How successfully do green facades enhance urban environmental conditions and to what extent is this approach sustainable?

Context

‘Unless one merely thinks man was intended to be an all conquering and sterilizing power in the

world, there must be some wise principle of coexistence between man and nature, even if it has to

be a modified kind of man and a modified kind of nature.

This is what I understand by conservation’ (Elton, 1958; p. 145).1

IN ALL RESPECTS A BARRIER OF SORTS? AGAINST PARTICULATE POLLUTION, NOISE POLLUTION, HEAT, HEAT TRANSFER, WIND,

The concept of greening our buildings with plants is by no means a new and original idea. Indeed, the concept has been integral to buildings for thousands of years. First, we may consider the Mediterranean region, where 2000 years ago palaces were found to be covered with vines for they were known to shade the buildings, their evapotranspirative cooling properties, economic value for the growth of fruits, and aesthetic improvement through ornamentation.2 In Northerly regions of the world though, vernacular architectures developed using turf to line roofs and walls as a readily available and effective insulation. This can be seen in the ancient structures of Jarlshof in the Shetland Isles. Here the settlement, of which the earliest parts are 4000 years old, small tight stone structures were erected which were once covered with turf to protect against the harsh North Sea wind.

In more recent years however, the greening of facades re-emerged in the 19 th century as a response to the industrialisation of cities. This was also the inspiration for Ebenezer Howard’s Garden City Movement which addressed the deterioration of cities and quality of life through a rationalised plan

balancing industry, agriculture and housing for the ‘ideal’ city arrangement.3 The implementation of vegetation within cities was the “first ecological reaction to industrialisation”4

Contemporary thought however originates from the increased awareness of global warming, climate change and the potential for sustainability, which was promoted by German

1 ‘Urban reconciliation ecology: The potential of living roofs and walls’ from Journal of Environmental Management (2009) Francis, R. A., Lorimer, J.

2Köhler, M. “Green Facades – a view back and some visions” Urban Ecosyst (2008) p.4233http://www.architecture.com/HowWeBuiltBritain/HistoricalPeriods/TwentiethCentury/ GardenCityMovement/PlanOfAnIdealGardenCity.aspx#.UlkunlC-2uI [Accessed: 12/10/13]4 Kaltenbach, F., “Living Walls, Vertical Gardens – from the Flower Pot to the Planted System Facade,” Detail 12 (2008): 1455.

artists such as Hundertwasser in the 1970s, as suburbanisation increased and inner cities demanded redevelopment.5

http://www.hundertwasser.at/images/hundertwasser/oeko/apa_0343.jpg

The construction of green facades was thus endorsed, especially in Germany, with a strong incentive programme initiated in Berlin resulting in the construction of 245 584m2 green facades installed in the years 1983-97.6

Green buildings – appearance of green – misconceptions associated with this – misleading to both general public and professionals alike.

Reduce history to 100 words, and then:- psychological benefits

It may not be actively acknowledged or considered however it is an essential part of our instinct for human survival that vegetation affects our psychological well-being. - global scale/urban landscape- outline spectrum of attitudes:< aesthetic to scepticism >; glossy Patrick Blanc > Paradise Passage

http://2.bp.blogspot.com/-S7CU7gWyFnc/TcL7V-emKeI/AAAAAAAAAXM/3ux1pTGMmQ0/s1600/caixa_forum_madrid_9.jpg

http://www.architectsjournal.co.uk/pictures/636xAny/2/9/1/1232291_DSDHA_Paradise_Park.jpg5 Köhler, M. “Green Facades – a view back and some visions” Urban Ecosyst (2008) p.4246 Ibid.

Various systems:

First patented system – US Stanley Hart White 1938, Vegetation-Bearing Architectonic Structure and System

The various environmental conditions affected by the implementation of green vertical systems:

Thermal – cooling /insulation Light (seasons) Air

o Movemento Pollutiono Wind / bioprotection

Moisture/rain Sound – dampens noise pollution - insulation but also has its own noise to cover

Passive energy system for:

Shade Insulation by vegetation and substrate Evaporative cooling Barrier to wind

In this section the aim is to demonstrate the potential benefits and weaknesses of vertical green systems. A pressing issue in the analysis of this performance is however the variables which may influence the results, which is the source of difficulties in offering clear comparisons to the different systems.7 It should be noted that the following represents an overview of the effects of green walls but that in each case climate, plant variety, immediate context, type and material of system implemented should also be considered.

