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Green roofs in Australia: review of thermal performance and associated policy development
Abstract
In Australia, there is an increasing interest in using extensive green roofs to make buildings more sustainable and provide a number of social, ecological, aesthetic and thermal benefits to cities. The potential of green roofs to reduce building energy consumption has been extensively studied overseas in a variety of different climates. However, in Australia the green roof industry is relatively new. There is still very little information on the thermal properties of Australian green roofs and their performance. Further, as a relatively new industry, there is a general lack of specific policies and initiatives to promote green roofs. In this paper, we briefly review the research investigating green roof thermal performance in various climates and analyse policies and actions that have been implemented internationally to foster green roofs with an emphasis on their thermal performance.
The results showed that most policies were focused on ecological benefits, such as stormwater runoff reduction, rather than thermal benefits. Many green roof policies had difficulty interpreting the thermal performance of green roofs, because of the dynamic nature of green roof R-values. In this study, the effectiveness of overseas green roof policy is discussed and recommendations how they could be adapted for Australian cities are provided.
... Findings are varied and sometimes in contrast to one another. Common findings are that green roof thermal performance depends on the climate zone, the building materials, the seasonality and the green roof material selection [16,[18][19][20][21][22][23]. For example, in cold climate areas, a thick substrate enhances thermal performance compared to a thin substrate [24]. ...
... Because of these wide ranging results, it is not possible to specify one "optimum" green roof build-up (drainage layer, substrate or growing medium and plants) that will maximise green roof thermal benefits in all countries or climate zones. In situ research is therefore necessary to help select substrates and plants for green roofs in various locations [19]. ...
... Sailor's model comprise many variables and parameters, some relevant to the vegetation layer, and others to the substrate layer, called soil in the model. As a result of a review of the literature [19], we selected three vegetation parameters and three soil parameters as the most relevant to a green roof thermal performance for further investigation. These are: (i) Leaf Area Index (LAI); (ii) Leaf reflectivity (LR); (iii) Minimum stomatal resistance (SR); (iv) Substrate thickness (ST); (v) Conductivity of dry soil (CDS); and (vi) Saturation volumetric moisture content (MC). ...
Green roofs are consistently being used to reduce some of the negative environmental impacts of cities. The increasing interest in extensive green roofs requires refined studies on their design and operation, and on the effects of their relevant parameters on green roof thermal performance. The effects of two design parameters, substrate thickness (ST) and conductivity of dry soil (CDS), and four operating parameters, leaf area index (LAI), leaf reflectivity (LR), stomatal resistance (SR), and moisture content (MC), were investigated using the green roof computer model developed by Sailor in 2008. The computer simulations showed that among the operating parameters, LAI has the largest effects on thermal performance while CDS is a more influential design parameter than ST. Experimental investigations of non-vegetated and sparsely vegetated green roofs in Melbourne were principally used to understand the effect of the substrate and enable better understanding of dominant heat transfer mechanisms involved. Investigated green roofs had three substrate thicknesses (100, 150 and 200 mm), and their performance was compared to a bare conventional roof. In contrast to the computer simulations, the experimental results for summer and winter showed the importance of MC and ST in reducing the substrate temperature and heat flux through the green roof.
... Green roofs can regulate roof surface temperatures through shading, insulation, direct solar reflection and heat loss from the evapotranspiration of leaves and substrates but the benefit of this varies considerably with climate (Pianella et al., 2016a). When irrigated, vegetated green roofs in SE Australia reduce temperatures at the substrate surface and at depth and in summer these are generally cooler than ambient air temperature during the day and warmer at night because of their capacity to retain heat (Razzaghmanesh et al., 2016). ...
... Irga et al. (2017) suggested that government influence can play a significant role in encouraging installation of green roofs, and related the number of green roof installations in different local government areas to the existence and type of green roof policies. However, even with increasing research findings directly relevant to the Australian context, most green roof policies focus on providing information and encouragement for installation, rather than regulating or mandating their inclusion in new developments (Pianella et al., 2016a). Arguably, this period of policy encouragement and support has been important in skilling up and developing industry capacity as well as increasing the 'visibility' and interest in green roofs amongst policy makers. ...
Australian cities have been slow to implement green roofs. This is because there are many potential barriers to their widely acceptance as a nature based solution that can make cities more liveable and help them adapt to, and mitigate, climate change. Due to significant differences in rainfall, temperature, available substrates and suitable vegetation relying on northern hemisphere research and experience is problematic as many of the environmental and economic benefits of green roofs are location specific. This paper aims to 1. synthesise a decade of Australian green roof research that has sought to overcome these barriers, 2. assess the current status of the Australian green roof industry and remaining knowledge gaps, and 3. provide a roadmap for future progress developed in multidisciplinary industry workshops. Many of these insights will be applicable to areas with similar seasonally hot and dry climates or emerging green roof markets.
We identified that significant progress has been made in addressing the barriers to green roofs in Australia. Research has focused on developing green roofs for local conditions and quantifying their benefits. Substrate research has investigated the suitability of locally available materials with a focus on how water retention additives and organic waste materials can increase plant available water and therefore survival. By taking a plant physiology approach Australian researchers have gained a strong functional understanding of suitable green roof plants and the benefits they provide, considerably expanding the available palette beyond the succulents commonly used internationally. Research has quantified green roofs’ stormwater retention and building insulation and energy benefits and provided evidence that they benefit well-being and performance, important for employee productivity.
... However, many studies have shown that building energy savings from green roofs are strongly dependant on climate and location [17], as well as building characteristics and existing insulation [18,19]. Given that Australia is conventionally divided into eight different climate zones [20], the thermal performance and consequent building energy savings would be different across the country, and specific recommendations on the best green roof build-up should be tested and provided locally [21]. ...
... However, we recognise that further studies are required to model the thermal performance of buildings using these substrates across Australia climate zones [21]. We also understand that the green roofs should be judged on the multiple benefits they provide, and not only for their insulating properties. ...
