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COMPARISON OF RESOURCE USE AND ENVIRONMENTAL PERFORMANCE OF

GREEN WALLS WITH FAÇADE GREENINGS AND EXTENSIVE GREEN ROOFS

CLARA GERHARDT, BRENDA VALE

School of Architecture, Victoria University of Wellington, New Zealand

TYPE OF PAPER

Student Paper, scientific ABSTRACT

In recent years green walls or “vertical gardens” have become more widely used. They have been praised as they increase biodiversity in the urban environment, sequester CO2, retain water, bind dust and pollutants, can produce food and contribute to reducing the heat-island effect. Being integrated into the facade they are highly visible and take on an important emblematic function as a symbol of a green mind set. Their appeal to corporations, public clients and the media is therefore not surprising. However, from an architectural perspective their environmental impact should be assessed and compared to alternatives such as green roofs and greening of facades before promoting their use in sustainable projects for their emblematic character. In this paper the potential of green wall systems to enhance sustainability and to contribute to an improved urban environment is discussed through looking at their resource use compared to green roofs and greened facades. A material intensity analysis using the material per service unit concept (MIPS) of five different options is carried out using data from a hypothetical case study. The results enable a direct comparison of the resource use of a panel-based green wall system, a hydroponic green wall, a greened facade, an extensive green roof and a conventional envelope for a 60 year life. Furthermore the material intensity of vegetables grown on two green wall systems made for food production is compared with commercially grown vegetables. Compared to a conventional envelope, green walls show an impressive set of beneficial functions that range from air filtration to creating natural habitats and reducing the internal temperatures of buildings in summer. However, the analysis carried out here suggests that the extensive green roof and greened facade substantially outperform all of the analysed green wall systems in terms of resource use while providing a similar or greater benefit to the urban landscape. Furthermore green walls seem to use a lot more resources for producing local food when compared with commercially grown food sourced within a 500 kilometre distance.

KEYWORDS

green wall; green roof; material intensity analysis; building envelope; life-cycle-wide resource use

INTRODUCTION

In the last two decades the potential of vegetative building cover to mitigate many of the pressing environmental issues associated with dense urban environments has become more widely known and the number of green roofs and greened facades has slowly increased. Research has shown that green roofs and façade greening can significantly reduce urban environmental problems such as increased storm water runoff caused by large areas of impervious surfaces, and have a positive impact on the urban heat island effect, loss of wildlife habitat and decline of air and water quality (Bass, 2001), (Dürr, 2005), (Minke, 2006). While addressing these problems is of concern for the whole community, the individual homeowner can also profit from green facades as vegetation can improve the acoustic and thermal performance of

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building envelopes resulting in less energy consumption and impacting positively on the wellbeing of people with minimal or no additional costs (Kosareo, 2007), (Niachou, 2001). Some greening options, such as the extensive green roof or the greened façade, have evolved over centuries and have been adapted to modern buildings from the 1970s onwards. These have been widely used for environmental reasons (Germany’s green roof policy implemented in the 1970s resulted in 10 million m² of green roofs in 1996). In contrast the green wall is a more recent addition that emphasizes the aesthetic value of vegetation in the built environment (Bass, 2001). Furthermore green walls are praised as an option for growing fresh food in limited urban spaces. They are seen as an opportunity for the individual to contribute to reducing the material flows associated with transportation of food supply into cities and also as an opportunity for disadvantaged communities to supply fresh food to their members (Wong, 2008).

Figure 1: Greened facade, green wall (with permission of Yong-Kwan) and extensive green roof

While it seems that the qualitative and quantitative benefits of green roofs, greened facades and green walls are slightly different yet comparable (Table 1), the question arises as to whether this is also true of their life-cycle-wide resource use (Bass, 2001). Table 1: Positive influence of vegetation on the building envelope and the urban environment green wall extensive green roof greened facade

substrate based hydroponic festuca sedum evergreen deciduous Heat island effect ++ +++ ++ ++ ++ ++ water retention +++ +++ +++ +++ + + thermal insulation (winter)

+ + +++ ++ + -

thermal insulation (summer)

