved from the research can be illustrated as follows:

• • • •

• Heat penetration from the building roofs in summer can be mitigated by about 80% via green roofs. • Green roofs consume 2.2–16.7% less energy than traditional roofs in summer time. • Heating demand of buildings can be reduced by 10–30% through greenery surfaces. • The temperature difference between conventional and greens roofs • • •

3.7–11.3 °C while increasing the percentage of foliage between 13% and 54%. The temperature difference between living wall and bare wall is 1–31.9 ℃. The range of the heat flux reduction is reported to be 30–70 W/m2 during daytime and 1.5 W/m2 during night. Wind speed within foliage decreases nearly 0.43 m/s compared to 10 cm distance from bare wall and the wind speed inside vegetation is found to be zero. Greenery systems can provide an energy saving of about $215/year depending on regional and climatic conditions. Annual carbon capture of green roofs is in the range of 0.375–30.12 kg carbon/m2, whereas it is between 0.99 and 0.14 kg carbon/m2 for vertical greenery systems. Green roofs and facades do not require complex irrigation, waterdraining and hydro-insulation systems. Irrigation is usually conducted through time-controlled and low-power DC/AC pumps. Flexible coats are utilised for hydro-insulation. Water-draining is performed by conventional piping elements, which is low-cost.

Overall, green roofs and facades are sustainable, energy-efficient and eco-friendly structures toward low/zero carbon building standards. They are also attractive in terms of architectural and aesthetic aspects. The characteristic turf houses of Iceland [156] shown in Fig. 28 proves this output. It is useful to identify the interaction between the thermal effects brought by the trees to cities and the thermal influences of green roofs and facades on buildings. Urban forests are capable of keeping cities within a healthy temperature range as reported by Lenart [157]. Expanding the greenery surfaces in cities by about 10% or more can help minimise the local temperature rise projected for the upcoming future. In this respect, a similar tendency is expected for buildings when retrofitted with green roofs and facades.

in winter is found to be about 4 °C, whereas it is about 12 °C in summer. Annual energy demand of buildings is notably affected by plant intensity. Annual energy demand is determined to be 23.6, 12.3 and 8.2 kW h/m2 for extensive, semi-intensive and intensive greenery surface. Bare roof albedo of 0.15, in comparison with 0.30 of green roof, 936

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