There has been growing interest in using extensive green roofs for commercial and residential buildings in urban areas. Green roofs provide many benefits, including adding an additional insulation layer. The potential of this benefit depends on many factors, including the thermal properties of the green roof substrate. Thermal conductivity values of three substrates comprised primarily of scoria, crushed roof tile and bottom-ash were measured with steady-state and transient techniques under three moisture conditions. Specific heat capacities of the green roof substrates were also measured with a transient technique. Steady-state measurements were performed with a “k-Matic” apparatus while transient measurements with KD2 Pro needles. In general, the steady-state measurements showed more consistency than transient measurements. Thermal conductivity differed among the three substrates: crushed roof tile had the highest conductivity values across all moisture contents. Substrate moisture content consistently increased thermal conductivity across all substrates, but this was significantly greater for the crushed roof tile substrate. Steady-state thermal conductivity curves were fitted using the thermal conductivity model for green roof substrates adopted by Sailor (2011). The coefficients obtained are presented and can be used in green roof models to quantify the thermal performance of green roofs and building energy savings.
... This strategy is backed up by accepted design standards and legal backing. As of 2005, Germany had planted 13.5 million m 2 of roof spaces [54,62]. Munich, the Bavarian regional capital (Germany), is implementing a variety of living roof-related initiatives [63]. ...
Concerns about climate change and rising energy demands have grown as a result of fast population rise and global industrialization. The construction industry has a huge impact on the energy and environmental sectors, accounting for about 40% of global energy consumption and a large portion of overall territorial emissions. There is a need for a shift in mindset when it comes to energy usage, as well as enhanced energy efficiency approaches and radical energy efficiency initiatives. As an energy-saving solution, the green roof, also known as the living roof has suitability and environmental benefits on many levels, while also strengthening aesthetic features and provoking structural innovation. Moreover, drought-prone areas, e.g. Saudi Arabia, have significant household energy demands. The Saudi building sector consumes more than 76% of the country's total electric power generation. As a result, the purpose of this study is to provide a general overview of living roof technology and its potential in Saudi Arabia as an energy-saving strategy. An overview of the building envelope, the impact of cladding design considerations on power usage, the benefits of a living roof, cost-benefit analysis, green policies, and examples from other countries are included in the paper. Other environmental benefits, besides the energy-saving potential of living roofs, were shown to boost the quantitative benefits of the living roof idea. A more detailed study is needed, among other things, to evaluate the energy-saving potential of living roofs based on the weather of various locations.
... These cities have different drivers for the implementation of green roofs and walls, most often related to issues of increasing the resilience and livability of the city. Cities with more developed living architecture industries have a range of policy approaches to encourage and/or mandate green roofs and walls (Pianella et al. 2016). The approaches adopted in these cities are expanded and critiqued in the Case Study report accompanying this report. ...
A large-scale and coordinated effort to retrofit green infrastructure into our built environment could see residential property values increase by 6 to 15 per cent as well as providing social, health and environmental benefits. A team of researchers at the University of Technology Sydney analysed the business case for green roofs and walls in Australia. The literature review included a US Cost Benefit Analysis (CBA) which found a viable case for boosting urban greenspace. It showed increases in residential property values with good amounts of green infrastructure between 6 and 15%. The review also found that widescale adoption of green roofs in Toronto, Canada could reduce temperatures in urban areas by up to 5 degrees Celsius. ‘Expanding the Living Architecture in Australia’ explores whether a mandatory or voluntary approach to green roofs and walls would work best in Australia, and draws on scientific literature and international case studies to illustrate how it could work in a localised setting. It found green roofs and walls offer great potential to expand living architecture in Australia and could deliver a suite of benefits from improved air quality and reduced storm water impacts, in addition to boosting community interaction and urban aesthetics.
... Because of these wide ranging results, it is not possible to specify an 'optimum' green roof build-up (plants and substrate or growing medium) that will maximise green roof thermal benefits in any country or climate zone. In situ research is therefore necessary to help select green roof materials in different areas (Pianella et al., 2016a). ...
The growing interest in extensive green roofs prompts more refined studies on green roof design and operation. To assist in the design, installation and operation of green roofs, the effects of design and operating parameters on green roof thermal performance need to be fully understood. The effects of two design parameters: substrate thickness (ST), conductivity of dry soil (CDS) and four operating parameters: leaf area index (LAI), leaf reflectivity (LR), stomatal resistance (SR), moisture content (MC) were investigated using the green roof model developed by Sailor in 2008. The sensitivity analysis revealed that among the operating parameters, LAI has the largest effects on thermal performance. On the other hand, between the two design parameters CDS is more influential than ST. An experimental investigation in Melbourne of non-vegetated green roofs with three substrate thicknesses (10 cm, 15 cm and 20 cm) and on a bare conventional roof was employed to isolate the effect of plants and enable better understanding of heat transfer mechanisms involved. In contrast with the sensitivity analysis, the experimental results for summer and winter showed the importance of the ST in reducing the substrate temperature and heat flux across the green roof. They also showed how the green roof substrate alone reduces the heat flux compared to the conventional roof. Finally, the thermal performance of sparsely vegetated green roofs in summer revealed a contribution to cooling effect of plants.
Green roofs can deliver multiple environmental and social benefits by reducing the urban heat island effect, reducing building energy use and greenhouse gas emissions, improving air quality, providing habitat for biodiversity and access to the biophilia effect. Green roofs provide these benefits to differing degrees in different climate zones globally. Despite known benefits, uptake of green roofs has been slow. Different cities, globally, adopt various policies and programmes to increase their green roofs; the question is which approach is best? This research used an in-depth review, site visits and qualitative methods, to determine whether mandatory or voluntary approaches produced greater uptake. Green roof policies and practices from selected global cities, London, Toronto, Singapore, Rotterdam and Stockholm, Sydney and Melbourne were examined. Singapore’s voluntary approach led to the greater uptake of green roofs. The mandatory approach taken by Toronto, with financial grants provided meaningful outcomes. London and Rotterdam implemented useful voluntary programmes, and Stockholm required more time to evaluate the effectiveness of its voluntary approaches in increasing green roofs. A voluntary approach for retrofit and a mandatory approach for new build developments are suggested as recommendations for Australian cities. Given the increases in green roofs internationally, similar increases can occur in Melbourne and Sydney in Australia, and these findings may be transferable to other global cities investigating different approaches to the increased adoption of retrofitted green roofs.