++ ++ +++ +++ ++ ++

sound absorption ++ ++ ++ ++ ++ ++ wild life +++ ++ +++ +++ ++ ++ dust absorption ++ ++ +++ ++ ++ ++

Legend: - no influence + small influence ++ significant influence +++ strong influence

Many different green wall systems exist ranging from low- to high-tech solutions, with use of varying resources (Kaltenbach, 2009). Using a material intensity analysis (MAIA), this paper establishes the resource use of four different green wall systems (three panel systems and one substrate fleece system) over a 60 year life span (Hegger et al., 2005), (Minke, 2006) and compares them with an extensive green roof and a greened façade. Life cycle wide material flows are taken into account. These were derived for a more comprehensive case study which established the resource use of different building envelopes and claddings, assuming the theoretical building was located in Graz, Austria (Gerhardt, C., 2009). BACKGROUND TO GREEN WALLS

Developed in Japan, where urban green space is often limited due to dense urbanisation and high property values, the concept of bringing gardens into the vertical has proved successful. The modern green wall is different from the traditional greening of a facade where plants grow in the ground or pots and climb to grow over facades. Unlike green roofs and facade greening, modern green walls

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bring the plant roots into the vertical and hence need irrigation systems. While this increases the installation complexity the advantage is that walls can be planted evenly and to a greater height. Plant diversity is potentially greater and compared to greened facades the variety of designs or patterns that can be achieved is greater, as Figure 2 illustrates. Furthermore, some systems make food production possible. While green roofs are mostly only accessible to a few people, a benefit of facade greenery and green walls is that their effect is highly visual and can be enjoyed by a wide range of people.

Figure 2: Examples of green wall designs

For contemporary architecture the green wall presents a combination of an unusual facade (pattern, material, texture, and colour) which attracts attention to a building and at the same time symbolises a human, eco-friendly face. It is therefore ideal for public and sophisticated commercial clients that are looking to create a green and innovative image. Distributors of green walls therefore often refer to the walls as “feature walls” thus drawing attention to their aesthetic rather than their other advantages. However, the green wall is slowly becoming more attractive for the residential market. The benefits of green walls are numerous and are comparable to the benefits of the traditional options. They range from filtering water, cleaning air, and binding dust to cooling buildings and cities and providing habitat for wild life. Depending on the thickness of the vegetation layer and construction method, they can provide significant sound insulation (Dunnett, 2008). While any form of vegetation in the built-up urban environment has positive effects on micro-climate, wild life and humans and is therefore welcome, before promoting the wider use of green wall systems they should be analysed in case the new trend for vertical gardens triggers significantly larger resource flows than other options (such as green roofs of greened facades). MATERIAL INTENSITY ANALYSIS

The overall resource uses of the different green façade systems were assessed through a resource analysis using the material intensity per service unit approach (MIPS), an assessment tool developed by the Wuppertal Institute for Climate, Environment and Energy. MIPS is an analytical tool that aims to provide a solid estimation of the impact of goods and services on the environment (Ritthoff, 2002). The assumption behind MIPS is that the environmental stress potentials of a product are linked to the quantity of the material flows it triggers and can hence be deduced from the life-cycle wide material input. The greater the resource use, the greater the change in a certain ecosystem or natural flow, the higher the potential environmental damage and degradation. The material intensity analysis thus enables the comparison of the potential of products and materials to impact on the environment and allows the user to rank products and services accordingly. The link between eco-efficiency and material input can be described as follows. Resource Productivity=Material Input/ Service Unit Material Input refers to the lifecycle wide input of resources that will have to be moved, consumed and refined during production, use and recycling or disposal of a product (Schmidt-Bleek, 1993). For a material intensity analysis all the material input, raw and refined, that has to be sourced from the environment or has been moved (stone, sand, earth) in order to produce a certain industrial product will have to be considered and summarised. For this study this included the main material flows triggered directly or indirectly during manufacture and operation of the green wall and roof options.

