Life cycle assessment of green roofs: A literature review of layers materials and purposes

Global warming potential comparison between green and conventional roofs in cold climate using life cycle assessment

Highlights There were no significant differences (a = 0.05) in rainfall retention between blue and green roofs. Blue roofs appear to be a viable option to green roofs when the priority is stormwater management. Blue and green roofs need to be studied on larger scales to gain more confidence in the treatment provided. Abstract. Impervious surfaces caused by urbanization alter hydrologic conditions. In ultra-urban areas (75% or more impervious), rooftop stormwater control measures (SCMs), such as green roofs, exploit underutilized rooftop space. Green roofs reliably attenuate rainwater, but associated water quality treatment results are not as consistent. This study explored the viability of blue roofs, an emerging technology, as an alternative to green roofs. Nine adjacent plots consisting of three each of conventional (control), green, and blue roofs were constructed in Raleigh, NC. As a part of hydrology monitoring, on-site rainfall was measured using a tipping bucket and verified using a manual gauge. Water quantity data were collected using water level loggers in storage bins placed under the roof plots to record outflow. Measured rainfall retention was similar for blue and green roofs, and both systems were significantly (a = 0.05) and substantially (50%) more retentive than control roofs. A comparison of evaporation rates suggested blue roofs may be more effective at regenerating storage capacity than green roofs. Thirteen water quality events were sampled, and the mean total nitrogen (TN) concentration from the green and blue roofs was 5.83 and 2.18 mg/L, respectively. Similarly, the mean total phosphorus (TP) concentration from the green and blue roofs was 2.75 and 0.25 mg/L, respectively. Total suspended solids (TSS) data were collected, but seven of 13 samples were below the practical quantitation limit (2.5 mg/L), and the data were not statistically analyzed. Total kjeldahl nitrogen (TKN), TP, and orthophosphate (O-PO4 3-) concentrations from green roof treatments were significantly (a = 0.05) larger than concentrations from blue roofs. Except for nitrate/nitrite nitrogen (NO2,3-N), there were no significant differences between blue and control roof pollutant concentrations. However, green roofs often discharged substantially higher pollutant loads than control roofs for every pollutant other than NO2,3-N and total ammonical nitrogen (NH3-N). While blue roofs may lack the aesthetic appeal of green roofs, this study suggests blue roofs are a relatively cost-effective option, likely preferable, when solely focused on the benefits of stormwater management from rooftops. Keywords: Blue roofs, Conventional roofs, Green roofs, Hydrology, Monitoring, Stormwater control measures, Water quality.

Green (vegetated) roofs have gained global acceptance as a technologythat has the potential to help mitigate the multifaceted, complex environmental problems of urban centers. While policies that encourage green roofs exist atthe local and regional level, installation costs remain at a premium and deter investment in this technology. The objective of this paper is to quantitatively integrate the range of stormwater, energy, and air pollution benefits of green roofs into an economic model that captures the building-specific scale. Currently, green roofs are primarily valued on increased roof longevity, reduced stormwater runoff, and decreased building energy consumption. Proper valuation of these benefits can reduce the present value of a green roof if investors look beyond the upfront capital costs. Net present value (NPV) analysis comparing a conventional roof system to an extensive green roof system demonstrates that at the end of the green roof lifetime the NPV for the green roof is between 20.3 and 25.2% less than the NPV for the conventional roof over 40 years. The additional upfront investment is recovered at the time when a conventional roof would be replaced. Increasing evidence suggests that green roofs may play a significant role in urban air quality improvement For example, uptake of N0x is estimated to range from $1683 to $6383 per metric ton of NOx reduction. These benefits were included in this study, and results translate to an annual benefit of $895-3392 for a 2000 square meter vegetated roof. Improved air quality leads to a mean NPV for the green roof that is 24.5-40.2% less than the mean conventional roof NPV. Through innovative policies, the inclusion of air pollution mitigation and the reduction of municipal stormwater infrastructure costs in economic valuation of environmental benefits of green roofs can reduce the cost gap that currently hinders U.S. investment in green roof technology.