Green infrastructure could provide a suite of benefits, including stormwater management and climate change impact mitigation. While European countries, the USA and Canada are increasingly adopting green roofs, Australia is lagging behind. This study specifically reviews the performance of green roofs in terms of stormwater retention, runoff quality, building energy consumption and life cycle cost analysis, as well as policy development in the Australian context. Research studies on these topics within Australia are identified, collated and analysed with respect to other studies reported by reputable research groups in other countries, where the green roof system has already matured. It should be mentioned that the results related to energy consumption were from simulation studies and empirical evidence derived from limited case studies, as reported in the literature. This study found that the average water retention capacity (worldwide) of a green roof is around 66.2%, which is within to the values (60–70%) reported in Australian studies. Because of the high water retention capacity, green roofs limit the export of pollution for most storm events. But when green roofs are saturated, they may be a source of pollution. However, the concentration of the pollutant decreases significantly over time. The temperature reduction by green roofs in buildings ranges between 4 and 6 °C. There is a potential for energy savings in building heating and cooling, but the findings from the literature are highly variable (ranges between 9 and 50%). The average payback period reported for a green roof is about 16 years. However, this depends on the initial cost, maintenance and assumed discount rate in the study. It has been shown that government policies and public awareness are important to increase the green roof adoption rate. Finally, current challenges in adopting this technology and future research directions are summarised.
Introduction. The era of high technologies and economy disrupts interaction between man and nature, worsening the state of the environment and living conditions on Earth. In Russia, the construction industry follows a classical development model and applies stereotypical patterns of urban design. Finding problem solving methods means identifcation of problem triggers that help to clearly understand and develop rational problem tackling mechanisms. Green roofs suggest an advanced approach to architecture and urbanization whereby green spaces take the place of new buildings. Supplementary dynamic space is not reduced to its decorative and environmental functions.
Materials and methods. We applied such research methods as analysis, the system approach, synthesis, deduction, and comparative analysis. The initial review of the state of affairs in the Ryazan region was performed with regard for the theoretical nature of this study; correlation and regression analysis were employed to assess territories and spaces.
Results. The history of green roof systems is analyzed in the article. Systematization of the effective regulatory and technical framework enabled the co-authors to assess the widespread applicability of the green roof technology in Russia. Special attention is paid to the issues that are not covered by effective domestic regulations. The research work has shown that the first edition of GOST (All-Russian State Standard) 58875 is an attempt to consolidate previously issued manuals and recommendations. There is no information available about the seasonal efciency of green roof solutions in different Russian regions. A number of issues remain unresolved. The “green roof” policy has not been developed.
Conclusions. Russia is not ready for large-scale construction of green roofs. Further research into green construction should be carried out with regard for unstable climatic conditions in different regions of the country to confrm the feasibility of green roofng at the legislative level.
Green roof is an effective energy efficiency measure to reduce the building cooling load in summer and heating load in winter, in addition, it can add ecological benefit and landscape value to the community. Therefore, it has attracted extensive attention worldwide. This paper studies the selection of planting materials, plant configuration patterns and plant growth medium of the green roofs in China, and presents the researches on ecological benefits, thermal performance, and applications of the green roofs in China. This paper also introduces and analyzes the green roofs development policies in China, including the incentive mechanism, laws and regulations, and finally presents the analysis and suggestions on their application prospects.
Green roofs could act as a thermal buffer in buildings and offer potential energy savings. However, the energy benefits from green roofs are not usually properly recognised by traditional building energy regulations. Building energy regulations are traditionally over-simplistic during the assessment of the energy performance of complex building constructions. In the case of green roof designs, it is essential that the assessment mechanisms should not ignore the complex heat and moisture balances within the green roof layers. In this paper, dynamic energy modelling that considers the complexity within the green roof layers is adopted to guide policy makers in China on the relationship between using specific thicknesses of roof insulation against green roof layers. Simulations are run for a residential building type by also considering different thermal envelope characteristics across eight large Chinese cities and within the five main climatic zones of China. Results that link the green roof characteristics with respective traditional insulation layers are produced for all cities and it is found that optimising the plant and soil characteristics of green roofs in some climates could substitute more than 125 mm of roof insulation, while less optimum green roof types could only replace about 25 mm of roof insulation.
Green roofs are generally seen as a desirable building element, providing numerous benefits where water availability does not restrict their implementation. However, most Mediterranean locations have long, dry summers, requiring irrigation to sustain vegetation throughout extended dry periods. The cooling effect and water use of several types of plants suitable for extensive green roof systems were assessed using small test cells, which were insulated and equipped with internal thermal mass to provide a thermal response comparable to that of real buildings. The water requirements of the plant species tested ranged from 2.6 to 9.0 L/m2 per day. Aptenia cordifolia was the most efficient in its use of water, providing the highest cooling benefit per unit water required for irrigation. However, the cooling efficiency of all roof variants studied was very low, and the reduction in the sensible heat load of the model building attributed to the green roof system was less than 5% of the latent heat content of the water lost to evapotranspiration. In this context, it is hard to justify green roofs in such environments on the basis of their contribution to building energy conservation, although other benefits may nevertheless make green roofs attractive.