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The sum of all these material flows, which are triggered by the final product or the service, represents the ecological rucksack of the product (or indirect resource use), which equals its material intensity (MI) minus the weight of the product (direct resource use). In order to derive a meaningful estimate, material inputs are differentiated into abiotic raw materials

such as fossil fuel energy carriers or mineral raw materials such as sand, biotic raw materials such as biomass, earth movement such as erosion or mechanical earth movement, water use, meaning use of ground or surface water (from which tap water is derived), and air. Service Unit is a service provided by products that has to be defined by the user, for example the retention of a specified amount of storm-water on site. MIPS is especially useful for comparing the material associated with the provision of a service unit by different means as done in this study. Comparisons allow sensible conclusions to be drawn within the established boundaries of the analysis. Comparisons of alternative specifications are therefore preferable to attempting to determine the actual resource flow associated with a Service Unit. Material input in relation to weight is defined as Material Intensity (MI), with units of kg/kg. If the MI is derived in relation to the service provided it will indicate the global environmental impact of that service. The MIPS indicator can therefore provide quantitative information on direct and indirect resource use or the resource efficiency of a product. MIPS measures the scarcity of resource components indirectly by presuming that a scare material will require a larger indirect material flow than an abundant element. One shortcoming of the Material Intensity analysis is that it does not register toxicity. Therefore toxic materials have either to be avoided during planning/design or be measured using another indicator. MIPS is currently used by many European governments, and in Japan and New Zealand. However, while there are data available for the embodied energy and embodied CO2 for many building materials used in New Zealand, for example (Baird, G. et al, 1997) and (Alcorn, A. 1998), Material Intensity (MI) data has so far not been collected. However, MI data has been widely published in Europe (especially in German speaking countries) for example by (Bergmann, I., Weiss, W., 2002), (Schmidt-Bleek, F., Manstein, C., 1999), (Wuppertal Institute for Climate, Environment and Energy, 2003). This explains why this comparative analysis is based in Graz, Austria. As Austrian and mainly German data were used for the material intensity analysis, other underlying parameters such as building geometry and size were subsequently adapted to European standards in order to ensure the consistency of the case studies. Therefore both the envelopes and the material intensity of their components reflect local building standards and product intensities and are not immediately transferable to another climate or location, where transportation, energy mix, or simply production processes are different.

METHOD

Analysed options For the material intensity analysis four different green wall options were considered and compared with an extensive green roof and a greened facade. Three of the green walls options were substrate-based panel systems which function like shelves filled with soil. Water drips from the top of these panels and soaks the soil. Each shelf is perforated to allow for root growth as well as for water penetration. Run-off water is collected underneath the lowest shelf, filtered and pumped back into the water tank.

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60 17mm water supply hose 61 vegetation 62 150 mm substrate 63 green wall perforated stainless steel panels 64 8mm drip irrigation 65 water proofing membrane, PE 66 spacer,HDPE, filled with polyethylene 67 mounting bracket, aluminium 68 6mm/50mm flathead pan wood screw, stainless steel 69 batten 40mm/60mm, compressed straw-fibre

Figure 3: Isometric view of a substrate based panel, view of a green wall using panels (with permission of Sharp, R. 2009), and section through the substrate based green wall of the case study

The three panel systems analysed are made from different materials: aluminium (152 mm deep, weighing 35 kg/m² (panel) + 166 kg/m² (substrate)), stainless steel (102 mm deep, weighing 67 kg/m² + 250 kg/m²) and high density polyethylene with a depth of 100 mm (weighing 18 kg/m² + 226 kg/m²). Both the plastic and the stainless steel panels can be used for food production. The fourth green wall system analysed was a hydroponic green wall. While all other analysed options were chosen as they represent typical products available on the market, the hydroponic wall was included as it is extremely low-tech and therefore cheaper and potentially applicable in a residential context. In the hydroponic system, in place of soil, plant roots grow between layers of synthetic polyester needle felt and get all their nutrition from an agricultural drip irrigation system that soaks the felt evenly with liquid nutrients. The entire green wall system has a depth of 40 mm and weighs roughly 30 kg/m² with a life span of approximately 10 years.

