Green Roof Installation Research Articles

Connecting the resource nexus to basic urban service provision – with a focus on water-energy interactions in New York City

Urban water and energy systems are crucial for sustainably meeting basic service demands in cities. This paper proposes and applies a technology-independent “reference resource-to-service system” framework for concurrent evaluation of urban water and energy system interventions and their ‘nexus’ or ‘interlinkages’. In a concrete application, data that approximate New York City conditions are used to evaluate a limited set of interventions in the residential sector, spanning from low-flow toilet shifts to extensive green roof installations. Results indicate that interventions motivated primarily by water management goals can considerably reduce energy use and contribute to mitigation of greenhouse gas emissions. Similarly, energy efficiency interventions can considerably reduce water use in addition to lowering emissions. However, interventions yielding the greatest reductions in energy use and emissions are not necessarily the most water conserving ones, and vice versa. Useful further research, expanding the present analysis should consider a broader set of resource interactions, towards a full climate, land, energy and water (CLEW) nexus approach. Overall, assessing the impacts, trade-offs and co-benefits from interventions in one urban resource system on others also holds promise as support for increased resource efficiency through integrated decision making.

Ten-year persistence of native plant species on a green roof in Northeast US

We evaluated persistence of 28 northeastern or Midwestern US native plants for 10 y after installation on the green roof of the Wellesley College Water Treatment Facility. After 10 y, 50% of the original 28 species had perished and 29% had greatly expanded their presence on the green roof. Allium cernuum Roth (nodding onion; Liliaceae), Antennaria plantaginifolia (L.) Richardson (pussytoes; Asteraceae), Arctostaphylos uva-ursi (L.) Spreng. (kinnikinnick; Ericaceae), Campanula rotundifolia L. (bluebell bellflower; Campanulaceae), Carex pensylvanica Lam. (Pennsylvania sedge; Cyperaceae), Eurybia divaricata (L.) G.L. Nesom (white wood aster; Asteraceae), Fragaria virginiana Duchesne (Virginia strawberry; Rosaceae), ( Oligoneuron album (Nutt.) G.L. Nesom (prairie goldenrod; Asteraceae), Schizachyrium scoparium (Michx.) Nash (little bluestem; Poaceae), Solidago sciaphila Steele (shadowy goldenrod; Asteraceae), and Symphyotrichum laeve (L.) A. Love & D. Love (smooth blue aster; Asteraceae) were able to persist and grow in response to the challenging green roof conditions, and we recommend them for use on green roofs. Twenty-seven emergent species were present on the green roof at the end of the study. Emergent Solidago L. (goldenrod; Asteraceae) species, Solidago caesia L. (wreath goldenrod), Solidago odora Aiton (anise scented goldenrod), and Solidago rugosa Mill. (wrinkleleaf goldenrod) should be investigated further for their suitability in green roof installations. Lubell JD, Connolly B, Jones KN. 2017. Ten-year persistence of native plant species on a green roof in Northeast US. Native Plants Journal 18(3):227–234.

Optimal location selection for the installation of urban green roofs considering honeybee habitats along with socio-economic and environmental effects

This study proposes a new framework for the selection of optimal locations for green roofs to achieve a sustainable urban ecosystem. The proposed framework selects building sites that can maximize the benefits of green roofs, based not only on the socio-economic and environmental benefits to urban residents, but also on the provision of urban foraging sites for honeybees. The framework comprises three steps. First, building candidates for green roofs are selected considering the building type. Second, the selected building candidates are ranked in terms of their expected socio-economic and environmental effects. The benefits of green roofs are improved energy efficiency and air quality, reduction of urban flood risk and infrastructure improvement costs, reuse of storm water, and creation of space for education and leisure. Furthermore, the estimated cost of installing green roofs is also considered. We employ spatial data to determine the expected effects of green roofs on each building unit, because the benefits and costs may vary depending on the location of the building. This is due to the heterogeneous spatial conditions. In the third step, the final building sites are proposed by solving the maximal covering location problem (MCLP) to determine the optimal locations for green roofs as urban honeybee foraging sites. As an illustrative example, we apply the proposed framework in Seoul, Korea. This new framework is expected to contribute to sustainable urban ecosystems.

