Benefits and Ecosystem Services Provided by Green Roofs: Comparison
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Water-sensitive urban design (WSUD) strategies have been widely used in cities to mitigate the negative consequences of rapid urbanization and climate change. Whilst WSUD is a popular term in Australia and a few other countries, blue-green infrastructure (BGI) is also a commonly used term. Green roofs (GR) have been regarded as a promising BGI strategy to deal with these globally growing concerns of urbanization and climate change. This BGI strategy, sometimes called a living roof or vegetated roof, offers a wide range of environmental, social, and economic benefits compared to conventional roofs (CRs). The stormwater management and the mitigation of heat-related issues are the two primary GR benefits that have attracted the attention of researchers the most. Additionally, GR is capable of reducing energy consumption, improving air and runoff quality, and alleviating noise pollution.

  • WSUD
  • green roofs
  • ecosystem services
  • quantify benefits
  • large-scale implementation

1. Introduction

Water-sensitive urban design (WSUD) strategies have been widely used in cities to mitigate the negative consequences of rapid urbanization and climate change. Whilst WSUD is a popular term in Australia and a few other countries, blue-green infrastructure (BGI) is also a commonly used term. Green roofs (GR) have been regarded as a promising BGI strategy to deal with these globally growing concerns of urbanization and climate change. This BGI strategy, sometimes called a living roof or vegetated roof, offers a wide range of environmental, social, and economic benefits compared to conventional roofs (CRs) [1]. The stormwater management and the mitigation of heat-related issues are the two primary GR benefits that have attracted the attention of researchers the most. Additionally, GR is capable of reducing energy consumption, improving air and runoff quality, and alleviating noise pollution [2]. The vegetated roof also enhances the aesthetic aspects of a building and urban ecology by converting impervious roof surfaces to green spaces [3].
The configurations of a GR vary according to its geographical location, requirements, and the purposes for which it is built. Generally, a typical GR consists of the following layers (from the bottom to the top): waterproofing membrane, drainage layer, filter layer, substrate (growing medium), and vegetation layer. The insulation layer is optional and added when GRs are implemented on existing roofs (i.e., retrofitting a green roof). In the event that long-rooted plants are applied, anti-root layers are compulsory to protect both the GR system and its underneath structure [1]. In terms of GR types, GRs are categorized into intensive GRs (IGRs) and extensive GRs (EGRs). The main difference between these two groups is the substrate depth. While the IGR substrate is more than 20 cm thick, the EGR growing medium is thinner, with less than 15 cm [4]. Consequently, IGRs are suitable for the vast majority of plants, whereas EGRs are only able to support the survival of drought-resilient plants, such as succulents. By contrast, EGRs are much more prevalent than IGRs for several reasons, including lesser efforts for maintenance, lighter weights, and lower construction costs [5,6][5][6]. A semi-intensive GR (SIGR) is a type of GR with an intermediate substrate depth between those of EGRs and IGRs. A moderate substrate thickness allows a SIGR to accommodate small shrubs [7].

2. GR Types

As can be seen in Figure 1, the number of papers applying EGRs are 6–7 times more than those applying IGRs and SIGRs. It is well known that EGRs have plenty of advantages that have led to their widespread implementation. On the contrary, the implementation of IGRs faces many challenges, as they require high structural load bearing capacities and high amounts of maintenance. However, with the consideration of the capability of providing ecosystem services alone, IGRs remarkably outperform EGRs. It is also worth noting that SIGRs appear to have a combination of advantages taken from both EGRs and IGRs. Their growing media are thinner and lighter than that of IGRs, and they support a broader range of appropriate plants than what EGRs can. Therefore, SIGRs not only eliminate the disadvantages of IGRs and EGRs but also optimize the GR benefits. Further research should study the potential of intensive and semi-intensive types of GRs, so that they are not disregarded. More efforts to explore alternative materials for light-weight GR systems are also encouraged.
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Figure 1. Distribution of papers across each type of GR (EGR: extensive green roof, IGR: intensive green roof, and SIGR: semi-intensive green roof).
Attempts to establish hybrid GR-based experiments were found to be negligible in this work. Such hybrid GRs are highly recommended because of the encouraging results achieved by them. For example, Hui and Chan [99][8] found that the Ts of a hybrid photovoltaic GR (PV GR) was 5 °C cooler than that of a traditional GR, due to the shading effect of the PV panels. An increase of 4.3% in power output from the PV panels also resulted from this combined system. Another example of the enhanced performance of hybrid GRs is provided by Chemisana and Lamnatou [93][9]. They found a maximum ΔTs of 14 °C as well as a maximum increase of 3.33% in solar energy from the PV system. Yeom and La Roche [37][10] evaluated the combination of a GR and a radiant cooling system by setting up various test cells. They found a 2-°C lower temperature inside the cell with a GR and radiant pipes as compared to GRs with and without an insulation layer. Additionally, the combination of GRs and green walls provide outstanding thermal and energy reductions as compared to stand-alone GR systems [33,34,35,88][11][12][13][14]. In an attempt to improve the runoff retention rate, a system integrating GRs with blue roofs was developed and the runoff outflow rates from the green–blue roof and a control roof were 0.1 L/s and 0.3 L/s, respectively, in the study by Shafique, Lee and Kim [28][15]. It is worth noting that the outflows from the green–blue roof only occurred a few hours after the beginning of a rainfall event, which helps to alleviate the stress on the urban drainage system.

