Highlights
• Review of existing research on green roofs and green walls benefits and costs.
• Focus on building scale benefits, urban scale benefits and life-cycle costs.
• Variability identification and average quantification assessment.
• Fewer studies have quantified green walls benefits and costs.
• There is high variability in data across all benefits and costs.
Abstract
... Green infrastructures, like green roofs and green walls, have multiple associated environmental, social and economic benefits that improve buildings performance and the urban environment. Yet, the implementation of green roofs and green walls is still limited, as these systems often have additional costs when compared to conventional solutions.
Recent studies have been comparing these greening systems to other solutions, balancing the long-term benefits and costs. Also, there is significant research on green roofs and green walls benefits. Although, green roofs and green walls economic analyses don't include all benefits due to measuring difficulties. The associated uncertainty regarding the quantification of the benefit makes it difficult to compare the research outcomes.
This paper aims to provide a research review of existing benefits and costs of different types of green roofs and green walls. These were divided between building scale benefits, urban scale benefits and life cycle costs, focusing on the identification of results variability and assessment of their average quantification.
The analysis shows that in general, there [is little] ... data regarding intangible benefits, as the promotion of quality of life and well-being. Also, there are still few studies quantifying green walls benefits and costs. High variability in data is mostly related to the different characteristics of systems, buildings envelope, surrounding environment and local weather conditions.
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Green roofs, also known as ecoroofs, living roofs or vegetated/vegetative roofs, refer to all systems which enable greening roofs, allowing the growth of different types of vegetation on top of buildings. Green roofs include a set of layers that protect the support and improve system performance. They commonly include the vegetation, growing medium (substrate), filter layer and drainage layer. These solutions are normally applied over waterproofed roofs with a root barrier and the insulation layer. Also, the green roof system must be applied over a layer with a minimum slope of 2% to drain the excessive rainwater along the roof. Compared to conventional roofs, green roofs tend to be more expensive, requiring extra maintenance depending on the vegetation type and irrigation needs. If the substrate thickness is significant the system may need an increased weight load capacity of the roof to be able to be implemented.
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In general, green roofs are more energy-efficient than black roofs in all climates. Table 1 shows that maximum energy savings are obtained when comparing intensive green roofs to black roofs, especially over non-insulated roofs, reaching to 84% energy savings in the cooling season and 48% in the heating season. Also, in Csa climate, green roofs can be more effective than white roofs in the heating season, especially in buildings that are not insulated. However, in this climate green roofs are not as effective in the cooling season, except for intensive green roofs due to substrate thickness.
In the Tropical climate (Af), where only cooling is necessary, tests developed by Wong et al. in a commercial building in Singapore, demonstrate that extensive green roofs show higher energy savings than black roofs also in non-insulated buildings, obtaining average energy savings of 63%. In the Tropical desert climate (Bwh) the application of extensive green roofs was tested by Zinzi and Agnoli. Results from Cairo city in Egypt, demonstrate that in the heating season extensive green roofs are more effective than black and white roofs, reaching 22% and 52% energy savings, respectively. However, in the cooling season results are not as promising, as white roofs reveal to be more effective. In the Semi-arid hot climate (Bsh) a semi-intensive green roof was analysed by Ascioni et al.. Energy savings for cooling of this green roof reached an average of 7,25% compared to a traditional roof and were not as effective as white roofs. In cold climates (Cfb and Dfb), where winters require more heating loads, all types of green roofs have proven to be more effective than black and white roofs. In summer green roofs demonstrate to reduce energy loads when compared to black roofs but not as much as in warmer climates. In Table 1 maximum energy savings were also obtained with extensive green roofs in the Oceanic climate (Cfb) revealing to be highly effective in the cooling season reaching to 84% in insulated buildings and 100% in non-insulated buildings, but not in the heating season.
More recently some authors have studied green walls potential to improve energy efficiency in buildings, due to surface temperature reduction and shadowing provided by plants. Studies demonstrate that, in Csa climate, when compared to a conventional wall, green façades can have an energy efficiency of 34% and living walls 59% to 66%, during the cooling season.