Use of compost as amendment for soilless substrates of plants in green roof installations

Use of compost as amendment for soilless substrates of plants in green roof installations

Probabilistic private cost-benefit analysis for green roof installation: A Monte Carlo simulation approach

Green roofs are known as one of the environmentally-friendly applications and also as a sustainable approach in developing countries. Although many researchers have proven the environmental benefits of installing green roofs all around the world, they have not been used widely in many countries due to the lack of knowledge about cost-benefit issues. This paper places an emphasis on all the private factors affecting cost-benefit analysis. Installation, operation and maintenance costs are compared with the benefits such as energy saving, the increase in property value, and the acoustic effect in order to determine two indicators namely “net present value” and “pay-back period,” using the Monte Carlo simulation. Two scenarios are considered in the analyses: using the property, and selling the property after construction. Moreover, correlation and regression sensitivity analyses are also conducted. The capital of Malaysia, Kuala Lumpur, is selected for the case study due to the lack of cost-benefit analysis in developing countries. The results show that there is low probability of loss in the installation of both types of green roofs during their lifespans. Moreover, net present value for intensive green roofs is found to be higher than extensive ones, whereas the payback period for installing extensive green roofs is lower than intensive green roofs. It is concluded that the probability of loss for the owner is higher than that of benefit in the scenario of selling the property after construction resulting from the installation of both types of green roofs.

Parameterizing a Water-Balance Model for Predicting Stormwater Runoff from Green Roofs

Crop coefficients (kc) were calculated for three different species of common green roof succulents from March to November in 2011, to parameterize the Food and Agricultural Organization of the United Nations (FAO) Penman-Monteith equation for use in a mechanistic green roof water-balance model. Seasonally averaged kc values for each species for 2011 were used to predict plant evapotranspiration (ET) in 2012. The adjusted FAO Penman-Monteith equation predicted the total annual ET within 3–13 mm, a substantial improvement over model predictions with kc set to 1, which overpredicted ET by 100 mm or more, depending on the species. The adjusted equation was inserted in water-balance models, which predicted runoff within 2–13% of measured totals for 2012. This discrepancy may be explained by variability in maximum water-holding capacity, which is difficult for two-dimensional models to predict. Nevertheless, these results provide increased confidence in the use of models to predict stormwater runoff from green roofs, and evaluate performance. Monitoring multiple green roof installations with cost-effective sensor networks will increase the ability to identify the key components to enhance green roof function, reduce stormwater runoff, and inform future design.

Soil-roots Strength Performance of Extensive Green Roof by Using Axonopus Compressus

Green roof technology has been proven to provide potential environmental benefits including improved building thermal performance, removal of air pollution and reduced storm water runoff. Installation of green roof also involved soil element usage as a plant growth medium which creates several interactions between both strands. This study was carried out to investigate the soil-roots strength performance of green roof at different construction period up to 4 months. Axonopus compressus (pearl grass) was planted in a ExE test plot with a designated suitable soil medium. Direct shear test was conducted for each plot to determine the soil shear strength according to different construction period. In addition, some basic geotechnical testing also been carried out. The results showed that the shear strength of soil sample increased over different construction period of 1st, 2nd, 3rd and 4th month with average result 3.81 kPa, 5.55 kPa, 6.05 kPa and 6.48 kPa respectively. Shear strength of rooted soil samples was higher than the soil samples without roots (control sample). In conclusion, increment of soil-roots shear strength was due to root growth over the time. The soil-roots shear strength development of Axonopus compressus can be expressed in a linear equation as: y = 0.851x + 3.345, where y = shear stress and x = time.

