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Journal of Cleaner Production 69 (2014) 91e99

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Journal of Cleaner Production

journal homepage: www.elsevier .com/locate/ jc lepro

Lifecycle assessment of living walls: air purification and energyperformance

Haibo Feng, Kasun Hewage*

School of Civil Engineering, University of British Columbia, Kelowna, Okanagan, Canada

a r t i c l e i n f o

Article history:Received 10 June 2013Received in revised form5 November 2013Accepted 12 January 2014Available online 22 January 2014

Keywords:Living wallsLifecycle analysisChemical emissionsEnergy sustainability

* Corresponding author. School of Engineering (Ok1137 Alumni Avenue, Kelowna, BC, Canada. Tel.:2508079850.

E-mail addresses: Haibo.Feng@ubc.ca (H. Fe(K. Hewage).

0959-6526/$ e see front matter � 2014 Elsevier Ltd.http://dx.doi.org/10.1016/j.jclepro.2014.01.041

a b s t r a c t

Covering a building envelope with green walls is considered a sustainable construction practice. Greenwalls can be classified as green facades or living walls based on their purpose and characteristics. Livingwalls are built with different layers and variable planting styles depending on the geographic location,function, and weather conditions. This paper discusses a comparative lifecycle assessment (LCA) of threeliving wall systems: trellis system, planter box system, and felt layer system. Chemical emissions andenergy consumption of the living wall materials are evaluated in the whole lifecycle, and compared withthe chemical absorption and energy savings of operational living walls. The results demonstrated thatthe felt layer system is not environmentally sustainable in air cleaning and energy saving compared tothe trellis system and modular panel system. The environmental performance of living walls is influ-enced by the types of materials and plants chosen for the systems, as well as the external factors, such asclimate and building type. The LCA indicates the need of environmental friendly materials for sustainableliving walls.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

It is recognized that construction practices are one of the majorcontributors of environmental problems. United State Energy In-formation Administration estimated that buildings account for 72%of total electricity consumption, and 38.9% of total carbon dioxideemissions (Buildings Energy Data book, 2008). In order to addressthe environmental concerns, such as global warming, deforesta-tion, waste generation, the concept of sustainability has beenintroduced to the building construction sector. Research shows thatsustainable building practices can considerably reduce the build-ing’s environmental impact in energy consumption. For example, asurvey of 99 green buildings in the United States showed that anaverage of 30% less energywas used in green buildings compared tothe conventional buildings (The Economist, 2004). Other casestudies show that energy-efficient designs can reduce a building’senergy consumption by as much as 50% (The Economist, 2004).Increasingly, vegetation is being used as an important new con-struction material to make the buildings more sustainable(Eumorfopoulou and Kontoleon, 2009; Fioretti et al., 2010).

anagan Campus), EME 4227,þ1 2508078176; fax: þ1

ng), Kasun.Hewage@ubc.ca

All rights reserved.

Integration of vegetation in buildings, through green roofs or greenwalls, increases the building’s ecological and environmental ben-efits (Castleton et al., 2010; Eumorfopoulou and Kontoleon, 2009).

1.1. Types of living walls

Green walls can be divided into two main types: green facadesand living walls. Green facades are systems in which climbingplants or hanging shrubs are grown using special support struc-tures to cover a desired area (Pérez et al., 2011). The plants can beplaced directly on the ground, at the base of the structure, or in potsat different heights of the facade. Green facades are simply based onthe use of climbing plants without the complexity and technifica-tion of the living wall systems (Pérez et al., 2011). Ecological ben-efits of green facades, such as energy savings, thermal insulation,and building protection, are not as pronounced as they are withliving walls (Weinmaster, 2009).

Living walls are made of pre-vegetated panels, vertical modules,or planted blankets that are fixed vertically to a structural wall orframe. The panels and geotextile felts provide support to the plants.These panels are generally made out of plastic, expanded poly-styrene, synthetic fabric, clay, metal, or concrete (Pérez et al., 2011).There are many commercially available living wall systems, andthey can be categorized in terms of different parameters. Loh(2008) classified the living walls into three systems: trellis,modular panel, and felt layer systems. This classification is based on

H. Feng, K. Hewage / Journal of Cleaner Production 69 (2014) 91e9992

the characteristics of the plant box. Perini et al. (2012) classifiedliving walls with different features of growingmedium. The pottingsoil is used as substrate in the planter box living wall system, theform is used as the growing medium in the form substrate livingwall system, and the felt layers are used as substrate and water-proofing in the felt layer living wall system.

1.2. Benefits of living walls in air cleaning and energy savings

In the recent literature, many claims were made about thepositive influences of living walls. The environmental benefits ofliving walls are; increasing the thermal performance of buildings(lowering energy costs), improving air quality, mitigating the UrbanHeat Island effect (UHI), reducing noise pollution, improving watersensitive urban design (WSUD), increasing urban biodiversity andurban food production, and improving human health and well-being (Cheng et al., 2010; Wolverton and Wolverton, 1993;McCarthy et al., 2001; Getter and Rowe, 2006).