3. GR Benefits

As mentioned in previous sections, urban heat mitigation and runoff reductions have been found to be the most-studied GR benefits. On the other hand, the mitigation of air pollution through the direct CO2 sequestration of GR plants is a GR benefit that has been inadequately investigated. A barrier to undertake research on this GR benefit is the limited options for the vegetation layer. While EGR keeps receiving the most attention during the last ten years due to several advantages, shallow-root plants in EGRs have a smaller capacity to capture gaseous pollutants and dust particles in the atmosphere [1,2][1][2]. Large plants, such as trees with deep roots, could absorb greater amounts of air pollutants, but face many challenges in GR construction and operation during its lifespan. Other than that, the insignificant quantity of CO2 sequestrated by GR plants could be an explanation for their lesser preference among researchers. Last but not least, the simplest way to conduct such projects is by utilizing the factor values already computed in the literature, which could not produce dependable results. Otherwise, the investigation of this benefit (reduction in air pollution) requires a complex experimental set-up and specific knowledge about the phytology of plants.
Noise reduction is also inadequately researched due to various constraints. One of them could be attributed to the fact that a GR system can only lessen the noise transmitted from traffic into the indoor environment if it is installed on a low-rise building [2]. Along with the rapid urbanization and population growth, multi-storey buildings are being constructed. Consequently, the GRs of both existing and new buildings are positioned far away from the ground level. This reduces the possibility of the application of GRs for noise reduction. Another difficulty could be the lack of standards for measuring the sound transmission through roofs [112][16]. Moreover, methods for testing the transmission loss are mainly developed for other building components, such as interior walls and exterior facades.
Finally, other GR benefits relating to runoff quality, the ecosystem, and social and economic benefits have been researched more often than the mitigation of noise and air pollution. However, they are still in experimental stages due to various constraints. For example, the question of whether GRs improve or degrade the runoff quality should be properly answered before proceeding to implementation. Additionally, the cost–benefit analysis of GRs pointed out remarkable long-term savings, and many experts have confirmed that the GR benefits outweigh their potential costs. Nevertheless, numerous important benefits were not considered in the economic evaluation because most of them are difficult to convert into monetary values. Consequently, this hinders researchers from carrying out such projects. Hence, future studies are suggested to be conducted with a well-designed approach.

4. Innovative GR Construction Techniques and Materials

In spite of unsolved issues, the potential of GRs remains remarkably huge. Considering only the GRs’ capability of providing ecosystem services, most studies illustrate positive outcomes. In order to facilitate the widespread implementation of GR, it is suggested that future studies should be directed towards identifying optimal GR designs to satisfy not only the ecosystem services provided by GRs but also their affordability for installing, operating, and maintaining. According to the literature review, it was determined that there exist attempts to explore alternative GR materials. Some of them succeeded to enhance the GR benefits. For example, Carpenter, Todorov, Driscoll and Montesdeoca [48][17] obtained a noticeable averaged retention rate of 96.8%, which exceeded the previously-reported values mainly due to the application of an effective drainage layer. Another high retention rate from Todorov, Driscoll and Todorova [45][18] was also a result of a drainage design that was never applied in other studies. Altering the substrate composition by adding biochar improved both the retention capacity and quality of the GR discharge [74][19]. On the other hand, the application of modelling software to simulate GR performance was observed during the last decade. Though the accuracy of simulation results compared well with actual field data, their application for the large-scale implementation of GRs is still negligible. The costs and benefits of large-scale implementation of GRs need to be clear to authorities and stakeholders, such as building owners, builders, and developers. Therefore, this is a recommendation for future research studies.