Applying artificial intelligence modeling to optimize green roof irrigation

Recent increase in green-roof installation has increased irrigation water consumption which could be wasteful using conventional watering management protocol. The knowledge gap in irrigation optimization to achieve water conservation could be filled. The complicated conventional approach uses weather and soil sensors to calculate watering needs, which is impractical and not cost-effective. This study employs artificial intelligence algorithms composed of artificial neural network and fuzzy logic, using weather data to simulate soil moisture changes to develop an optimal irrigation strategy. The artificial neural network is trained to predict soil moisture based on four daily weather variables: real-time air temperature, relative humidity, solar radiation, and wind speed. Fuzzy-neural network is applied to determine the irrigation time and watering volume. The simulation model successfully mimics the human brain in making irrigation decision. The artificial intelligence irrigation could maintain adequate soil moisture ranging from 0.13 to 0.22m3/m3 and reduce 20% of water use with improved plant coverage. Since the evapotranspiration from living vegetation plays a key role in the passive cooling mechanism, better plant coverage could increase the thermal-energy performance of green roofs. The low-cost and effective technique can motivate the adoption of green roofs by alleviating the water consumption obstacle.

Drought versus heat: What's the major constraint on Mediterranean green roof plants?

Green roofs are gaining momentum in the arid and semi-arid regions due to their multiple benefits as compared with conventional roofs. One of the most critical steps in green roof installation is the selection of drought and heat tolerant species that can thrive under extreme microclimate conditions. We monitored the water status, growth and survival of 11 drought-adapted shrub species grown on shallow green roof modules (10 and 13cm deep substrate) and analyzed traits enabling plants to cope with drought (symplastic and apoplastic resistance) and heat stress (root membrane stability). The physiological traits conferring efficiency/safety to the water transport system under severe drought influenced plant water status and represent good predictors of both plant water use and growth rates over green roofs. Moreover, our data suggest that high substrate temperature represents a stress factor affecting plant survival to a larger extent than drought per se. In fact, the major cause influencing seedling survival on shallow substrates was the species-specific root resistance to heat, a single and easy measurable trait that should be integrated into the methodological framework for screening and selection of suitable shrub species for roof greening in the Mediterranean.

How do green roofs mitigate urban thermal stress under heat waves?

AbstractAs the climate warms, heat waves (HWs) are projected to be more intense and to last longer, with serious implications for public health. Urban residents face higher health risks because urban heat islands exacerbate HW conditions. One strategy to mitigate negative impacts of urban thermal stress is the installation of green roofs (GRs) given their evaporative cooling effect. However, the effectiveness of GRs and the mechanisms by which they have an effect at the scale of entire cities are still largely unknown. The Greater Beijing Region is modeled for a HW scenario with the Weather Research and Forecasting model coupled with a state‐of‐the‐art urban canopy model (Princeton Urban Canopy Model) to examine the effectiveness of GRs. The results suggest that GR would decrease near‐surface air temperature (ΔT2max = 2.5 K) and wind speed (ΔUV10max = 1.0 m s−1) but increase atmospheric humidity (ΔQ2max = 1.3 g kg−1). GRs are simulated to lessen the overall thermal stress as indicated by apparent temperature (ΔAT2max = 1.7°C). The modifications by GRs scale almost linearly with the fraction of the surface they cover. Investigation of the surface‐atmosphere interactions indicates that GRs with plentiful soil moisture dissipate more of the surface energy as latent heat flux and subsequently inhibit the development of the daytime planetary boundary layer (PBL). This causes the atmospheric heating through entrainment at the PBL top to be decreased. Additionally, urban GRs modify regional circulation regimes leading to decreased advective heating under HW.