1.2.1. Thermal performanceLiving walls contribute to the cooling and insulating benefits of

a building. The air layer between the façade and the living wall hasan insulating effect, which makes the living wall as an extrainsulator for the building envelope (Perini et al., 2011a). Thephototropism effect created by the living walls can ensure acooling effect in warmer climates. Of the sunlight falling on theleaves, 5%e30% is reflected, 5%e20% is used for photosynthesis,10%e50% is transformed into heat, 20%e40% is used for evapo-transpiration, and only 5%e30% passes through the leaves(Krusche et al., 1982; Ottelé et al., 2011). The green vertical clad-ding can also mitigate potential solar heat impact, which affectsthe indoor spaces even after the sunset. Computer simulationmodels showed that the shading provided by living walls in colderclimates can decrease indoor temperatures significantly in sum-mer, and may save 23% of energy costs (Bass and Baskaran, 2011).Eumorfopoulou and Kontoleon (2009) made an investigation inGreece, during the winter, to compare the thermal performance ofa bare concrete wall and a plant-covered building façade. Theresults demonstrated that the surface temperatures of plant-covered wall sections were considerably lower than those of thebare wall sections. The effect was about 10.8 �C. Another recentstudy by Wong et al. (2010a), on a free standing wall in Hortpark(Singapore), with vertical greening, showed a maximum tem-perature reduction of 11.6 �C. Alexandri and Jones (2008) indi-cated that covering the building envelope with vegetation is animportant method to save cooling and heating energy consump-tion. Depending on the climate type and the amount and positionof vegetation on a building, the energy savings can vary from 35%to 90% (Alexandri and Jones, 2008).

1.2.2. Air qualityIt is well known that the outdoor plants can absorb toxic com-

pounds from the air. Wolverton and Wolverton (1993) explainedthat potted-plants can significantly improve indoor air quality, notonly because plants can absorb carbon dioxide and release oxygenthrough photosynthesis, but also plants can reduce air-borne con-taminants such as nitrogen oxides, volatile organic compound(VOCs), and dust. Another experiment conducted by Ottelé et al.(2010) in the Delft University of Technology, demonstrated thatgreen vegetation can reduce number of particulates (<10 mm) inthe air, which have a long-term threat to human health. In addition,living walls can help in absorbing toxic gas emitted by vehicles, andimprove the air quality. In a UK based study of air quality, with anindoor gas heater, Coward et al. (1996) found that houses with sixor more potted-plants showed reductions of over one third in NO2

levels. In a study of Korean native indoor species, Lee and Sim(1999) showed that indoor plants absorb and metabolise SO2.Some additional studies showed that plants effectively reducedlevels of benzene, ammonia, formaldehyde, nitrogen oxides, andparticulate matter (Lohr and Pearson-Mims, 1996). Plants have alsobeen shown to increase indoor relative humidity, by releasingmoisture into the air, thus increasing the comfort level in sealedenvironments (Aydogan and Montoya, 2011).

1.3. Objectives

Living walls have environmental, social, and economical bene-fits such as reducing greenhouse gas emission, adaptation toclimate change, air quality improvement, habitat provision, aes-thetics perfection, and energy savings by insulation (Weinmaster,2009; Ottelé et al., 2010; Perini et al., 2012). However, from thelifecycle point of view, the sustainability of living walls has rarelybeen analyzed. One notable attempt was made by Ottelé et al.(2011), where the environmental burden analysis of living wallsfrom the entire lifecycle was conducted. Global warming potential,human toxicity, and fresh water aquatic eco-toxicity were consid-ered by Ottelé et al. (2011), as the environmental burden profile, toconduct a comparative analysis on energy savings. However, thesustainability of the livingwall was not demonstrated clearly due tothe limitations and variations.

This paper attempts to investigate the sustainability of livingwalls from a new perspective. Since the two major quantitativebenefits of livingwalls are energy savings and air cleaning (Bass andBaskaran, 2001; Eumorfopoulou and Kontoleon, 2009; Cowardet al., 1996; Lee and Sim, 1999), this study not only consider theenergy savings benefit, but also the air cleaning benefit of livingwalls, which makes the investigation more comprehensive.Furthermore, the environmental burden in this study, energyconsumption and chemical emission, is based on the burdencreated by all the components of the living wall, in its entirelifecycle.

The objective of this paper is to compare chemical emissionsand energy consumptions, with air purification and energy savingsof living walls, over the product lifecycle. The paper discusses thesustainability of living walls in terms of energy savings and airpurifications.