60 17mm water supply hose 60 a 200 mm compressed straw panel, water resistant 60 b root protection layer, water tight 60 c 5 mm felt layer, polyester 60 d drainage membrane, perforated PE 61 vegetation 64 8mm drip irrigation 65 water proofing membrane, PE 68 wood screw, stainless steel 69 batten 40mm/60mm, compressed straw-fibre

Figure 4: Image of the hydroponic green wall (with permission of De Geus, 2006) and a section through the hydroponic green wall of the case study

The irrigation system for all four options is similar. A pump transports a mixture of rain water, tap water and nutrients through the drip irrigation that soaks either the soil substrate or the felt. Run-off water is filtered and returned into the cycle as shown in Figure 5.

a extensive green roof b water tank c tap water d over flow e fertiliser f pump with control g nutrient solution for plant facade h return run

a

b

c d

e

f

g

h

Figure 5: Irrigation circuit of the case study after (Kaltenbach, 2008)

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While soil-based systems only require a boost of fertiliser from time to time, the hydroponic system needs a constant supply of nutrients which include many more components than in the average fertiliser, and hence is more material intensive. For the comparison an extensive roof construction with a light-weight substrate of 150 mm was chosen, which meant the roof did not need extra irrigation and could be slightly pitched which facilitated the collection of run-off water.

10 vegetation

11 150 mm of substrate

12 filter fleece, Polypropylene (PP)

13 11 mm drainage layer, Polystyrol (HIPS)

14 protective layer, Polythene (PE-HD)

15 Root protection layer (PE-LD)

Figure 6: Photograph of a green roof (with permission of United States Environmental Protection Agency) and details of the extensive roof used in the case study

The envisaged vegetation layer consisted of the recommended fescues and herbs (festuca rubra genuia, ovina and commutata) to maximise thermal insulation benefits which were taken into account for the material intensity analysis (Minke, 2006).

The sixth analysed option was a façade greening consisting of a Henry’s honeysuckle (Lonicera henryi) growing on a climbing support structure made from a stainless steel grid system (4.3 kg/m²). The plant was selected for its resistance to frost, evergreen foliage and tolerance to availability of sun and water. It can cover buildings over 8 metres high and, unlike ivy (an alternative evergreen, shade tolerant plant) produces a decorative flower and does not damage the building surface..

60 climbing aid, fixings, chrome-nickel 61 vegetation 62 climbng aid, grid, 300x400 mm, 9mm diameter, chrom-nickel 65 water proofing membrane, PE

Figure 7: Image of the Henry’s honeysuckle (with permission of Chorlton) and a section through the greened facade of the case study

System boundaries

Without boundaries to the analysis the assessment of material flows through the life of a building or building element can become very complicated. Clearly defined boundaries that constrain data gathering and analysis are critically important in comparative case studies. Results depend directly on boundary definition, which also determines whether the results may be compared with those of other studies. This study included data for raw material extraction and processing (for both product and process materials), transporting processed materials to manufacturing facilities, manufacturing and use. As the material intensities of most elements of the envelope were retrieved from literature and were hence not product-specific, the manufacturing location could not be established more specifically by country (Wuppertal Institute, 2003), (Schmidt-Bleek, 1999), (Bergmann, 2002). Therefore, the transport of all elements to the fictitional building site in Graz, whether for initial building or replacement during the 60 year life, was excluded from the calculation. A study (Kellenburger, 2008) for normalised operating energy for different walls in Switzerland over 80 years

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showed that for a lightweight insulated timber frame wall the transport to the building site was 5% of total impact rising to 10% for a heavy insulated wall construction. Although these values cannot be directly related to the study in this paper they indicate that other factors likely to be far more important than transport between the place of manufacture and the building site. The material flows associated with recycling of materials and elements that have reached the end of their service life were not counted separately as these are by definition counted by MIPS in new products made from recycled materials (Table 2). As the presented data was part of a more comprehensive material intensity analysis of an entire building envelope, the impact of the different options (e.g. green roof) on for example the thermal performance of the overall envelope (resulting in slightly lower heating requirements and hence fuel consumption) were taken into account. Only the electrical energy needed to operate the pumps of the green walls was not included as the amount used depends on the flow rate and construction height and the energy mix used and despite contacting the manufacturers of the systems it was impossible to discover typical electricity uses. However, since the results of the analysis show that the extensive green roof, which uses no pump has a lower impact than the irrigated façade types that do, including the electricity use for the latter will not change the final rankings. The material flows triggered by the pre-grown plants used in the green wall systems were not included as there are no MIPS data for these. This omission was consistent for all selected wall and roof systems, so will not change the final rankings. Data sources