Green roofs have multiple environmental benefits and are widely used around the world. In keeping with this mainstream movement, the use of green roofs has been increasing in Taiwan in recent years. This paper reviews policies promoting green roof development in Taiwan, and compares the environmental and economic performance of green roofs in Taiwan to those in East Asian countries and worldwide. National and regional government policies have stimulated the development of green roofs by establishing goals for reducing carbon emissions of cities, promoting green construction, mitigating heat island effects, and increasing urban flood control. Local studies of green roof performance are few, other than thermal investigations. These studies have shown that green roofs significantly contribute to thermal reduction and moderate temperature variations around buildings. One study sampled stormwater runoff from green roof sites and found that sediment and nutrient concentration on these roofs are up to ten times higher than on conventional bare roofs; however, acid rain can be neutralized by green roofs. Hydrographs have shown that reductions in runoff from green roofs are not as great as expected because retention and detention are affected by high rainfall intensity, which is the typical precipitation pattern in Taiwanese cities. Without additional maintenance, green roofs can contribute to nonpoint source pollution in urban cities in wet and hot weather zones, because of high runoff and associated mass loading. Moreover, the environmental benefits of green roofs in Taiwan may not be as significant as those in other countries in which utility costs are higher, where decreasing energy consumption and CO2 emissions would be of greater benefit.
The urban environment has distinctive biophysical features in relation to surrounding rural areas. These include an altered energy exchange creating an urban heat island, and changes to hydrology such as increased surface runoff of rainwater. Such changes are, in part, a result of the
altered surface cover of the urban area. For example less vegetated surfaces lead to a decrease in evaporative cooling, whilst an increase in surface sealing results in increased surface runoff. Climate change will amplify these distinctive features. This paper explores the important role
that the green infrastructure, i.e. the greenspace network, of a city can play in adapting for climate change. It uses the conurbation of Greater Manchester as a case study site. The paper presents output from energy exchange and hydrological models showing surface temperature and surface
runoff in relation to the green infrastructure under current and future climate scenarios. The implications for an adaptation strategy to climate change in the urban environment are discussed.
This paper addresses the impact of roof-to-envelope ratio on overall energy savings of a green roof design over conventional
roof designs. Simulations were performed using a modified version of the Environmental System Performance program simulator,
developed at the University of Strathclyde. The modified design employed a model developed by Columbia University and the
Goddard Institute of Space Science which models the evapotranspiritive effect of a green roof calculated using the Bowen ratio;
that is, the ratio of sensible heat flux to the surrounding air to the latent heat flux resulting from evapourative energy
losses. The resulting heat flux term is proportional to the external surface convection, but inversely proportional to the
surface Bowen ratio, which is held constant and chosen to match experimental results obtained for a given roof design. The
present study performed simulations for the month of July in a Toronto climate on square warehouse style one, two, and three-story
buildings, with windows occupying 10% of the area of each wall. For the first set of simulations, the internal building load
of each story was set to zero, and the roof–envelope ratio was increased by increasing the building width and length. For
the final simulations, several roof–envelope ratios were chosen, and the internal load of each story was increased from 0
to 50,000W. As the roof–envelope ratio increases, the cooling load of the upper floor for multi-story designs approaches
the entire building cooling load. This indicates the importance of upper zone cooling in total building energy reductions.
Furthermore, the total energy savings of a green-roofed building over a conventional roofed building were far more significant
for single-story structures. A 250 × 250m green-roof design with 50,000W internal loading was found to have percentage energy
savings of 73%, 29%, and 18%, for a one, two, and three-story design, respectively.
The realisation that much of conventional, modern architecture is not sustainable over the long term is not new. Typical approaches are aimed at using energy and materials more efficiently. However, by clearly understanding the natural processes and their interactions with human needs in view, designers can create buildings that are delightful, functional productive and regenerative by design. The paper aims to review the biomimetics literature that is relevant to building materials and design. Biomimetics is the abstraction of good design from Nature, an enabling interdisciplinary science, particularly interested in emerging properties of materials and structures as a result of their hierarchical organisation. Biomimetics provides ideas relevant to: graded functionality of materials (nano-scale), adaptive response (nano-, micro-, and macro-scales), integrated intelligence (sensing and actuation at all scales), architecture and additional functionality. There are many examples in biology where emergent response of plants and animals to temperature, humidity and other changes in their physical environments is based on relatively simple physical principles. However, the implementation of design solutions which exploit these principles is where inspiration for man-made structures should be. We analyse specific examples of sustainability from Nature and the benefits or value that these solutions have brought to different creatures. By doing this, we appreciate how the natural world fits into the world of sustainable buildings and how as building engineers we can value its true application in delivering sustainable building.
An Introduction to Australian Public Policy: Theory and Practice is the first book to comprehensively address both the theoretical and practical aspects of policy making in Australia. Written in an accessible style, this text is designed to introduce students to the real world challenges and skills involved in working in a range of policy roles. Drawing on their own experiences, the authors ground public policy theory in a number of key controversies to illustrate the contestable nature of the policy process. Each chapter features case studies that outline contemporary policy issues, such as the deregulation of the financial system, ‘Knowledge Nation’, paid maternity leave, and the Northern Territory intervention. Including practical exercises on how to write policy briefs and media releases, this book is essential reading for anyone who needs to know how public policy is developed in Australia.
The application of green roofs on urban buildings is considered to have a positive impact on their thermal behaviour and local microclimatic conditions. According to the literature, their ability for attenuation of storm water run-off as well as their contribution to the building's thermal protection is among the most important benefits of this technique. However, despite the development of computer models that can assist towards analysing the nature of their behaviour, there is still a relative gap in measured data representing long-term period thermal performance. In this paper, the results of a long-term experimental analysis are presented, which attempt to identify the thermal behaviour of a green roof in comparison with a bare flat roof. The results show that in Mediterranean countries, a green roof can contribute substantially to building's energy conservation mainly during the warm period of the year, while its influence during the cold period is negligible.