By performing the LCA technique, the environmental impacts ofthe living walls could be assessed. The major stages of an LCA studyare raw material acquisition, materials manufacture, production,use/reuse/maintenance, and waste management (USEPA, 2012).The goal of this LCA is to evaluate the environmental performanceof living walls in manufacturing, constructing, maintaining anddisposing of 1 m2 living walls. Furthermore, the air cleaning andenergy saving performance of living walls, in the product lifecycle,are quantified with comprehensive statistical analysis.

Since there aremany commercially available livingwall systems,product costs could vary for different designs and functions. In thisLCA research, three living walls systems are adopted from thegreening systems presented by Ottelé et al. (2011). The first one is atrellis system, where the climber is planted on the ground, andgrown on the stainless steel mesh. The second one is a planter boxliving wall system, and the third one is a felt layer living wall sys-tem. The materials data of the living walls required for the in-ventory analyses were cited from Ottelé et al. (2011).

2. Methodology

This paper evaluates the lifecycle sustainability of living walls,by comparing air pollution and energy consumption in the materialproduction, construction, maintenance, and disposal stages, with

H. Feng, K. Hewage / Journal of Cleaner Production 69 (2014) 91e99 93

air purification and energy savings in the operation phase. The“balance years” refer to the time needed for achieving equality inchemical emissions and energy consumptions.

2.1. Basic approach

As shown in Fig 1, the three living wall systems were chosenfrom Ottelé et al. (2011) to conduct the LCA.

1)The trellis system: The trellis system is composed of evergreenclimbing plants and a stainless steel frame (which is used as thesupport). The plants are directly grown on the ground.2)The planter box living wall system: The planter box living wallsystem is based on the plastic modules, which are made out ofHDPE. The box is filled with soil and onwhich evergreen speciesare planted with evergreen species. Water and nutrients for theplanter box living wall system are supplied by an irrigationsystem.3)The felt layer living wall system: The felt layer living wall sys-tem is based on several felt layers as substrate, which is supportedby a PVC sheet. A separate irrigation system is attached to feltlayers to supply water and nutrients for the plants.

In this comparative LCA, chemical emissions and energyconsumption were calculated by considering the following life-cycle phases of the three living wall systems: 1) raw materialdepletion, 2) manufacturing, 3) transportation, 4) installation, 5)maintenance, and 6) disposal. A functional unit for LCA should bedefined to serve as a basis for comparison of the greening al-ternatives. The functional unit selected in this research is a 1 m2

of living wall. As the basis for calculating the materials andproducts involved in each selected system, a building façade of200 m2 was considered.

The following steps were undertaken to assess the benefits ofliving walls. The chemical absorption data presented by Yang et al.(2008) was referred to quantify the air cleaning potential. Energy

Fig. 1. Three living wall s

savings of three typical living wall systems were calculated in threesteps:

Step 1: The percentage of energy savings (with the use of livingwalls) was calculated based on Ottelé et al. (2011);Step 2: The energy consumption of a standard building, indifferent climatic conditions, was evaluatedwith the EnergyPlussimulation; andStep 3: The total energy savings of a building, due to living walls,were obtained by multiplying the results of step one and steptwo above. The results of step three were used to calculate therequired time period to balance the initial energy usage in thematerial production, with the energy savings in the operationalphase.

2.2. Tools

The following tools were used for data analysis.

2.2.1. SimaPro modellingSimapro modelling (7.3.3) was used to evaluate the chemical

emissions and energy consumptions of living wall materials, in themanufacturing process. The chemical emission results of Simaprowere compared with the chemical abatement values of the livingwalls presented by Yang et al. (2008).

2.2.2. EnergyPlus modellingEnergyPlus is a building energy simulation tool which can be

used to model hourly energy consumption of a building, subject touser-specified construction, internal loads, schedules, and weather(Kneifel, 2010). The building energy simulations were run inEnergyPlus V7.1 (EnergyPlus Example File Generator, 2011) toobtain the annual energy use in electricity and natural gas forheating and cooling. The results of energy savings, which werecalculated with the multiplication between the data from Ener-gyPlus, and the energy saving percentage of Ottelé et al. (2011),

ystems for the LCA.

H. Feng, K. Hewage / Journal of Cleaner Production 69 (2014) 91e9994

were compared with the energy consumption results of Simapro toattain the balance years.

2.3. Data inventory

For the LCA, all the components of the living wall systems wereexamined. The trellis system involves only a stainless steel supportfor the plant. The planter box living wall system has planter boxes,which are filled with potting soil. The felt layer living wall systemhas several layers for rooting, waterproofing, and supporting. Thematerials of the three living walls used for the inventory were citedfrom Ottelé et al. (2011).