The outdoor microclimate benefits and energy saving resulting from green roofs retrofits

This paper focuses on the benefits on the local microclimate and the building energy saving resulting from green roof retrofits. The research investigates a case study located in a university campus in Toronto, Canada. After completing a detailed energy audit of the building, an assessment of the benefits resulting from the installation of an extensive green roof was performed. A virtual model validated using multiyear data of a local network of different weather stations was used to simulate the effects of the green roof retrofit over the outdoor microclimate. Then, a building energy model was used to compare the energy saving of several green roof designs. Results indicate that increasing the leaf area index (LAI) would lead to an increased cooling effect of the air temperature up to 0.4°C during the day at pedestrian level, while a more significant temperature reduction would be obtained only at the rooftop level. This confirms the potential of green roofs as urban heat island mitigation strategy. The adoption of a green roof retrofit resulted in a building energy demand reduction by 3%, and in significantly improved indoor comfort levels in the floor below the green roof. Finally, the parametric analysis of different green roof options showed that for building energy savings, increasing the soil depth is more important than increasing the LAI.

INVESTIGATING THE ENVIRONMENTAL IMPACTS OF GREEN ROOF INSTALLATION

Green roof installation has been used as a sustainable approach for many years in order to reduce the adverse impacts of environmental issues all around the world. These environmental issues include: global warming, air pollution, flood, carbon footprint and so on. Green roofs are practical as a sustainable approach, especially in developed countries; however, there is an increasing trend of implementing green roofs in developing countries. This paper is reviewing the benefits of green roof installation and how green roof might contribute to reduce the negative environmental issues. The environmental problems that can be solved or reduced by these benefits are discussed separately in this paper. It is concluded that due to the environmental benefits of green roof installation, it leads to many economic benefits. Moreover, green roof implementation is beneficial in both urban areas and rural places.

Identifying keystone meteorological factors of green-roof stormwater retention to inform design and planning

Green roofs have received considerable attention for multiple ecosystem services which include stormwater amelioration. Offering sustainable stormwater source-control, green roofs utilize otherwise unused impervious urban surfaces to restore pre-development hydrologic functions such as infiltration and retention. Its ability to retain stormwater is related to external and internal factors. Meteorological conditions prior to a rainfall event can influence antecedent evapotranspiration, the only water-exit pathway of the green-roof system besides discharge. Whereas evapotranspiration may be difficult and costly to measure directly, other common meteorological conditions can be monitored conveniently and inexpensively even prior to actual green-roof installation. Identifying salient on-site meteorological factors may provide valuable insights into hydrologic dynamics, and inform green-roof design and planning decisions. A statistical regression approach identified potential antecedent meteorological factors and moisture indicators of extensive green-roof retention. Continuous field-monitoring data revealed the combined effects of rainfall depth, wind speed, solar radiation, and antecedent dry weather period to explain the measured stormwater retention under a humid subtropical rainfall regime. Amongst the studied environmental factors, solar radiation and wind speed contributed notably to green-roof stormwater retention and may provide a dependable basis to assist green-roof site selection when resources are limited. It is important to incorporate site-specific planning and assessment prior to green infrastructure design and implementation to maximize hydrologic and evaporative cooling services in cities with complex topographical features.

Microclimate and substrate depth influence green roof plant community dynamics

The depth of growing media on green roofs can strongly influence plant growth and survival. The setting of the roof, including wind exposure, shading and the nature of the building below the green roof can also impact microclimate and plant performance, but the effects of roof setting have been little investigated. We conducted a five-year experiment in which green roofs differing in substrate depth and the nature of the underlying and surrounding structures (heated/air-conditioned vs. non-air-conditioned space underneath; height of green roof relative to roof surface) were planted with the same mixture of 11 species. After five years with no irrigation or weeding, planted species were still present in all treatments, but communities were dominated by ruderal species that colonized from the surrounding area. Both substrate depth and exposure affected plant community composition with exposure making a larger difference. Plant community change was greatest after the driest year of the study, suggesting that substrate depth and exposure largely exerted influence over plant communities via drought. The system installed over non-air-conditioned space had much more variable substrate temperatures and higher maximum and lower minimum temperatures. Installation of green roofs over non-air-conditioned spaces and over well-insulated buildings is common and the effects on plants of more extreme microclimates caused by these roof settings should be incorporated into plant selection criteria.