There is a lack of evidence on regulatory effectiveness available to support policy makers with the selection of appropriate instruments to deliver better environmental regulation. We identify the types of evidence required to enable regulatory reform, characterize evidence gaps, and explore how these may be filled through future research. A typology of regulatory instruments is presented, and evidence of what has worked when and why is examined, drawing on international experience and recent cases from the United Kingdom (UK). Evidence of the capabilities of good environmental regulators for regulatory effectiveness is lacking, and it is proposed that ethnographic research that captures the nuances of regulatory practice will prove necessary to address this. This paper is of value to policy makers and regulators around the world considering the selection and deployment of the full range of environmental regulatory instruments to respond to environmental risks and in support of economic growth. It can inform the selection of suitable approaches and the design of institutions capable of delivering them. Copyright © 2012 John Wiley & Sons, Ltd and ERP Environment.
The overall assessment of an intensive green roof located in a sub-tropical region has been undertaken. The results showed a fairly good agreement between the published and measured solar radiation data and also confirmed July and January as the hottest and coldest periods, respectively, for the region. The soil was established as a silt type with good planting medium properties for green roofs. The overall thermal performance showed that the green roof provided an average temperature reduction of 3.3℃ (i.e. 50% temperature reduction) through the roof in July. Equally, its performance was remarkable during the coldest period of January. A maximum differential temperature of 15.5℃ was achieved with the soil contributing to 24% of the temperature difference through the roof. Further studies are, however, needed to cover a wider area of influence such as effects of different types of construction materials, plants, locations and soil. In view of the limitation of the theoretical model, it would also be useful to consider some of the factors which were either neglected or assumed to be constant in any future comparative studies.
A green roof is a specialized roof system that supports vegetation growth on rooftops. This technology is rapidly gaining popularity as a sustainable design option for buildings. In order to contribute to an understanding of green roof in regions with cold winters and snow, an on-site experimental investigation was present with a focus on the assessment of green roof performance during the winter. This field experiment took place on a six small buildings during the winter of 2010–2011. The work monitored three buildings with green roofs, two buildings with reference roofs and one building with a bare soil coverage for the roof. These six buildings were identically constructed and instrumented with sensor networks to provide heat flux data through the roofs. The 15 min averaged data were statistically analyzed for a week under the two separate periods, first without a snow cover and second with a snow cover. The results show that the roof type is a significant factor in affecting the thermal performance of these buildings. Most importantly, green roofs reduce heat flow through the roof and thus reduce the heating energy demand during the winter. However, the energy savings for buildings with the green roofs are reduced under snow conditions because the snow diminishes thermal resistance of the roof and increases the heat transfer process through the roofs.
The relative contribution of substrate depth and vegetation type on temperature mitigation and stormwater runoff reduction was studied in an experimental green roof in North eastern Italy. Two substrate depths (120 and 200 mm) and two vegetation types (herbaceous plants and shrubs, respectively) were used, and compared to control modules with similar substrate depths but left bare of vegetation. Experimental observations showed that: a) green roofs substantially reduce thermal load over the rooftop, with significant effects of substrate depth and no apparent impact of vegetation type; b) thermal effects are strongly influenced by substrate water content; c) green roofs strongly reduce water runoff with significant substrate x vegetation effects. Our data suggest that green roof design addressed to optimization of the thermal functions should take into account adequate planning of substrate depth. Moreover, our data show that vegetated modules out-competed medium-only ones in terms of runoff reduction capacity, in accordance with some previous studies. Both shrub-vegetated and herbaceous modules intercepted and stored more than 90% rainfall during intense precipitation events, with no significant difference between the two vegetation types despite different substrate depths.
This study developed municipal green roof promotion strategies by reviewing successful green roof initiatives of the international municipalities. This paper consists of three major parts: the first introduced some green roof initiative policies in some cities. The second part of this paper employs the key findings of some municipal governments have set green roof policies and program to assist the government policy makers can better determine which policies suit their needs. The third part of this paper is to propose some green roof promotion strategies for the municipalities to pursue the goals of energy-saving carbon reduction and ecological-compensation for the cities.
Green roofs have been increasingly enlisted to alleviate urban environmental problems associated with urban heat island effect and stormwater quantity and quality. Most studies focus on extensive green roofs, with inadequate assessment of the complex intensive type, subtropical region, and thermal insulation effect. This study examines the physical properties, biological processes, and thermal insulation performance of an intensive green roof through four seasons. An experimental woodland installed on a Hong Kong building rooftop was equipped with environmental sensors to monitor microclimatic and soil parameters. The excellent thermal performance of the intensive green roof is verified. Even though our site has a 100cm thick soil to support tree growth, we found that a thin soil layer of 10cm is sufficient to reduce heat penetration into building. Seasonal weather variations notably control transpiration and associated cooling effect. The tree canopy reduces solar radiation reaching the soil surface, but the trapped air increases air temperature near the soil surface. The substrate operates an effective heat sink to dampen temperature fluctuations. In winter, the subtropical green roof triggers notable heat loss from the substrate into the ambient air, and draws heat upwards from warmer indoor air to increase energy consumption to warm indoor air. This finding deviates from temperate latitude studies. The results offer hints to optimize the design and thermal performance of intensive green roofs.
The passive cooling effect of green roofs in humid, tropical Hong Kong was investigated with reference to three vegetated
plots, grass, groundcover herb, and shrub, with contrasting growth form and biomass structure and a bare control plot. Temperature
was monitored at 15-min intervals for a year at seven levels: high (H) at 200 cm, middle (M) at 60 cm, low (L) at 20 cm, surface,
soil, rockwool (water storage), and roof-tile surface. The findings indicated the crucial roles played by biomass quantity
and structural complexity in passive cooling functions. Temperature variations of vegetated roofs occurred mainly during the
day, with lower maximum and minimum than the control, but they did not cool air at night better than the control. Control
and grass surfaces were warmed above the ambient temperature, but groundcover and shrub surfaces followed the ambient. Despite
complex biomass structure, shrub created the most extreme diurnal air temperature regime. Despite simple biomass structure,
grass cooled air more effectively than groundcover and shrub. Four anomalies in the vertical temperature profile were detected.