2.3.1. Manufacturing stageIn this stage, the raw material depletion, product fabrication,

and the related transportation were all covered. As shown inTable 1, the materials used for the inventory were based on Otteléet al. (2011). Since the bare façade, where the living wall isattached, is not considered in this LCA analysis, the components ofthe bare facade are not listed in Table 1.

2.3.2. Construction stageIn the construction stage, transportation and machine usage in

the installation process are the major contributors of chemicalemissions and energy consumption (Zabalza Bribián et al., 2009;Baek et al., 2013). In terms of the living wall installation, once thematerials are delivered to the construction site, only simple toolssuch as a driller and screwdriver are needed to fasten the compo-nents of the living wall systems. In this research, chemical emis-sions and energy consumptions of the machines were notconsidered because of their minor influence. For the materialtransportation, from the manufacturing factories to work sites, allthe distances for transporting vegetation, steel components,planter box, and felt layers were estimated for the City of Delft,where the living walls were installed. These data was cited fromOttelé et al. (2011) and listed in Table 1. Chemical emissions and

Table 1Components, Material weight (kg/m2), and service life (years) of living wall.

System name Components Material Weight Distances Servicelife

(kg/m2) (km) (years)

Trellis system Bolts Stainless steel 0.015 18 50Spacer brackets Stainless steel 0.045 18 50Structural supportmember

Stainless steelmesh

1.55 18 50

Vegetation H.helix 2.7 30 50Planter box

living wallsystem

Bolts Steel S235 0.27 15 50Spacer brackets Steel S235 0.315 15 50Supporting Usection

Steel S235 4.62 15 50

Planter boxes HDPE 13.2 15 50Growing material Potting soil 75.6 30 50Vegetation Pteropsida 8 30 10Watering system PE 0.26 35 7.5

Felt layerliving wallsystem

Bolts Steel S235 0.13 65 50Spacer brackets Steel S235 0.19 65 50Supporting Usection

Steel S235 4.62 65 50

Foam plate PVC 7 65 10White fleece Polypropylene 0.3 65 10Wool fleece Polyamide 0.6 65 10PE fleece Polyethylene 0.045 65 10Black fleece Polypropylene 0.27 65 10Vegetation Pteropsida 7.5 30 3.5Watering system PE 0.09 35 7.5

Derived from Ottelé et al. (2011).

energy consumption in the transportation process was calculatedwith the SimaPro software, and the 3.5-ton truck was chosen forthe analysis.

2.3.3. Maintenance stageRepairing and replacing would be major works in living wall

maintenance. It is mainly horticultural work to maintain the livingwalls. Therefore, the impact of chemical emission and energyconsumption in the maintenance stage is negligible. The nutrientsupplied in the maintenance stage should be calculated as thenutrient producing process. This process creates chemical emis-sions and consumes energy (Berndtsson et al., 2006). VegTech(2002) estimated that the total fertilizer applied to a 1 m2 vegeta-tion roof is 17.24 g/year. As the vegetation has a similar perfor-mance, either in the living wall or in the green roof, the sameamount of nutrient consumption is assumed to calculate thenutrient requirement of a living wall.

2.3.4. Disposal stageIn the disposal stage, the materials could be recycled, reused, or

landfilled. The steel components can be reused, and the planter boxcan be recycled. However, many cities do not have the requiredfacilities to undertake the recycling process. Therefore, the worstscenario was considered in this analysis: i.e. the living wall com-ponents have to be landfilled.

The steel materials were assumed to be disposed in inert ma-terial landfill, and the plastics, such as the planter box and feltlayers, in a sanitary landfill. The vegetation and soil were assumedto be naturally disposed as they are mostly organic. The trans-portation process in the disposal stage makes a significant contri-bution to the chemical emissions and energy consumptions. In thisstudy, all thewastematerials were considered to be delivered to thelandfill centre in Amsterdam, which is about 66 km away from thecity (ICOVA, 2013).

2.3.5. AssumptionsThe assumptions made in this analysis are mainly about the

service life of living wall components. The living wall system is anewly developed green technology. Scientific research on greenfacades is relatively new and peaked in 1980s (Köhler, 2008), andthe stainless steel cable system for green facades was first intro-duced in 1988 (Greenroofs, 2008). Patrick Blanc, the grandfather ofgreen walls, invented the first hydroponic system for living wallsonly about 20 years ago (Weinmaster, 2009). Therefore, the actualservice life of living walls has not been confirmed yet.

Since the service life of a conventional bare wall is about 50years, the service life of steel components were assumed as 50years, and the lifespan of the vegetation in the trellis system wasalso assumed as 50 years. The replacement frequencies of plants inplanter box and felt layer living wall systems were assumed as 10and 3.5 years respectively. The life expectancy for the plasticplanter box (HDPE) was assumed as 50 years (Ottelé et al., 2011). Asper the analysis performed by Riedmiller and Schneider (1992), thelifespan of PVC layers in the felt layer system is about 10 years. Thelife expectancy of the felt layers is similar to the PVC, which wasalso assumed as 10 years. The irrigation system for the planter boxand felt layer living walls was assumed to be replaced every 7.5years due to the crystallizing of salts (Ottelé et al., 2011). The serviceperiods of all the components were listed in Table 1.