A green roof by broad definition can be defined as any planted open space that is detached from the earth by a building or other structure; they are a contained green space on top of a human made structure (Beecham, 2003). They offer many benefits including: a reduction in stormwater runoff; better insulated buildings; an increased roof membrane life span; reduction of the urban heat island effect in built-up areas; create more natural green spaces; and, enhanced air quality.A qualitative and quantitative assessment of green roofs has been undertaken to contextualise the importance of green roofs for South East Queensland (SEQ). The qualitative aspect of this thesis was conducted through research of the benefits of green roofs globally, and then adapted to the local climate of SEQ. A Life Cycle Assessment (LCA) of a conventional roof, extensive green roof and an intensive green roof was then used to quantify the benefits of green roofs on an economic level. The research conducted is seen as a vital step towards the product, which is to realise the benefits associated with green roofs for the sustainability of SEQ.In order to undertake the LCA a few assumptions had to be made. Fytogreen, a green roof supplier for SEQ was able to give an indicative costing for the extensive and intensive systems based on a model with 232 m2 of roofing with substrate depths of 100 mm and 1000 mm respectively. The cost of materials, labour and some machinery were taken into consideration and the dollar value of the extensive system was $39k and the intensive system was $49k. However, waterproofing, site compliance costs, freight of material and crane or scissor lift hire were omitted. All other material costs such as those needed for a conventional roof and stormwater detention basins were taken from Cordell’s Building Cost Guide.It was found that although there was a substantial increase in the initial investment for a green roof compared to conventional roof , after a study period of 25 years it was seen that an extensive green roof is the optimum roof structure both economically and environmentally. Initially an extensive green roof and an intensive green roof cost the investor $17k and $27k more than a conventional roof. However, taking into consideration the life expectancy for a conventional roof, the cost of stormwater quality and quantity, energy use and other consideration within the LCA, the net present value of the extensive green roof was $46k which is 8% more economically viable than the other models. The qualitative environmental benefits of green roofs where also found to be of considerable benefit to the sustainability of SEQ.Even though it has been shown that green roofs are both financially and environmentally viable their successful implementation and up-take would require a combined approach based on media promotion, training, research and leadership by industry and Government. Further research, by the construction of green roof case studies in SEQ, need to be implemented to fully quantify the benefits.

The body of life cycle assessment (LCA) literature is vast and has grown over the last decade at a dauntingly rapid rate. Many LCAs have been published on the same or very similar technologies or products, in some cases leading to hundreds of publications. One result is the impression among decision makers that LCAs are inconclusive, owing to perceived and real variability in published estimates of life cycle impacts. Despite the extensive available literature and policy need formore conclusive assessments, only modest attempts have been made to synthesize previous research. A significant challenge to doing so are differences in characteristics of the considered technologies and inconsistencies in methodological choices (e.g., system boundaries, coproduct allocation, and impact assessment methods) among the studies that hamper easy comparisons and related decision support. An emerging trend is meta-analysis of a set of results from LCAs, which has the potential to clarify the impacts of a particular technology, process, product, or material and produce more robust and policy-relevant results. Meta-analysis in this context is defined here as an analysis of a set of published LCA results to estimate a single or multiple impacts for a single technology or a technology category, either in a statisticalmore » sense (e.g., following the practice in the biomedical sciences) or by quantitative adjustment of the underlying studies to make them more methodologically consistent. One example of the latter approach was published in Science by Farrell and colleagues (2006) clarifying the net energy and greenhouse gas (GHG) emissions of ethanol, in which adjustments included the addition of coproduct credit, the addition and subtraction of processes within the system boundary, and a reconciliation of differences in the definition of net energy metrics. Such adjustments therefore provide an even playing field on which all studies can be considered and at the same time specify the conditions of the playing field itself. Understanding the conditions under which a meta-analysis was conducted is important for proper interpretation of both the magnitude and variability in results. This special supplemental issue of the Journal of Industrial Ecology includes 12 high-quality metaanalyses and critical reviews of LCAs that advance understanding of the life cycle environmental impacts of different technologies, processes, products, and materials. Also published are three contributions on methodology and related discussions of the role of meta-analysis in LCA. The goal of this special supplemental issue is to contribute to the state of the science in LCA beyond the core practice of producing independent studies on specific products or technologies by highlighting the ability of meta-analysis of LCAs to advance understanding in areas of extensive existing literature. The inspiration for the issue came from a series of meta-analyses of life cycle GHG emissions from electricity generation technologies based on research from the LCA Harmonization Project of the National Renewable Energy Laboratory (NREL), a laboratory of the U.S. Department of Energy, which also provided financial support for this special supplemental issue. (See the editorial from this special supplemental issue [Lifset 2012], which introduces this supplemental issue and discusses the origins, funding, peer review, and other aspects.) The first article on reporting considerations for meta-analyses/critical reviews for LCA is from Heath and Mann (2012), who describe the methods used and experience gained in NREL's LCA Harmonization Project, which produced six of the studies in this special supplemental issue. Their harmonization approach adapts key features of systematic review to identify and screen published LCAs followed by a meta-analytical procedure to adjust published estimates to ones based on a consistent set of methods and assumptions to allow interstudy comparisons and conclusions to be made. In a second study on methods, Zumsteg and colleagues (2012) propose a checklist for a standardized technique to assist in conducting and reporting systematic reviews of LCAs, including meta-analysis, that is based on a framework used in evidence-based medicine. Widespread use of such a checklist would facilitate planning successful reviews, improve the ability to identify systematic reviews in literature searches, ease the ability to update content in future reviews, and allow more transparency of methods to ease peer review and more appropriately generalize findings. Finally, Zamagni and colleagues (2012) propose an approach, inspired by a meta-analysis, for categorizing main methodological topics, reconciling diverging methodological developments, and identifying future research directions in LCA. Their procedure involves the carrying out of a literature review on articles selected according to predefined criteria.« less