First, the grass roof cooled daytime near-ground air to create a suspended temperature inversion. Second, the stagnant air
within the shrub biomass trapped heat to generate a daytime canopy temperature inversion. Third, the elevated branch-foliage
biomass of groundcover and shrub brought passive cooling to form a perched thermal discontinuity. Fourth, the air gap of the
plastic drainage layer arrested downward heat transmission in all vegetated plots to form a subsurface thermal discontinuity.
The findings provide hints on species choice and design of green roofs.
To model the impacts of ecoroofs on building envelope heat transfer accurately, thermal property data for ecoroof soils are needed. To address this need we have measured thermal conductivity, specific heat capacity, thermal emissivity, short wave reflectivity (albedo) and density for ecoroof soil samples over a range of moisture states. To represent a wide range of commonly used ecoroof soils we created eight test samples using an aggregate (expanded shale or pumice), sand, and organic matter in varying volumetric composition ratios. The results indicate significant variability in properties as a function both of soil composition and soil wetness. Thermal conductivity ranged from 0.25 to 0.34W/(mK) for dry samples and 0.31–0.62W/(mK) for wet samples. Specific heat capacity ranged from 830 to 1123J/(kgK) for dry samples and 1085–1602J/(kgK) for wet samples. Albedo was consistently higher for dry samples (0.17–0.40) decreasing substantially (0.04–0.20) as moisture was added. Thermal emissivities were relatively constant at 0.96±0.02 regardless of soil type or moisture status. These results are discussed in the context of their impacts on building energy consumption and the importance of including daily and seasonal property variation within models of the ecoroof energy balance.
The use of passive cooling on roofs holds a significant unfulfilled potential in hot-arid regions. In this study, the contribution of a watered soil with two types of shading for roof cooling was assessed. Two test cells of approximately 4m2 in area and 2.5m high were monitored during the summer season. Both cells were covered with a 16-cm layer of soil. One was untreated while the other was watered and shaded consecutively by means of an overhead shading mesh and a layer of lightweight gravel. Temperature profiles were measured across the section of each roof, from the top surface of the soil to the ceiling inside the chamber, and embedded heat flux plates were also used to evaluate the cooling effect. A comparison of the two shading strategies demonstrated that while the mesh provided more cooling over a daily cycle, the daytime cooling potential, which is crucial in a desert climate, was higher with lightweight gravel.
The research deals with the experimental assessment of the yearly thermal performance of a green roof compared to other passive cooling technologies under temperate climate. All roofs are installed on highly insulated slabs (U-value < 0.25 W/m2 K), in order to understand whether in summer the passive cooling effects are inhibited by the low thermal transmittance recently introduced in many southern Europe countries to meet the demands of the energy saving regulations for the winter heating season. Even though many studies have focused on the performance of green roofs, there is little knowledge regarding their potential in highly insulated roofs. Most of the studies derived from analytical simulations, but the thermal behaviour of a green roof is a complex phenomenon and involves combined heat and mass transfer exchanges difficult to assess with analytical models, while more experimental data are necessary. Optical properties of the roofs covering materials were experimentally measured, and the thermal transmittance of the roofs was experimentally evaluated. Field measurements show how in winter the green roof is able to guarantee further insulation even in saturation conditions. In summer, the green roof mitigates incoming heat fluxes and ceiling temperatures. Its performance is however partially hindered by high insulation.
Air pollution problems caused from the development of infrastructures are getting serious, in which air flow is reduced and heat is trapped among high-rise buildings. In order to mitigate these problems, various methods have been developed in previous studies. Extensive green roof has been identified as one of the most important means to mitigate these problems and implement sustainable development principles in the building features. Governments world-wide have been introducing various policies and regulations for promoting extensive green roof particularly for building projects. However, the existing buildings in many large cities such as Hong Kong display few extensive green roof features. Hong Kong is one of the most densely populated cities with many high-rise buildings. This paper examines the major barriers encountered in promoting extensive green roof systems for the existing buildings in Hong Kong. Case study approach is adopted to investigate how and why the barriers can hinder the implementation of extensive green roof features. Research results show that lack of promotion and incentives from governments and the increase maintenance cost are identified as the top barriers to the implementation. The paper concludes by providing further suggestions and actions that can help mitigate these existing barriers.
Modular green roofs were investigated to better understand surface and membrane level temperature expectations of unirrigated green roofs during hot summer conditions in south-central Texas. We used three succulent monocultures, Sedum kamtschaticum, Delosperma cooperi, Talinum calycinum syn. Phemeranthus calycinus and one unplanted control module, each replicated 3 times. Media surface and below media temperatures were monitored, as well as soil water content and general weather conditions (RH, air temperature). Temperatures at the surface and below the media surface were compared with temperatures of a standard roof surface. We found that diurnal surface temperature reductions were very stable throughout the summer. Much larger temperature reductions were achieved below the modules than at the soil surface. Temperature reductions at the soil surface were predominantly driven by soil volumetric water content (VWC) and, to a lesser degree, air temperature while species and percent cover had small modifying effects through interactions with VWC and air temperature. Temperature reductions below the modules were driven by surface soil temperature, while increasing VWC led to a small decrease in temperature reductions at the membrane level. Mean daily temperature reductions achieved were 18.0 °C at the soil surface and 27.5 °C below the module, thus demonstrating that unirrigated, succulent-based green roofs can provide significant rooftop temperature reductions during hot, dry summer conditions.
This paper presents the results of a comparative study aiming to investigate the suitability of materials used in outdoor urban spaces in order to contribute to lower ambient temperatures and fight heat island effect. The study involved in total 93 commonly used pavement materials outdoors and was performed during the whole summer period of 2001. The thermal performance of the materials was measured in detail using mainly infrared thermography procedures. The collected data have been extensively analysed using statistical techniques. Comparative studies have been performed in order to identify the major advantages and disadvantages of the materials studied. Materials have been classified according to their thermal performance and physical properties into 'cool' and 'warm' materials. The impact of color, surface roughness and sizing has been analysed as well. The study can contribute to selection of more appropriate materials for outdoor urban applications, and thus assist to fight the heat island effect, decrease the electricity consumption of buildings and improve outdoor thermal comfort conditions.