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Table 2 shows some research studies demonstrating that the addition of an extensive green roof to a building can have a significant impact on sound transmission reduction. The decrease in noise levels varies from 5 dB to 20 dB, depending on the frequencies. However, sound transmission reduction may vary according to the type of support, substrate composition and its depth, water content, and types and stage of plant species development
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Some authors have proven that green walls also have good sound absorption properties compared to other cladding materials. Like green roofs, green walls sound absorption depends on green coverage type, variety of plant species and materials used in the system. Pérez et al. obtained also a 2 dB increase in soundproofing with a living wall and a 3 dB increase with a green façade
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New solutions and technologies for water recovery and water treatment can significantly contribute to buildings reduction of potable water consumption. Greywater decentralized treatment and recycling requires simpler treatment systems and reduces the impact on urban wastewater management. Local greywater recycling can achieve potential water savings of 9%–46% within the household.
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Results indicate that green walls can remove 80%–90% of total suspended solids (TSS), over 90% of biological oxygen demand (BOD), 30%–50% of total nitrogen (TN), 15%–30% of total phosphorus (TP), 30%–70% of chemical oxygen demand (COD) and 20%–80% of Escherichia coli (E. coli).
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Current conventional roofs include a waterproofing membrane that usually lasts between 10 and 20 years, while green roofs could ensure an in-service life of 50 years or more. However, old green roofs exist in Berlin for more than 90 years without being replaced
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Current conventional roofs include a waterproofing membrane that usually lasts between 10 and 20 years, while green roofs could ensure an in-service life of 50 years or more. However, old green roofs exist in Berlin for more than 90 years without being replaced
Green roofs and green walls add property value to buildings. Several authors use several methodologies (e.g. hedonic pricing method) to estimate how the presence of green spaces, like green roofs and green walls, influence property value. Fig. 4 presents the results obtained by different authors regarding the average property value increase due to the presence of green areas, including green roofs and green walls. Based on these results an average increase of 8,24% was determined. Ichihara and Cohen estimated an increase of 16,2% in rental prices in buildings with green roofs. Perini and Rosasco identified a 2%–5% increase in property value due to the presence of green walls.
... Solutions such as green roofs or green walls, can contribute to evaporative cooling from evapotranspiration, shading, increase the surface albedo (0,7 to 0,85 versus 0,1 to 0,2) and emissivity, and complement the building insulation. The [Urban Heat Island] (UHI) mitigation potential of greening systems is conditioned by several variables as i) climate conditions (solar radiation, outdoor temperature and humidity, wind and precipitation); ii) optical variables (surface albedo and emissivity); iii) thermal variables (thermal capacity and thermal transmittance); iv) and hydrological variables (latent heat loss through evaporation by plants and soil) ... Overall, the average reduction of the surrounding temperature collected in these studies is l, 34 °C, varying between a minimum and a maximum average of 1 °C to 2,3 °C.
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Table 3 shows the results obtained by different authors regarding the potential of living walls to contribute to urban noise reduction. Results indicate a variation along different frequencies and between systems. A total average urban noise attenuation of 5,5 dB was obtained, ranging between 0 and 10 dB.
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Regarding extensive green roofs, high variability was identified across studies, ranging between an average minimum stormwater runoff reduction of 33% and a maximum average of 81%. Overall, extensive green roofs contribute in average 57% to decrease stormwater runoff.
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Table 3 shows the results obtained by different authors regarding the potential of living walls to contribute to urban noise reduction. Results indicate a variation along different frequencies and between systems. A total average urban noise attenuation of 5,5 dB was obtained, ranging between 0 and 10 dB.
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Regarding extensive green roofs, high variability was identified across studies, ranging between an average minimum stormwater runoff reduction of 33% and a maximum average of 81%. Overall, extensive green roofs contribute in average 57% to decrease stormwater runoff.
Concerning to intensive green roofs, Fig. 7 also shows a 79% average stormwater runoff retention, which represents a 22% higher water retention capacity compared to extensive green roofs. This increase may be due to substrate depth. This difference is also notorious when comparing stormwater runoff average results between green roof solutions in similar climates (Cfa and Cfb), representing a 31% difference in Cfa and 10% in Cfb climates.