In terms of the benefits of the green layer, the trellis systemperforms different from the living wall systems. For the trellissystem, the full covering of the façade by H.helix is estimated after20 years (Ottelé et al., 2011). For both living wall systems, thebenefits could be calculated after the installation due to the amountof plants and the material layers.

H. Feng, K. Hewage / Journal of Cleaner Production 69 (2014) 91e99 95

3. Results

Comparative analysis of chemical emissions and energy con-sumption of living walls in the entire lifecycle, and air purificationand energy savings in the operations phase, are discussed in thissection. In the manufacturing process, the chemical emissionsinclude all the emissions from raw material depletion, trans-portation to the factory, and fabrication. In the construction stage,chemical emissions of different components are due to the trans-port emissions, where the weight of the component and thetransport distance are the key factors. In the maintenance stage,nutrient is applied twice a year to the living walls, and the chemicalemissions are mainly due to the manufacturing of fertilizer. In thedisposal stage, transportation to the landfill and the landfillingprocess both create emissions. It is the same in energyconsumption.

3.1. Chemical emissions

The impact assessment (Simapro based) of different living wallmaterials (presented in Table 1) is shown in Fig 2. The Eco-indicator99 was used to conduct the impact assessment of the living wallcomponents. The Eco-indicators express the total environmentalload of a product or a process. The standard Eco-indicator valuescan be regarded as dimensionless figures. The scale 1Pt means onethousandth of the yearly environmental load of one average Eu-ropean inhabitant (Eco-indicator 99, 2000). Based on the compar-ison results in Fig 2, PVC foam has the highest environmentalimpact compared to other materials, mainly because of the highercarcinogens emission. Steel and stainless steel have high respira-tory inorganic emissions. Further, PE pipes, HDPE plastics, andfleece consume more fossil fuels. It is evident that the PVC foamlayer is the least environmentally friendly material.

According to the inventory results, around 900 emissions werenoted in the full lifecycle of a living wall. In order to match thechemical absorption data of Yang et al. (2008) with the inventoryresults, Nitrogen Dioxide (NO2), Sulphur Dioxide (SO2), Ozone (O3)and Particulates (<10 mm) were considered. Table 2 shows theamount of substances that are released to the environment (air,water, and/or solid) by producing 1 kg of each component of livingwalls.

By multiplying the weight of each component in Table 1 withthe emissions per kilogram of the relevant component in Table 2,chemical emissions for creating 1 m2 of living wall could beattained. Ottelé et al. (2011) noted that the expected operating life

Fig. 2. Comparison of product stages between different materials (Method: Eco-indicator 99 (E) V2.08 /Europe EI 99 E/E/ Single score).

for the trellis system and the planter box living wall system is about50 years. Therefore components of both systems, which have anoperational life of less than 50 years, such as the watering systemwith 7.5 year service life, needed to be replaced during the livingwall’s lifetime. Therefore, the emissions of those components weremultiplied by the number of replacement cycles. With the air pu-rification rate cited by Yang et al. (2008), years required to balancethe chemical emissions created by the living walls in the wholelifecycle was calculated. Table 3 lists total emissions released indifferent stages by 1m2 living wall system. Table 3 also presents thenumber of operational years needed to balance the emissions.

3.2. Energy

According to SimaPro inventory results, Table 4 lists all the en-ergy requirements, from different lifecycle stages, to make 1 kg ofdifferent components of living walls.

By multiplying the weight of each component in Table 1, withthe energy requirement per kilogram of the livingwall componentsin each lifecycle stage, the energy requirements for a 1 m2 of livingwall was calculated and summarized (Table 5). Similar to thechemical emissions calculations, materials which have a service lifeof less than 50 years in the trellis system and planter box living wallsystem, and less than 10 years in the felt layer living wall system,needed to be replaced. Therefore the initial energy consumption ofthese materials was multiplied by the number of replacementcycles.

In order to evaluate the time required for a living wall to balancethe energy consumption in its lifecycle, the energy savings of livingwalls were compared. Alexandri and Jones (2008) estimated theenergy saving percentages in Mediterranean and temperate cli-mates by conducting an energy simulation of a building(40 m � 10 m � 5 m), by installing a 200 m2 living wall (Table 6). Inorder to calculate the energy saving percentages (in Table 6), thesame building dimension was assigned for EnergyPlus simulation.According to the eco-region map created by USEPA (2006), LosAngeles, California was selected for weather inputs in a Mediter-ranean climate; and Albany, New York was selected for weatherinputs in a temperate climate. Other parameters, such as buildingactivity (people density, electrical plug intensity etc.), buildingfenestration, and HVAC system, were defined with the smartdefault models in EnergyPlus. Results show that the total heatingand cooling energy consumption for a Mediterranean climate is6.7 GJ/year and 23.9 GJ/year respectively, and the total heating andcooling energy consumed for the temperate climate is 70.77 GJ/yearand 17.59 GJ/year respectively.