5. Inconsistent Impact of Parameters on GR Performance

It is not challenging to find studies on the benefits of GRs relating to UHI mitigation and energy reduction. These two benefits of GRs are quite apparent during the period of the strongest solar radiation (SR). Conversely, they are less desirable at night and in cold weather conditions. Moreover, seasonal and daily thermal behaviours of GRs are even more complicated. He, Yu, Ozaki and Dong [18][20], He, Yu, Ozaki, Dong and Zheng [80][21], Bevilacqua, Mazzeo, Bruno and Arcuri [82][22], and Foustalieraki, Assimakopoulos, Santamouris and Pangalou [89][23] found a higher Ts of GRs than that of traditional roofs at nighttime in both hot and cold seasons. Additionally, although the maximal Ts reduction in winter is not as impressive as that in summer, ΔTs still remains positive during winter daytime. Those research outcomes are contrary to others’ findings. More precisely, the Ts of a non-vegetated roof was higher than that of a GR at night-time, following Morakinyo, Dahanayake, Ng and Chow [26][24] and Cascone, Catania, Gagliano and Sciuto [21][25]. A warmer roof deck underneath the GR was observed in winter at daytime, following Boafo, Kim and Kim [81][26] and Cai, Feng, Yu, Xiang and Chen [85][27]. A combined effect of great heat storage and thermal inertia of GR components is likely to explain a warmer skin temperature of outer roof decks at night [18][20]. Oppositely, Cascone, Catania, Gagliano and Sciuto [21][25] stated that the evapotranspiration process allows GRs to release the heat accumulated during a hot summer day, which helps to maintain a lower Ts of a GR at night. The negative ΔTs at night during summer could lead to a higher cooling demand, though this is preferable to save the electricity for heating during cold seasons. On the other hand, attempts to study this topic have not yet been made. Consequently, solutions from future research are needed to avoid any unexpected impacts of GRs.
Among many attempts to study the HTC improvement, the vast majority of them analyzed the indoor dry-bulb temperature (DBT), which is also known as air temperature. On the other hand, the wet-bulb globe temperature (WBGT), which is a combined effect of air temperature and relative humidity, has received less attention. Human discomfort is primarily affected by WBGT and, hence, this variable should be involved in future works (rather than simply analyzing DBT). Moreover, the studies about whether the Tair.in improved due to the construction of GRs has been limited. Though energy savings after GR installation has been generally agreed upon, it is worthwhile to investigate the possibility of a GR-based passive-cooling system.
Another noteworthy finding is the differences in the experimental setups of measuring devices from one study to the other. It is understandable that the position of measuring devices strongly depends on the specific research aims. However, this work suggests that future research needs to apply consistent measurements for accuracy of result comparisons and performance evaluations between different studies. For example, this work detected a difference in the height of sensors for measuring the air temperature above the plant canopy. Additionally, previous studies published ΔTs values with various positions of thermal sensors. The explanations for those chosen sensor positions are also missing. In order to properly understand the effects of GRs, an adequate number of studies with identical and appropriate experimental designs are required.
Palermo, Turco, Principato and Piro [22][28] and Gregoire and Clausen [43][29] stated that the inconsistency in published runoff reduction was due to differences in the catchment size, the length of the study period, the data-analysis approach, and the hydraulic characteristics of the GR materials. Nevertheless, no consensus about how those variables influence the GR capability of reducing runoff have been reached yet. For example, Zhang, Miao, Wang, Liu, Zhu, Zhou, Sun and Liu [49][30] and Razzaghmanesh and Beecham [69][31] highlighted the importance of an antecedent dry weather period (ADWP) in the retention capability of GRs. In contrast, Zhang, Szota, Fletcher, Williams and Farrell [47][32], Ferrans, Rey, Pérez, Rodríguez and Díaz-Granados [57][33], and Hakimdavar, Culligan, Finazzi, Barontini and Ranzi [51][34] concluded that the substrate storage capacity and initial substrate moisture content were more related to the retention performance, whereas the impact of ADWP was negligible. They also found that the selection of high water-use plants, followed by the high evapotranspiration (ET) rate, was not as important as the substrate type. In contrast, Johannessen, Muthanna and Braskerud [59][35] raised another debate, as they reported opposite results as the ET process made greater variations in GR performance than different GR configurations did. Kaiser et al. [114][36] highlighted the importance of ET by applying some solutions to increase the rate of ET. Such disputes could be resolved with extensive knowledge acquired from future works. Furthermore, Sims, Robinson, Smart and O’Carroll [55][37] maintained that the high retention rates in some studies resulted from the inclusion of rainfall events generating no runoff (100% retention) in the data analysis.

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