Hydrologic modeling of Low Impact Development systems at the urban catchment scale

Summary In this paper, the implementation of Low Impact Development systems (LIDs) as source control solutions that contribute to restore the critical components of natural flow regimes, is analyzed at the urban catchment scale. The hydrologic response of a small urban catchment is investigated under different land use conversion scenarios including the installation of green roofs and permeable pavements. The modeling is undertaken using the EPA SWMM; the “do nothing” scenario is calibrated and validated based on field measurements while the LID control modules are calibrated and validated based on laboratory test measurements. The simulations are carried out by using as input the synthetic hyetographs derived for three different return periods (T = 2, 5 and 10 years). Modeling results confirm the effectiveness of LID solutions even for the design storm event (T = 10 years): in particular a minimum land use conversion area, corresponding to the Effective Impervious Area reduction of 5%, is required to obtain noticeable hydrologic benefits. The conversion scenario response is analyzed by using the peak flow reduction, the volume reduction and the hydrograph delay as hydrologic performance indexes. Findings of the present research show that the hydrologic performance linearly increases with increasing the EIA reduction percentages: at 36% EIA reduction (corresponding to the whole conversion of rooftops and parking lot areas), the peak and volume reductions rise till 0.45 and 0.23 respectively while the hydrograph delay increases till 0.19.

Vegetation development on different extensive green roof types in a Mediterranean and temperate maritime climate

Green roofs offer a way to improve ecosystem services in urban areas. In the Mediterranean, specialized companies are still unfamiliar with green roof installation because they consider dry conditions and appropriate plant selection as obstacles. Also in a temperate maritime climate, green roof design and vegetation composition need to be adapted to account for the projected climate changes, including an increase in temperature and more irregular precipitation events. This paper discusses the results of a two-year experiment on unirrigated extensive green roofs which were constructed in both a Mediterranean (Avignon, France) and a temperate maritime climate (Heverlee, Belgium) and on which 18 drought-adapted plants with Mediterranean habitat affinities were tested. Experimental plots differed in solar exposure and substrate type (substrate depth of 5cm and 5 or 10cm above a water retention layer). Climatic conditions affected vegetation development in both locations, with fluctuating cover values in the Mediterranean reflecting the impacts of summer droughts and rainy autumn periods. Overall, the deepest substrate depth of 10cm with a water retention layer resulted in the highest cover and abundance values and the highest species diversity throughout the experiment. Both bryophytes and weeds established spontaneously over the two years, and their importance in terms of biodiversity value is discussed. To improve functional biodiversity of the extensive green roof, we advise to use succulent species together with annuals, perennial herbs and geophytes. The design and species recommendations shed some light on the possibilities for extensive green roof design in regions with summer drought and irregular precipitation and regions facing climate change.

Simulation of Water Regime and Sensible Heat Exchange Phenomena in Green Roof Substrates

The installation of green roofs has increased in the world's largest cities in the last 10 yr. Apart from aesthetic aspects, a main objective is to help regulate water fluxes in the urban environment and, under certain conditions, to reduce a building's heating requirements (by increasing thermal resistance) in winter and cooling requirements (by evaporation) in summer. We developed a theoretical approach using relevant thermal properties (heat capacity and thermal conductivity) for substrates being used for green roofs and propose an approach to modeling the interactions between water transport processes and heat flow and exchanges. Substrate water flow (using the van Genuchten approach) and heat flow were simulated for different mixtures of pozzolan and sphagnum peat. The main result was that substrate water retention properties and rain distribution throughout the year were more important for the rain/drainage balance than was substrate thickness. One‐dimensional simulations of water and heat transfer at the scale of a few days showed that the overall thermal properties of the substrate were driven by its capacity to store water. While water storage capacity could appear as an advantage for rainwater management in urban areas, it could be a drawback for insulation purposes in winter. In summer, at relatively low water contents, the substrate layer limits sensible heat transfer toward the roof; however, this transfer probably remains much lower than latent heat flow from a green roof to the atmosphere due to evapotranspiration under non‐limiting water uptake by plants.