All the material intensity data for the components and raw materials used in the different wall systems were sourced from the literature (Bergmann, 2002), (Schmidt-Bleek, 1999), (Wuppertal Institute, 2003). Similar to data for embodied energy and embodied CO2, the published MI data is specific to certain process standards that might vary according to location and regulations. The used data therefore mostly represents the material flows in European process chains. Table 3 gives a typical example of material intensities of a raw material as published by the Wuppertal Institute for Climate, Environment and Energy (2003). Table 2: Material Intensity of aluminium (Wuppertal Institute for Climate, Environment and Energy, 2003)

material material intensity (MI) [t/t]

aluminium abiotic material biotic material water air moved soil primary 37 1047.7 10.87 Europe secondary 0.85 30.7 0.948 Europe wrought alloy 35.28 996.8 10.375 Europe cast alloy 8.11 234.1 2.932 Europe average 18.98 539.2 5.909 Europe

Having quantified the direct resource use of all the materials used in the selected options, this was then multiplied with the given MI data as illustrated in Table 3 for a substrate based green wall aluminium panel and irrigation system. Table 3: Calculation of the resource use of 1 m² of one of the green wall systems (aluminium panel) excluding substructure

green wall aluminium, 154 mm deep, 1 m² abiotic material water air source MI

Considered time frame: 60 years MI product MI product MI product

Material/Pre-product: Unit Amount kg/unit kg/unit kg/unit kg/unit kg/unit kg/unit

16 mm water supply hose, (PE-LD) kg 0.08 2.49 0.19 122.20 9.49 1.62 0.13 *

150 mm substrate kg 208.82 1.00 208.82 - 0.00 - 0.00 **

perforated aluminium panels (secondary) kg 84.28 0.85 71.64 30.70 2,587.31 0.95 79.89 *

drip line, 8mm, (PE-LD) kg 0.27 2.49 0.67 122.20 33.12 1.62 0.44 *

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drip line insert, PVC kg 1.40 3.33 4.66 176.60 247.21 1.69 2.37 *

protective layer, Polythene (foil) kg 2.79 3.01 8.40 167.60 467.60 1.84 5.13 *

spacer, (PE-HD) kg 0.01 2.52 0.03 105.90 1.22 1.90 0.02 *

filled with polyurethane kg 0.00 6.31 0.01 505.01 0.84 3.56 0.01 *

mounting bracket, aluminium (secondary) kg 2.31 0.85 1.97 30.70 71.05 0.95 2.19 *

screw, stainless steel(17% Cr, 12%Ni) kg 0.07 17.94 1.31 240.30 17.51 3.38 0.25 *

300.03 kg 297.69 kg 3,435.34 kg 90.43 kg

* (Wuppertal Institute for Climate, Environment and Energy, 2003) ** (Ritthoff, private communication)

RESULTS

Figure 8 shows the resource use (excluding water) of one square metre of the different green wall systems, the extensive green roof and the greened façade. In order to indicate the ratio between the resource use of the green wall itself to the resource use of the additional common elements required, such as pumps, water storage tanks and irrigation systems, these are shown in a separate column. As can be seen, the greened facade provides the most resource efficient option by far in all categories. The green roof and the greened facade use fewer resources than any of the green wall systems analysed. The analysis shows that there is no fixed ratio between direct and indirect resource use, but that this is highly dependent on the materials used. Looking at the analysed green walls, it seems of less importance which type of system is used (hydroponic or substrate-based). The material choice plays a much more significant role. The stainless steel panels, for example, have a direct resource use (self-weight) of 0.36 tonnes which trigger 2.85 tonnes of abiotic material flows during production. The huge difference between the resource use of the aluminium panels and the stainless steel panels is partly caused by the fact that the manufacturer uses recycled aluminium but new stainless steel elements (as this is not recyclable). The hydroponic system seems to be the least resource intensive green wall option analysed (if water is not taken into account), although it accumulates almost the same direct weight as the plastic panels (filled with substrate) simply as it has to be replaced more often during the 60 year period. In terms of indirect resources it performs similar to or slightly better than the other systems. Among the substrate based systems, the plastic polyethylene system clearly outperforms all the others, as its components are less resource intensive. The additional elements such as pumps and water tanks have a minor influence on the overall resource use of the green wall systems analysed.