By multiplying the energy consumption data from EnergyPlusresults, with the energy saving percentages in Table 6, the annualenergy saving values for different living wall systems, in differentclimates, were calculated. The total energy required for an area of200 m2 living wall in the full lifecycle was calculated with the datain Table 5. With the annual energy savings value, the number ofyears needed to balance the initial energy consumption of livingwalls was calculated (Table 7).

The total balance years of the three living wall systems in termsof air cleaning and energy savings was illustrated in Fig 3.

4. Discussion of results

Analysis of total pollutants released in the living wallmanufacturing process shows that the felt layer system releases 3times more toxic substances to the environment than the modularpanel system and trellis system (Table 3). Additionally, the felt layersystem needs as many as 23 years to balance the pollution (emis-sions). As the expected operating life of a felt layer living wall is

Table 2Amount of substances released due to the production of 1 kg material in different stages (Derived from SimaPro results).

Process Unit Emission/kg

Steel Stainless steel PVC foam plate PE pipe HDPE plastic Fleece Vegetation Growing material Fertilizer

Manufacture g 12.24 28.53 16.74 7.96 7.77 10.41 0.00 0.00 0.00Construction g/km 1.66 1.66 1.66 1.66 1.66 1.66 1.66 1.66 0.00Maintenance g 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 22.04Disposal g 0.08 0.08 0.13 0.13 0.13 0.13 0.00 0.00 0.00

g/km 1.66 1.66 1.66 1.66 1.66 1.66 0.00 0.00 0.00

Table 3The emission released due to the production of 1 m2 living walls and years needed to balance it.

System Emission (g/m2) Total ofpollutants (g)

Air cleaning ability Balance years

Manufacture Construction Maintenance Disposal

Trellis system 45.93 0.05 19 0.15 65.14 8.46 g m�2 yr�1 (Cited fromYang et al. (2008))

8Planter box living wall system 180.8 1.78 19 2.12 203.69 24Felt layer living wall system 191.7 0.73 3.8 1.44 197.7 23

H. Feng, K. Hewage / Journal of Cleaner Production 69 (2014) 91e9996

about 10 years, the pollution removal benefit of the felt layer sys-tem cannot offset the pollution it initially created. Trellis systemand modular panel system could easily reach to balance air pollu-tion with purification, as their life expectancy is estimated as 50years (Ottelé et al., 2011). Therefore, among the three typical livingwall systems evaluated, the felt layer system is the least environ-mentally friendly living wall system, in terms of the air pollutionabatement (shown in Fig. 3).

Analysis of the total energy required shows that the felt layersystem consumes over 11 and 4 times more energy than the trellisandmodular panel systems respectively (Table 7). Furthermore, thefelt layer system needs nearly 10 years of energy savings to balancethe energy used in the Mediterranean climate, which is the fulloperational lifespan of the system. In a temperate climate, thebalance years are 3.6 times longer than the lifespan of the felt layersystem. Therefore, it is evident that, among the three typical livingwall systems, the felt layer system is the least environmentallyfriendly living wall system in terms of energy saving performance.

Based on these results, the felt layer system can be classified asenvironmentally unsustainable, in terms of air cleaning and energysavings points of view. The materials used in the felt layer systemare the main reason for its low performance. As shown in Table 1,the container and waterproofing are the only differences betweenthe modular panel system and the felt layer system. The HDPE

Table 4Energy needed to produce 1 kg material in different life stages (Derived from SimaPro r

Process Unit Energy consumption/kg

Steel Stainless steel PVC foam plate

Manufacture KJ 1450 29210 69110Construction KJ/km 6.07 6.07 6.07Maintenance KJ 0 0 0Disposal KJ 1.46 1.46 5.2

KJ/km 6.07 6.07 6.07

Table 5Energy required due to the production of 1 m2 living walls.

System Energy consumption (KJ/m2)

Manufacture Construction

Trellis system 47028.10 0.19Planter box living wall system 150667.45 6.61Felt layer living wall system 506209.15 2.67

planter box and the PVC foam plate are the containers in the twosystems. Table 2 shows that pollutants emitted by 1 kg PVC in themanufacturing process are around 2.2 times more than pollutantsemitted by HDPE. In addition, Table 4 shows that the energyconsumed to produce 1 kg PVC in the manufacturing is over 76times more than that for HDPE. Since containers have the highestweight density in the system (Table 1), the emission differences oftwo living walls is also mainly based on the containers. Therefore,the material chosen for container is extremely important in livingwall systems. As shown in Fig 2, it is evident that the PVC foamplatehas the highest environmental burden. If other materials, like PE orsteel, can be applied in the felt layer system, the balance yearsneeded for pollutant emission and energy consumption would bechanged significantly.