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Fig. 10 illustrates the pollutants removal capacity of green roofs obtained by different authors through dry deposition. All values were converted to grams (g) per area unit (m2) per year. These studies show higher average removal capacity of O3 (1,96 g/m2.year), PM10 (1,47 g/m2.year) and NO2 (1,03 g/m2.year). Significant average results were also obtained for SO2 (0,41 g/m2.year), CO (0,41 g/m2.year).
Fig. 10 illustrates the pollutants removal capacity of green roofs obtained by different authors through dry deposition. All values were converted to grams (g) per area unit (m2) per year. These studies show higher average removal capacity of O3 (1,96 g/m2.year), PM10 (1,47 g/m2.year) and NO2 (1,03 g/m2.year). Significant average results were also obtained for SO2 (0,41 g/m2.year), CO (0,41 g/m2.year).
Bianchini & Hewage [69] mention two examples of incentive policies. In New York City a tax reduction incentive is applied when building owners include extensive and intensive green roofs in their properties, covering at least 50% of the total roof area. This way can benefit from a tax reduction of 43€/m2 (49 USD/m2), reaching a maximum of 90.000 € (approximately 101.700 USD). Also, the city of Portland applies a stormwater fee discount of 35% for properties that reduce their impervious surfaces, including the application of green roofs.
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Fig. 11 presents the installation cost of extensive, semi-intensive and intensive green roofs respectively, obtained by different authors. Results demonstrate a high variability in systems cost between countries. An average installation cost of 99 €/m2 (112 USD/m2) was identified for extensive green roofs. Semi-intensive systems tend to be more expensive than extensive green roofs as they usually include a wider variety of plant species. Results indicate an average installation cost of 130 €/m2 (147 USD/m2) for semi-intensive green roofs. Intensive green roofs tend to be more expensive to install, as they include more material and heavier plants (e.g. small trees) reaching an average installation cost of 362 €/m2 (409 USD/m2).
Fig. 11. Installation cost (€/m2) of extensive green roofs
Fig. 12 refers to the results obtained by different authors regarding the installation cost of green façades and living walls. Significant differences were also obtained between green wall systems. Green façades have an average installation cost of 190 €/m2 (215 USD/m2), as they require less material. Living walls can have more differences in cost, as there are several different systems on the market. An average installation cost of 750 €/m2 (848 USD/m2) was obtained for living walls.
Fig. 13 shows the operation and maintenance costs of extensive, semi-intensive and intensive green roofs obtained by different authors. The analysed studies demonstrate that maintenance cost can vary significantly from country to country. An average maintenance cost of 4,28 €/m2/year (4,84 USD/m2/year), 7,77 €/m2/year (8,78 USD/m2/year) and 5,64 €/m2/year (6,37 USD/m2/year) is obtained for extensive, semi-intensive and intensive green roofs, respectively.
There is still few data regarding green walls maintenance cost. Cost may vary according to nutrients and irrigation needs and plants species used. Fig. 14 shows that, according to the most recent literature, the average maintenance cost for green facades is 5,57 €/m2/year (6,29 USD/m2/year) and for living walls is 18,98 €/m2/year (21,45 USD/m2/year).
Green roofs and green walls systems disposal costs normally include all system elements including the removal of all plants, substrate and support layers, transport to landfill and dump taxes [192]. Some studies take into consideration the roof replacement, including as well the recladding and renewal of the support surface.
Fig. 15 presents the disposal cost of extensive and intensive green roofs obtained by different authors. An average disposal cost of 12 €/m2 (14 USD/m2) was identified for extensive green roofs and 26 €/m2 (29 USD/m2) for intensive green roofs.
by MariavManso 2, InêsvTeotónio 2, Cristina Matos Silva 1 Carlos Oliveira Cruz 1
1. Instituto Superior Técnico, University of Lisbon, CERIS, Department of Civil Engineering and Architecture and Georesources, Av. Rovisco Pais, 1049-001, Lisbon, Portugal
1. Instituto Superior Técnico, University of Lisbon, CERIS, Department of Civil Engineering and Architecture and Georesources, Av. Rovisco Pais, 1049-001, Lisbon, Portugal
2. Instituto Superior Técnico, University of Lisbon, CERIS, Av. Rovisco Pais, 1049-001, Lisbon, Portugal
Renewable and Sustainable Energy Reviews via Elsevier Science Direct
Volume 135; January, 2021, 110111
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