Singapore is a Garden City. On the other hand, with increasing population and limited territory, the government had to adopt a high-density and high-rise residential strategy. The high concentration of buildings in cities has resulted in many environmental issues, such as the Urban Heat Island effect. The rising ecological concerns for building design attempt to create harmony between buildings and their surroundings through mitigating their negative impact on the environment. The greening of buildings is essentially one of the ecological measures. The National Parks Board (NParks) launched a skyrise greenery approach/programme recently to promote the greening of buildings.To explore the thermal benefits, a before and after measurement and an experiment were carried out. The green roof tends to experience lower surface temperature than the original exposed roof surface, especially in areas well covered by vegetation. A maximum temperature difference of 18 °C was observed. However, the substrate temperature measured can exceed the surface temperature of the original exposed roof when the substrate is very dry. In areas that tend to be sparsely covered by vegetation, the peak temperature recorded was up to 73.4 °C during the day time. Ambient air temperature, correspondingly, at 300 mm above the substrate surface can reach 40 °C. This is worsened by the lower wind speed recorded after the measurements. The resulting substrate moisture, when the green roof systems are well covered by vegetation, will tend to keep substrate temperature lower than the original exposed bare roof. The heat flux through the roof structure was greatly reduced due to the installation of extensive systems. Maximally, over 60% of heat gain was stopped by the system. The impact of different types of vegetation may vary as well. Those with relatively extensive greenery coverage led to better thermal performance.
Measurements of the thermal behaviour of two residential buildings equipped with a green roof system have been performed in Athens, Greece. Experimental data have been used to calibrate detailed simulation tools and the specific energy and environmental performance of the planted roofs system has been estimated in detail. Simulations have been performed for free-floating and thermostatically controlled conditions. The expected energy benefits as well as the possible improvements of the indoor thermal comfort have been assessed. It is found that green roofs have a limited contribution to the heating demand of insulated buildings operating under the Mediterranean climate. On the contrary, the green roof system is found to contribute highly to reduce the cooling load of thermostatically controlled buildings. For the considered residential buildings, a cooling load decrease of about 11% has been calculated. In parallel, it is found that green roofs contribute to improve thermal comfort in free-floating buildings during the summer period. The expected maximum decrease of the indoor air and roof surface temperatures is close to 0.6°C. Such a decrease contributes to reduce by 0.1 the summer absolute Predicted Mean Vote Comfort Index levels in the building. Copyright © 2009 John Wiley & Sons, Ltd.
Globally, buildings are responsible for approximately 40% of the total world annual energy consumption. Most of this energy is for the provision of lighting, heating, cooling, and air conditioning. Increasing awareness of the environmental impact of CO2 and NOx emissions and CFCs triggered a renewed interest in environmentally friendly cooling, and heating technologies. Under the 1997 Montreal Protocol, governments agreed to phase out chemicals used as refrigerants that have the potential to destroy stratospheric ozone. It was therefore considered desirable to reduce energy consumption and decrease the rate of depletion of world energy reserves and pollution of the environment. One way of reducing building energy consumption is to design building, which are more economical in their use of energy for heating, lighting, cooling, ventilation and hot water supply. Passive measures, particularly natural or hybrid ventilation rather than air-conditioning, can dramatically reduce primary energy consumption. However, exploitation of renewable energy in buildings and agricultural greenhouses can, also, significantly contribute towards reducing dependency on fossil fuels. Therefore, promoting innovative renewable applications and reinforcing the renewable energy market will contribute to preservation of the ecosystem by reducing emissions at local and global levels. This will also contribute to the amelioration of environmental conditions by replacing conventional fuels with renewable energies that produce no air pollution or greenhouse gases. The provision of good indoor environmental quality while achieving energy and cost-efficient operation of the heating, ventilating and air-conditioning (HVAC) plants in buildings represents a multi-variant problem. The comfort of building occupants is dependent on many environmental parameters including air speed, temperature, relative humidity and quality in addition to lighting and noise. The overall objective is to provide a high level of building performance (BP), which can be defined as indoor environmental quality (IEQ), energy efficiency (EE) and cost efficiency (CE). - Indoor environmental quality is the perceived condition of comfort that building occupants experience due to the physical and psychological conditions to which they are exposed by their surroundings. The main physical parameters affecting IEQ are air speed, temperature, relative humidity and quality. - Energy efficiency is related to the provision of the desired environmental conditions while consuming the minimal quantity of energy. - Cost efficiency is the financial expenditure on energy relative to the level of environmental comfort and productivity that the building occupants attained. The overall cost efficiency can be improved by improving the indoor environmental quality and the energy efficiency of a building. This article discusses the potential for such integrated systems in the stationary and portable power market in response to the critical need for a cleaner energy technology. Anticipated patterns of future energy use and consequent environmental impacts (acid precipitation, ozone depletion and the greenhouse effect or global warming) are comprehensively discussed in this paper. Throughout the theme several issues relating to renewable energies, environment and sustainable development are examined from both current and future perspectives.
The advantages of the planned roofs are undoubtedly numerous from both the ecological and the social point of view. They act positively upon the climate of the city and its region, as well as upon the interior climate of the buildings beneath them. They give protection from the solar radiation, which is the main factor in passive cooling. By reducing thermal fluctuation on the outer surface of the roof and by increasing their thermal capacity, they contribute, to the cooling of the spaces below the roof during the summer and to the increase of their heat during the winter. Due to the decrease of the thermal losses, the green roofs save the energy consumption.This paper refers to the analysis of the thermal properties and energy performance study of the green roof. The investigation were implemented in two phases: during the first phase, extended surface and air temperature 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 energy saving were examined, through a mathematical approach.