Green roofs have the potential to retain stormwater on the roof surface and lower the thermal loading on buildings. Because of this, the greatest environmental benefits from green roofs might be achieved in subtropical climates characterized by high temperatures and intense rain events. There is, however, little research to support this. In a replicated study in Texas, we compared the performance of six different extensive green roof designs vegetated with native species, to non-reflective (black) roofs, and reflective (white) roofs. Preliminary hydrologic and thermal profile data indicated not only differences between green and non-vegetated roofs, but also among green roof designs. Maximum green roof temperatures were cooler than conventional roofs by 38°C at the roof membrane and 18°C inside air temperature, with little variation among green roofs. Maximum run-off retention was 88% and 44% for medium and large rain events but some green roof types showed very limited retention characteristics. These data demonstrate indicate that: 1. Green roofs can greatly affect the roof temperature profile—cooling surface layers and internal space on warm days. 2. Green roofs can retain significant amounts of rainfall, this is dependent on the size of the rain event and design and can fail if not designed correctly. We suggest that as green roofs vary so much in their design and performance, they must be designed according to specific goals rather than relying on assumed intrinsic attributes.

The world is rapidly urbanizing, and many previously biodiverse areas are now mostly composed of impervious surface. This loss of natural habitat causes local bird communities to become dominated by urban dweller and urban utilizer species and reduces the amount of habitat available for migrating and breeding birds. Green roofs can increase green space in urban landscapes, potentially providing new habitat for wildlife. We surveyed birds and arthropods, an important food source for birds, on green roofs and nearby comparable conventional (non-green) roofs in New York City during spring migration and summer breeding seasons. We predicted that green roofs would have a greater abundance and richness of both birds and arthropods than conventional roofs during both migration and the breeding season for birds. Furthermore, we predicted we would find more urban avoider and urban utilizer bird species on green roofs than conventional roofs. We found that both birds and arthropods were more abundant and rich on green roofs than conventional roofs. In addition, green roofs hosted more urban avoider and utilizer bird species than conventional roofs. Our study shows that birds use green roofs as stopover habitat during migration and as foraging habitat during the breeding season. Establishing green roofs in urban landscapes increases the amount of habitat available for migrating and breeding birds and can partially mitigate the loss of habitat due to increasing urbanization.