0

0.5

1

1.5

2

2.5

3

3.5

4

Air 0.09 0.14 0.09 0.51 0.03 0.00 0.01

Biotic 0.01 0.00 0.02 0.00 0.00 0.00 0.00

Abiotic 0.27 0.29 0.31 2.85 0.10 0.16 0.04

Direct 0.12 0.16 0.30 0.36 0.01 0.15 0.01

Hydroponic

green wallPlastic panels

Aluminium

panels

Stainless steel

panels

Additional

elements

(pumps, water

storage)

Green roofGreened

facade

Figure 8: Resource use in tonnes of 1 square metre of green wall panels, green roof and greened façade over 60 years without taking irrigation, water or planting into account

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Looking at the water usage illustrated in Figure 9, a different picture in terms of efficiencies emerges. The diagram shows the water consumption during production of the analysed systems and for the irrigation water to water and feed the plants.

0

20

40

60

80

100

120

Irrigation during maintenance 109.5 65.7 65.7 65.7

Water during production 6.53 8.41 3.72 36.15 2.03 0.31 0.51

Hydroponic

green wall

Plastic

panels

Aluminium

panels

Stainless

steel panels

Additional

elements Green roof

Greened

facade

Figure 9: Water use in tonnes during production and maintenance of 1 square metre of green wall panels, green roof and greened façade over 60 years

While the green roof and the greened façade are dependent only on rainfall, all green wall systems require constant irrigation. Although the stainless steel panels are the least efficient option considering production by requiring 250 tonnes of water during manufacture, the felt panels require more water for irrigation due to higher evaporation levels and this creates the highest water demand overall. If the water demand can be satisfied by the use of storm- water this is not too critical, although peaks in water demand and precipitation are unlikely to occur at the same time making bigger storage facilities necessary. The water consumption for the substrate based wall was estimated based on recommendations from manufacturers of around 3 l/m²/day. Hydroponic walls were estimated to need 5 l/m²/day as they have a higher evaporation loss, cannot retain water in large quantities and need regular irrigation all year round, to supply nutrients to the roots. The absolute quantity is dependent on factors such as climate, plant species, shading by neighbouring buildings, and orientation to the sun.

Overall the green roof has the lowest water consumption, being similar to that of the greened façade. If irrigation is not taken into account, the green roof only needs 5% of the water required for the most efficient wall system (aluminium). With irrigation it needs only 0.5% of the water required for the aluminium system. The following picture emerges from comparing the resource use of vegetables grown on one square metre of green wall with a square metre of commercially grown vegetables. Presuming that the yield of the wall is 75% of the yield of horizontal agricultural land (as more radiation reaches the horizontal), each square metre of wall would produce 2.7 kg vegetables per year or 0.166 tonnes in 60 years (Statistik Austria, 2008, p. 11). This figure is slightly more optimistic than the estimates of 2.6 kg/m²/a of one of the manufacturers of food-grade stainless steel panels (Wong, 2008). The material intensity of the wall grown vegetables (excluding irrigation) can now be calculated as the resource use of a square metre of the plastic and (stainless steel) green wall systems including proportional material flows triggered by pumps, water storage and irrigation system is known. These are 0.16 tonnes (0.36) direct resource use, 0.29 tonnes (2.85) of abiotic resource use, 0.14 tonnes (0.51) of air and 8.41 tonnes (36.15) of water as can be seen in Figure 8. Table 4 shows the material intensity of one tonne of vegetables grown on a green wall in both plastic and stainless steel panels and compares it with

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commercially grown vegetables. It includes the material intensity of a 2.8t lorry per kilometre and transported tonne. Table 4: material intensity of wall grown vegetables and commercially grown vegetables material material intensity [t/t], [t/tkm]

abiotic biotic water air surface

[m/m]