In terms of air cleaning and energy savings, Tables 3 and 7 showthat both the trellis and modular panel system are relatively sus-tainable. As shown in Table 3, the years needed to balance thepollutants emitted by modular panel system is 3 times higher thanthat of trellis system. The reason can be found in Table 1; i.e. thetrellis system only has one simple layer to support the climber,while the modular panel system has three layers. However, thetrellis system uses stainless steel as a structure support, which hasthe highest pollutant emission rate in the manufacturing process(Table 2). Ottelé et al. (2011) conducted an environmental burden

esults).

PE pipe HDPE plastic Fleece Vegetation Nutrient

72110 900 1890 0 06.07 6.07 6.07 6.07 00 0 0 0 1181.615.2 5.2 5.2 0 06.07 6.07 6.07 0 0

Total of energy consumption (KJ)

Maintenance Disposal

1018.55 2.54 48049.381018.55 88.06 151780.66203.71 51.95 506467.48

Table 6Energy saving for heating and cooling in different climates (Ottelé et al., 2011;Alexandri and Jones, 2008).

Living wall system Benefit Mediterraneanclimate

Temperateclimate

Trellis system Energy saving for heating 1.20% 1.20%Energy saving for cooling 43% \

Planter box livingwall system

Energy saving for heating 6.30% 6.30%Energy saving for cooling 43% \

Felt layer livingwall system

Energy saving for heating 4% 4%Energy saving for cooling 43% \

Fig. 3. The total balance years of the three living wall systems in terms of air cleaningand energy savings.

H. Feng, K. Hewage / Journal of Cleaner Production 69 (2014) 91e99 97

analysis for different materials that can be used as living wallsupporting systems. Their results showed that stainless steel sup-port has roughly 10 times more pollutant emissions than recycledplastic (HDPE) supporting systems, hard wood, and coated steel.Therefore, stainless steel might not be a sustainable choice eventhough it has a lifespan of 50 years or longer.

As shown in Table 7, in a Mediterranean climate, the modularpanel system needs 3 times more time than the trellis system tobalance the energy spent in the manufacturing process. In atemperate climate, however, the trellis system needs more timecompared to the modular panel system, which has a more complexstructure. Table 6 shows that the trellis system makes a lowcontribution to the heating energy savings, which is less than 20%of the energy savings in the modular panel system. The containersand the growing medium of the modular panel system affect theinsulation properties of the building facade and lead to higherenergy saving for heating. The trellis system just has the climberattached to the mesh and covers the wall which will not make anybig change in energy savings for heating. However, the cooling ef-fects of living wall systems are mainly due to the shade andevapotranspiration created by the vegetation (Pérez et al., 2011),which make the modular panel system and felt layer system havethe same influence of energy savings in cooling.

In the product lifecycle, the manufacture process emitted themajority of the chemical pollution and consumed most the energy.As shown in Tables 3 and 5, nearly 85% of the chemical emissionswere in the manufacture stage. On average manufacturing processcontributes to 99% of energy consumption. As described in theSimaPro database, the manufacturing stage includes raw materialextraction, storage, transportation, material fabrication, wastetreatment etc. In order to make the living wall system more sus-tainable, thematerials with fewermanufacture processes should beconsidered.

In the construction stage of a livingwall, themajor contributionsto chemical emissions and energy consumptions are due to mate-rial transportation. A 3.5-ton van was chosen for the analysis, andthe chemical emission and energy consumption for transporting1 m2 living wall for 1 km was calculated. In reality, the vehicle ishardly fully loaded in delivering the components to the site, sincedifferent components might come from different companies, andthe amount also depends on the size of the living wall. Therefore,the pre-planning in material procurement could decrease theenvironmental impact of the living wall system. Since the

Table 7Years needed to balance the energy consumption.

Living wall system Saved value (GJ/year)

Mediterranean climate Temperate climate

Trellis system 10.36 0.85Planter box living wall system 10.7 4.46Felt layer living wall system 10.55 2.83

transporting distance is another key factor to mitigate the envi-ronment impact, the material resources that are close to the con-struction site should be given a priority. Since the living wallinstallation is simple and only the basic tools are needed toassemble the layers together, it is not taken into account in thisstudy.