Industrial symbiosis network (ISN) facilitation tools seek to holistically evaluate the environmental and economic performance of ISNs through life cycle assessment (LCA) and life cycle costing (LCC). ISNs have many stakeholders with diverse interests in the LCA and LCC results thus requiring multi-level analysis. The objective of this review was to examine the state-of-the-art methodologies used in LCAs and LCCs of ISNs and understand how multi-level analysis can be conducted. The systematic literature review methodology was applied to develop a corpus of peer-reviewed LCA and LCC studies of ISNs published between 2010 and 2019 without any geographic boundary. Abstracts were reviewed to shortlist studies that conducted an LCA or LCC of an ISN with numerical results. LCA and LCC methodologies used in the shortlisted studies were collected and categorized. Each methodology was examined to understand how the foreground and background systems are represented, how waste-to-resource exchanges are analyzed, and how the results can be computed at the network, entity, and flow levels. The review yielded 42 LCA studies and 11 LCC studies of ISNs that used eight different methodologies. Process-based LCA was used in 71% of the LCA studies, whereas tiered hybrid LCA was used in 14% of the studies. Waste-to-resource exchanges in ISN scenarios were represented either through process analysis or as a black box. Fewer LCC studies that evaluate the economic performance of ISNs exist compared with LCA studies. Economic studies often evaluated financial feasibility, net present value, profitability, or payback period of specific waste-to-resource exchanges or the network overall. The insights derived from this review chart future areas of research in multi-level modeling and analysis of the life cycle environmental and economic performance of ISNs. To improve the model construction and analysis process, research should be explored in developing a methodology for constructing a single model that represents multiple entities linked together by waste-to-resource exchanges and can provide LCA and LCC results for different stakeholder perspectives. The lack of LCC studies of ISNs merits the need for more research in this area at both the network and entity levels to quantify potential economic trade-offs between stakeholders. Developing a methodology for unified LCA and LCC modeling and analysis of ISNs can help ISN facilitation tool developers conduct simultaneous life cycle environmental and economic analysis of the potential symbiosis connections identified and how they contribute to the overall network.

Comparative environmental life cycle assessment of green roofs

Green roofs—rooftops that are partially or completely covered with vegetation growing in soil medium over a waterproof membrane—have gained momentum over the past six years as building owners recognize their advantages over conventional roofing in terms of better energy efficiency and reduced rain runoff. Now local governments are exploring incentives for moving the practice into the mainstream. A look at cities that are leading the country in green roof coverage reveals a growing range of policy tools.

Green roof application on real residential buildings in Tamilnadu, India is very limited and mostly concentrated in major cities mainly for visual purposes. There is not enough research has been conducted to boost up the benefits of green roof system in Warm and Humid weather in India. Green roofs have the potential to improve the thermal performance of a roofing system through shading, insulation, evapotranspiration and thermal mass, thus reducing a building’s energy demand for space conditioning. To quantify the thermal performance and energy efficiency of green roofs an experimental investigation was done in residential buildings of Madurai, Tamilnadu, India. This paper refers to the analysis of the thermal properties and indoor thermal performance study of the green roof. The investigation were implemented in two phases: during the first phase, extended surface, air temperature and relative humidity measurements were taken at the indoor and outdoor environment of the buildings where the green roof had installed and during the second phase of the study, the thermal properties of the green roof, as well as, the cooling potential were examined. Results showed vegetative roofs reduced heat gain compared to the white reflective roofs and conventional reinforced cement concrete due to the thermal mass, extra insulation, and evapo-transpiration associated with the vegetative roofing systems. The results also proved that green roofs provide acceptable indoor thermal performance with respect to the other conventional roofs while re-establishing the relationship between human and environment, which have been destroyed due to the rapid urbanization.

White and “green” (vegetated) roofs have begun replacing conventional black (dark-colored) roofs to mitigate the adverse effects of dark impervious urban surfaces. This paper presents an economic perspective on roof color choice using a 50-year life-cycle cost analysis (LCCA). We find that relative to black roofs, white roofs provide a 50-year net savings (NS) of $25/m2 ($2.40/ft2) and green roofs have a negative NS of $71/m2 ($6.60/ft2). Despite lasting at least twice as long as white or black roofs, green roofs cannot compensate for their installation cost premium. However, while the 50-year NS of white roofs compared to green roofs is $96/m2 ($8.90/ft2), the annualized cost premium is just $3.20/m2-year ($0.30/ft2-year). This annual difference is sufficiently small that the choice between a white and green roof should be based on preferences of the building owner. Owners concerned with global warming should choose white roofs, which are three times more effective than green roofs at cooling the globe. Owners concerned with local environmental benefits should choose green roofs, which offer built-in stormwater management and a “natural” urban landscape esthetic. We strongly recommend building code policies that phase out dark-colored roofs in warm climates to protect against their adverse public health externalities.