Plant performance on Mediterranean green roofs: interaction of species-specific hydraulic strategies and substrate water relations.

Recent studies have highlighted the ecological, economic and social benefits assured by green roof technology to urban areas. However, green roofs are very hostile environments for plant growth because of shallow substrate depths, high temperatures and irradiance and wind exposure. This study provides experimental evidence for the importance of accurate selection of plant species and substrates for implementing green roofs in hot and arid regions, like the Mediterranean area. Experiments were performed on two shrub species (Arbutus unedo L. and Salvia officinalis L.) grown in green roof experimental modules with two substrates slightly differing in their water retention properties, as derived from moisture release curves. Physiological measurements were performed on both well-watered and drought-stressed plants. Gas exchange, leaf and xylem water potential and also plant hydraulic conductance were measured at different time intervals following the last irrigation. The substrate type significantly affected water status. Arbutus unedo and S. officinalis showed different hydraulic responses to drought stress, with the former species being substantially isohydric and the latter one anisohydric. Both A. unedo and S. officinalis were found to be suitable species for green roofs in the Mediterranean area. However, our data suggest that appropriate choice of substrate is key to the success of green roof installations in arid environments, especially if anisohydric species are employed.

Economic evaluation of green-roof environmental benefits in the context of climate change: The case of Hong Kong

Green roof offers multiple environmental benefits to urban ecosystems. Some of these benefits can aid cities in addressing global and local climate-change challenges. Many studies have attempted to convert a wide variety of green-roof benefits into economic values but few of them focused on climate-change aspects. This study assessed and valued six climate-related benefits in the compact city of Hong Kong based on localized experimental and modeling studies. The six benefits are thermal insulation, urban heat island (UHI) mitigation, avoided upstream emissions of carbon dioxide (CO2) and air pollutants, CO2 sequestration and air pollutants removal. Two roof greening scenarios, extensive (EGR) and intensive (IGR), were evaluated for their annual benefits and lifecycle cost-effectiveness under district-scale installation. Results suggest that large-scale green-roof installation can substantially reduce energy consumption, upstream emission and atmospheric concentration of CO2. Establishment of EGR in the study district Yau Tim Mong (YTM) has an annual total value of USD 12.98 million with unit value of USD 10.77m−2 of green roof, and IGR, USD 22.02 million with unit value of USD 18.33m−2. EGR is more economically attractive than IGR in terms of benefit–cost ratio (BCR) and payback period. The 40-year lifecycle BCR of EGR is 3.84 with a payback period of 6.8 years, while the equivalent values for IGR are 1.63 and 19.5 years, respectively. The study suggests a better integration of green roof into the current policy schemes in Hong Kong to combat climate change due to the notable benefits and cost-effective characteristics.

Energy Saving Effects of Green Roof in Exiting Buildings according to Different Insulation Levels

Energy performance of building envelope components, including external walls, floors, roofs, windows and doors, is crutial for determining how much energy is required for heating and cooling in a building. Among various building technologies, a green roof system can be a good option for reducing heat gain and loss in new buildings as well as existing buildings for green remodeling. This paper evaluates the performance of green roof systems according to soil depth and Leaf Area Index (LAI) for existing buildings. It also attempts to quantify the energy saving effects on new and existing buildings with different insulation levels. Thermal performance of green roofs is mainly dependent on soil thickness and LAI. Installation of green roofs in deteriorated existing buildings can lead to improvements in roof insulation, due to the soil layer. An increase in soil depth leads to a decrease in heating load, regardless of conditions of vegetation on the green roof. Larger LAI values may reduce cooling loads in the cooling season. Installation of green roof in deteriorated existing buildings showed bigger energy saving effect in comparison to a case in new buildings. A simulation study showed that the installation of green roof systems in deteriorated existing buildings with low insulation levels, due to low thermal performance requirements when constructed, could improve the energy performance of the buildings similar or better to the peformance on new buildings with the most updated insulation standard. Thus, when remodeling a deteriorated building, green roofs could be a good option to meet the most recent energy requirements.