In a very hot climate equivalent to a Japanese summer, the reduction of heat coming into rooms is very important with respect to thermal comfort and energy efficiency. The objective of this study is to investigate the evaporative cooling effect from roof lawn gardens planted in non-woven fabric as one mode of passive cooling. It was confirmed by field measurements during the summer that the amount of heat coming into the rooms was reduced by a roof lawn garden. That is, the surface temperature of the roof slab decreased from about 60 to 30°C during day time, which was estimated to be followed by a 50% reduction in heat flux into the room by simple calculation. The evaporative cooling effect from roof lawn gardens is considered to play an important role in reducing heat flux. In order to evaluate the evaporative cooling effect of a roof lawn garden, analysis of the heat and moisture transport in the lawn garden is required. Thus, along with those field measurement, a wind tunnel experiment in a room was carried out in order to obtain the basic information for understanding and predicting the heat and moisture transport in the lawn. Furthermore, a numerical calculation by a simultaneous transport model of heat and moisture was carried out using the results of the wind tunnel experiment. The calculated results were in fairly good agreement with the measured values, and the evaporative cooling effect by the roof lawn garden was shown. For more accurate and quantitative evaluation and prediction, sensitivity analysis of the transport parameters and the examination of the proposed model including measurements are required.
Green roof utilisation has been known since ancient times both in hot and cold climates. Nowadays, it has been reconsidered at issue of energy saving and pollution reduction. In this paper, some measurement sessions on a green roof installed by the Vicenza Hospital are described. A data logging system with temperature, humidity, rainfall, radiation, etc. sensors surveyed both the parameters related to the green roof and to the rooms underneath. The aim is to evaluate the passive cooling, stressing the evapotranspiration role in summer time. Furthermore, the enhanced insulating properties have been tested during winter time. A predictive numerical model has been developed in a building simulation software (TRNSYS) to calculate thermal and energy performances of a building with a green roof, varying the meteorological dataset for a specific geographic zone.
There is increasing public, industry and government interest in establishing green roofs in Australian cities due to their demonstrated environmental benefits. While a small number of green roofs have been constructed in Australia, most are roof gardens or intensive green roofs. Despite their potential as a climate change adaptation and mitigation tool and their widespread use in the northern hemisphere, there are very few examples of extensive green roofs in Australia. One of the major barriers to increasing the prevalence of extensive green roofs in Australia is the lack of scientific data available to evaluate their applicability to local conditions. Relying on European and North American experience and technology is problematic due to significant differences in climate, available substrates and plants. This paper examines green roofs in Australia, discusses the challenges to increasing their use and the major information gaps that need to be researched to progress the industry in Australia.
Energy consumption in the residential and tertiary sectors is especially high in developed countries. There is a great potential for energy savings in these sectors. Energy conservation measures are developed for newly constructed buildings and for buildings under refurbishment. However, to achieve a significant reduction in energy consumption apart from the standard energy-efficiency methods, innovative technologies should be implemented, including renewable energy. Coherency of standard, modern energy efficiency and renewable options becomes necessities. To approach the idea of sustainable buildings, a few developmental steps are needed, regarding energy, water, land and material conservation, together with environmental loading, and the qualities of the indoor and outdoor environments.
This paper deals with the experimental investigation and analysis of the energy and environmental performance of a green roof system installed in a nursery school building in Athens. The investigation was implemented in two phases. During the first phase, an experimental investigation of the green roof system efficiency was presented and analysed, while in the second one the energy savings was examined through a mathematical approach by calculating both the cooling and heating load for the summer and winter period for the whole building as well as for its top floor. The energy performance evaluation showed a significant reduction of the building's cooling load during summer. This reduction varied for the whole building in the range of 6–49% and for its last floor in the range of 12–87%. Moreover, the influence of the green roof system in the building's heating load was found insignificant, and this can be regarded a great advantage of the system as any interference in the building shell for the reduction of cooling load leads usually to the increase of its heating load.
Vegetation strategically placed on roofs and walls can be considered as a complement of urban greens. It is actually an ecological solution to the concrete jungle in cities. The benefits of green roofs are unquestionable from the thermal point of view. But some quantitative data on this subject are still needed in the context of tropical climate. Therefore, a field measurement was conducted in Singapore to investigate the thermal impacts of rooftop garden. From the derived data, it has been confirmed that rooftop gardens contribute thermal benefits to both buildings and their surrounding environments.
Traditional construction practices provide little opportunity for environmental remediation to occur in urban areas. As concerns for environmental improvement in urban areas become more prevalent, innovative practices which create ecosystem services and ecologically functional land cover in cities will be in higher demand. Green roofs are a prime example of one of these practices. The past decade has seen the North American green roof industry rapidly expand through international green roof conferences, demonstration sites, case studies, and scientific research. This study evaluates existing international and North American green roof policies at the federal, municipal, and community levels. Green roof policies fall into a number of general categories, including direct and indirect regulation, direct and indirect financial incentives, and funding of demonstration or research projects. Advantages and disadvantages of each category are discussed. Salient features and a list of prompting standards common to successfully implemented green roof strategies are then distilled from these existing policies. By combining these features with data collected from an experimental green roof site in Athens, Georgia, the planning and regulatory framework for widespread green roof infrastructure can be developed. The authors propose policy instruments be multi-faceted and spatially focused, and also propose the following recommendations: (1) Identification of green roof overlay zones with specifications for green roofs built in these zones. This spatial analysis is important for prioritizing areas of the jurisdiction where green roofs will most efficiently function; (2) Offer financial incentives in the form of density credits and stormwater utility fee credits to help overcome the barriers to entry of the new technology; (3) Construct demonstration projects and institutionalize a commitment greening roofs on publicly-owned buildings as an effective way of establishing an educated roofing industry and experienced installers for future green roof construction.
Green Roof Policies-A guideline for decision makers and Green Roof supporters
- W Ansel
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Past-Present-Future: Green Roof Techniques in Changing Times, Green Roofs-Bringing Nature Back to Town. International Roof Congress
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Growing green guide: a guide to green roofs, walls and facades in Melbourne and Victoria, Australia, Victorian Department of Environment and Primary Industries
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Growing green guide: green roofs, walls & facades policy options background paper
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Thermal performance of green roofs through field evaluation
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