This study evaluated nutrient concentrations in runoff water from conventional roofs, green roofs, and urban streams, focusing on the impacts of compost addition at the industry standard of 15% by volume to green roofs at installation. Water samples were collected during selected rainfall events (n = 9) during calendar year (CY) 2008 and from the urban stream approximately monthly during baseflow conditions. Water samples were analyzed for ammonium-nitrogen (NH4-N), nitrite-N (NO2-N), nitrate-N plus NO2-N (referred to as NO3-N), total N (TN), soluble reactive phosphorus (SRP), total P (TP), and total organic carbon (TOC). The concentrations of SRP, TP, TN, and TOC were significantly greater in runoff from the green roofs that received compost during installation than from the conventional roofs or the green roof without compost addition, while the NH4-N, NO2-N, and NO3-N concentrations in stormwater runoff were generally not significantly different across the conventional or green roofs. Nutrient concentrations in the study streams, except for TOC, generally increased with the percentage of urban and pasture land use in the stream catchment, and the exponential relationship was generally strongest (higher R2) for NO3-N, TN, and P. Nutrient concentrations in stormwater runoff from the green roof without added compost were within the range observed across the study streams. However, nutrient concentrations in stormwater from the green roofs with compost were more variable when compared to the selected streams, and P concentrations were significantly greater in stormwater from the compost-amended green roofs compared to that measured in the study streams. The data collected in this study provide evidence that compost applied at the industry standard of 15% by volume to maintain plant growth is contributing to increases of nutrients in stormwater runoff 17 to 23 months after installation. Future studies should focus on compost additions to green roofs that maximize plant growth and survival while minimizing nutrient (particularly P) loss in stormwater.

Green roofs on small 1.8 by 2.4 m buildings consisting of a conventional flat roof covering, a root barrier, a 12 mm thick Enka drainage layer, 89 mm of growth medium, 25 mm of porous expanded polypropylene (PEPP), and Sedum spurium planted 76 mm on center were evaluated to determine their potential to reduce stormwater impacts and roof surface temperatures. Hydrology data were collected from three replicate buildings with experimental green roofs, and roof surface temperatures were collected from both green and conventional roofs, each with a 1:12 slope. The green roof media had an average porosity of 55 m3 m-3 and a field capacity of 34 m3 m-3. Rain and roof runoff data collected from seven rains during October and November 2002 showed that the green roofs delayed the start of runoff an average of 5.7 h. The green roofs retained an average of 45% (range 19% to 98%) of the rain from the seven storms evaluated and delayed the peak runoff by 2 h. Roof temperature data collected between April 2002 and February 2003 showed that the green roof maximum surface temperatures averaged 6°C higher in the winter and more than 19°C lower in the summer. The differences between the average maximum diurnal temperature change at the roof surface averaged 19°C during the cooling season (August and September) and 8°C during the heating season (October to February).

Green and cool roofs have significant potential to reduce energy consumption in buildings, but high initial costs and the need for local adaptation limit their adoption. This study aims to compare the life cycle energy assessment (LCEA) and life cycle cost analysis (LCCA) of green, cool, and standard (fibre cement) roofs in three Brazilian cities with different climatic and economic contexts. Computer simulations were carried out on a multifamily residential building model to assess the energy performance of the roofs. The simulation results and literature data were used to estimate the roofs’ energy consumption and cost over the life cycle. Over a 40-year life cycle, green and cool roofs reduced energy consumption by 13% to 22% compared to standard roofs. Cool roofs showed the lowest life cycle costs, while green roofs faced cost-effectiveness challenges due to high initial and maintenance costs. However, in areas with high energy demands and electricity tariffs, the life cycle cost of green roofs may be decreased. The study highlights the crucial role of material selection in embodied energy and emphasises the dominant impact of the operational phase on energy consumption and life cycle costs. These findings underscore the need for customised design strategies and localised assessments to support decision-making.