The maintenance requirement for a green roof is mostlydependent on the type of plants used and where the plants arelocated (Peri et al., 2012). The similar phenomenon could apply tothe living wall system. Watering, fertilizing, and vegetationreplacement are the major works in the maintenance stage. Wa-tering is not considered in the calculation due to the minor impactto the environment. However, fertilizer in living wall systems has asignificant impact in chemical emissions (in Table 3). The servicelife for the vegetation in the trellis system is about 50 years, whichmeans no replacement is required during the lifespan. However,the service life of the vegetation in the planter box and felt layerliving wall systems are 10 years and 3.5 years respectively, whichmeans a few replacements are needed during the lifespan (Otteléet al., 2011). This process needs extra transportation and con-struction activities. Therefore, vegetation which can survive withlow maintenance and less fertilizer requirements should beconsidered for living wall systems.

In the disposal stage, all the materials are generally landfilledwithout recycling or reuse. For example, the plastic materials suchas planter box and felt layers could be used as raw materials inmanufacturing new plastic materials, and the structure materialssuch as stainless steel and bolts could be reused in another livingwall system. Thus, the materials that can be recycled or reused afterthe lifecycle of a living wall should be promoted as sustainable.

Climatic conditions have a strong impact on the performance ofliving walls. As shown in Table 7, for all three living wall systems,the energy saved in a Mediterranean climate is more than the en-ergy saved in a temperate climate. Since the temperature is rela-tively cool in the temperate climate, no additional cooling load isneeded, even before the vegetation (living walls) is applied to thebuilding (Ottelé et al., 2011). Alexandri and Jones (2008) alsoconcluded that energy savings by living walls in a tropical climate

Total energy requiredfor the system (GJ)

The balance year

Mediterranean climate Temperate climate

9.61 1 1130.36 3 7

101.29 10 36

H. Feng, K. Hewage / Journal of Cleaner Production 69 (2014) 91e9998

(Brasilia) could be as much as 68% and only 37% in a cold climate(Beijing). Thus, the living wall is a potential application to usebroadly in warmer climates, to save cooling energy. In a coolerclimate, modular panel systems is a better option than trellis sys-tems as more heating energy can be saved.

5. Limitations

5.1. The LCA study conducted in this research has several limitationsand assumptions

In this study, three living wall systems were assumed to havethe same air pollutant cleaning capacity. However, different plantson different living walls have diverse performances. Wolverton andWolverton (1993) tested over thirty plants to determine the mosteffective ones in removing VOCs, and the results showed that themost effective plant is about 10 times better than the least effectiveplant. As different plant categories are suitable for different types ofliving walls, “ivy” on the trellis system might not have the same airpollutant removal rate as “sedum” on the modular panel system.Beside the plants, the pollution concentration, length of thegrowing season, weather type, and the behaviour of differentspecies in local weather conditions could be other factors thatmight impact the air pollutant removal rate (Nowak et al., 2006). InToronto, Canada, an annual air pollution removal rate of69 kg ha�1 year�1 was estimated by Currie and Bass (2008). InWashington, DC, an annual air pollution removal rate of77 kg ha�1 year�1 was reported by Deutsch et al. (2005). Therefore,the plant categories, pollution concentration, length of the growingseason, weather type, and the behaviour of different species in localweather conditions etc. are all factors that should be considered in acomprehensive analysis.

This study analyzed the living walls’ performance only in theMediterranean and Temperate climates. In addition, a singlebuilding type was considered. In reality, there are a wide variety ofbuilding types in different climatic regions. The CommercialBuildings Energy Consumption Survey (CBECS) separated thecommercial sector into 29 categories and 51 subcategories, and theenergy consumption for cooling and heating are totally differentamong these categories (Deru et al., 2011). Furthermore, the ori-entations of buildings, the locations of living walls, and the eco-regions are other identified parameters that will influence the en-ergy saving of buildings (Gratia and De Herde, 2007; Jim and He,2011; Alexandri and Jones, 2008). All these parameters should beexplored in future comprehensive studies.

6. Conclusion

The results of this lifecycle analysis provided insight to theenvironmental impact of the three living wall systems. The feltlayer system is not environmentally sustainable in air cleaning andenergy savings, compared to the trellis system and modular panelsystem. The trellis system has the best performance in air cleaningand energy savings. The LCA also indicated the need forenvironment-friendly materials for sustainable living walls. Thematerials that are closer to the site with less delivery requirementshould also be considered in the construction stage. In addition,materials that could be recycled or reused should be applied to thesystem asmuch as possible. Vegetation that consumes less fertilizerwith lower replacement should be used. The comparative analysisalso showed that the climatic conditions, building types, and plantcategories might impact the energy saving and air cleaning per-formance. Further research is essential to confirm or refute theassumptions made in this study, especially those related to thelimited number of living wall categories. The operational cost

analysis, such as durability, aesthetical value, and social factors,needed to be evaluated with related economic benefits. Moreover,chemical absorption and energy savings of living walls in differentweather conditions, building types, and plant categories need to befully investigated and analyzed.

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