Wall grown vegetables (plastic panels) 1.75 0 50.7 0.84 0.15

Wall grown vegetables (stainless steel panels) 17.16 0 217 3.07 0.15 Vegetables grown commercially (excl. transport) 1.4* 1.4* 8.89** 0.104** 1 Transport (Lorry, 2.8t)*** 0.00045 0 0.004124 0.00144 - * (Loske, R.et al., 1996, p. 104) **(Kaiser,C.et al., 2009) ***(Wuppertal Institute, 2003) The table does not include the water needed to irrigate the green wall but includes the water needed to irrigate the commercially grown vegetables so that the data is not completely comparable. It also does not take cooling and storing of commercially grown vegetables into account which would increase their material intensity. Nevertheless it gives a first indication of whether green walls can reduce or even compete with the resource use of commercially grown vegetables. Table 4 shows that growing vegetables in stainless steel panels increases the resource use significantly. Using the data from the table, a lorry could transport commercially grown vegetables for over 2000 km before reaching the same air consumption as the vegetables grown in stainless steel panels. In terms of abiotic and water usage the distances would over 35 000 and 50 000 km. For the plastic panels the boundary value would be a transport distance of 500 km (air), 777km (abiotic) or 10 000 km (water). This suggests that vertical farming is not going to be an answer to a growing hungry world population.

DISCUSSION AND CONCLUSION

The results from the MAIA provide an intial insight in the environmental impact of different types of available vegetative covers for building envelopes. The resource use of all of the analysed green wall systems is much higher than that of the green roof and the greened façade. Between the green wall systems resource use varies widely (with a maximum of a tenfold difference between plastic and stainless steel panels). Hence material choice seems of great importance as it can influence the resource-productivity of the chosen system greatly. One conclusion related to the above is that the promotion of food production on green walls for environmental reasons should be limited to food grown in plastic panels, as the stainless steel system requires up to ten times the resources of the plastic system. A first estimate shows that conventional food production and transportation would be more resource-efficient than growing food locally on green walls made from plastic panels within a radius of 500 kilometres for sourcing food. However, more detailed analysis is needed to incorporate all the material flows associated with the provision of fresh food (such as storage, cooling, super market operation etc.). Furthermore, green walls enable the production of food on unused vertical surfaces and therefore have the potential to increase the productive area for a given land surface. Given that the largest component of the total was the resource use of the wall panels (see to Figure 8) it would be worthwhile investigating the use of products with lower material intensities in order to make food production on walls more competitive with commercially grown alternatives. The results from the material intensity analysis underline the importance of a life cycle wide analysis of material flows in order to estimate the environmental disruption caused by the systems considered, as the self-weight of the product is not a sufficient indicator. It represents only a fraction (roughly 10% of the overall resource use in the case of the stainless steel green wall system) of the overall material flows triggered by the production and operation of products and services.

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However it has to be noted that the analysis used does not consider qualitative differences between the systems, such as visual appearance or improvement in air quality, but is limited to quantitative indicators. Therefore a detailed analysis of qualitative differences is also required to find the best solution for any project, as, for example, all systems perform slightly different in terms of thermal insulation and influence the micro-climate differently. Furthermore is has to be noted that this case study is limited to the analysed systems and resource uses are indicative estimates only. The absolute quantities of resources used can, to a limited degree, be influenced by careful planning. Using plants that require minimal irrigation and nutrition on the green wall would be environmentally sensitive. However, it seems that the chance to use a wide range of plant species independent of rainfall pattern forms part of the attraction of the green wall, which stays lush and green even in dry weather, while plants on a green roof or a greened facade usually rely on naturally occurring rainfall. Over time this naturally reduces the choice to those plants that are best suited for the climate and microclimate and does not allow for selection based on appearance. In summary it can be stated that the green roof outperforms the green wall systems in both resource use and synergistic effects, although potentially the green walls have a higher visual impact.

ACKNOWLEDGEMENTS

The research for this paper was kindly funded by the Bruno-Taut- Scholarship of the German Chamber of Architects and the German Academic Exchange Service (DAAD). Furthermore we would like to express our gratitude to Michael Ritthoff of the Wuppertal Institute for Climate, Environment and Energy for his personal support in sourcing some of the data used.

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