The thermal benefits of green facades and living wall systems, whether for the cooling or insulating properties of the wall are depended on the variables of foliage thickness, water content, structural properties and air spaces between the leaves.8

Shade

It is not a new concept to build near vegetation to aid in building cooling. Plants naturally grow with and respond directly to the seasons, thus it is logical to consider exploiting the intelligence of the natural façade as a way condition our buildings. “seasonal variation of the shading effect”9

7 Gabriel Pérez, Lídia Rincón, Anna Vila, Josep M. González, Luisa F. Cabeza, Green vertical systems for buildings as passive systems for energy savings, Applied Energy, Volume 88, Issue 12, December 2011, pp. 48548 Katia Perini, Marc Ottelé, A.L.A. Fraaij, E.M. Haas, Rossana Raiteri, Vertical greening systems and the effect on air flow and temperature on the building envelope, Building and Environment, Volume 46, Issue 11, November 2011, Pages 2287

9 Kenneth Ip, Marta Hoi-Yan Lam, Andrew Miller Assessing the shading performance of climbing plant canopies for the PLEA2007 The 24th Conference on Passive and Low Energy Architecture

The absorption of the solar radiation by the plants is also essential to their “biological functions” for photosynthesis, transpiration, evaporation and respiration, with only 5-30% of the solar radiation incident on the facade actually reaching the wall behind the foliage.10 Shading and evapotranspiration can considerably decrease the heat which would be re-radiated by other hard surfaces in the urban environment. INDEED “every 0.5° internal temp difference can reduce the electricity use for air conditioning up to 8%”11

Studies have shown that the solar radiation is two times lower when using a secondary façade of vegetation as opposed to a façade which only has blinds, not only because it is a more effective shading device but also because plant surfaces will never exceed 35°C, while blinds can reach above 55°C.12

The “Bioshader” Experiment conducted by the University of Brighton in 2003 investigated the “thermal shading performance of a vertical layer of deciduous climbing plant canopy”, in which two identical offices were selected as test and control rooms. To the test room’s windows external stainless steel frames were attached to the frame onto which Virginia Creeper plants were grown. Occupation of these rooms during the experiment also remained constant.

Tests concluded that solar transmittance varied between 0.43 and 0.14, for up to five layers of leaves and one layer respectively, and that the internal temperatures were reduced 3.5 – 5.6°C.

pp. 47-48 Green Design: Agnes Novak: the leaves “follows the daily and annual cycles optimally providing the following advantages: in summer – when the sun is high – leaves of the plant rise up operating as ventilating shutters providing a stack effect between the building and the plant and cooling the air that enters the house. Contrary to this, in winter – because of the low sun position –

10 Katia Perini, Marc Ottelé, A.L.A. Fraaij, E.M. Haas, Rossana Raiteri, Vertical greening systems and the effect on air flow and temperature on the building envelope, Building and Environment, Volume 46, Issue 11, November 2011, Pages 228811 Katia Perini, Marc Ottelé, A.L.A. Fraaij, E.M. Haas, Rossana Raiteri, Vertical greening systems and the effect on air flow and temperature on the building envelope, Building and Environment, Volume 46, Issue 11, November 2011, Pages 228812 Gabriel Pérez, Lídia Rincón, Anna Vila, Josep M. González, Luisa F. Cabeza, Green vertical systems for buildings as passive systems for energy savings, Applied Energy, Volume 88, Issue 12, December 2011, pp. 4856

leaves of the evergreen carpet turn down – low hydrostatic pressure can be observed – and stick together, enclosing a stationary insulating air layer.”

Manfred Köhler revealed that the density of leaves will vary the results; through experiment he concluded that Ivy is the most efficient in this respect, as it provides a shadow effect similar to that produced by trees.13

The results of a test carried out in Holland to determine the relative efficiencies of various vertical greenery systems (that is, direct greening system, indirect, and living wall system) recorded the wind speed and temperature differences induces by the differing systems.

Evaporative cooling by evapotranspiration

Dependent on the type of plant and exposure, and climate (dry/windy increase effect), and in the case of LWS the substrate will impact the effect of this also.

In referring to Wong, Perez et al. notes that external insulation is more effective than internal and thus VGS can work on two levels to reduce energy consumption: through reducing solar energy through shading, and reducing heat flow through evaporative cooling.14

“The cooling effect mediated by transpiration relies on the presence of a healthy plant cover sustained by adequate irrigation.”15 Current literature records that daily temperature changes can be dramatically reduced, with 40-80% of incident radiation reflected or absorbed, and converted to latent heat, which does not result in a rise of temperature.16 Such considerable temperature differentials are however likely to be more typical of hotter climates. This is nonetheless important especially in the discussion of heat mitigation in dense urban environments, where buildings will as common practice utilise air conditioning units. The same report referenced another study where energy reductions for air conditioning were reduced 23%, with an annual 8% saving in energy use17.

Insulation

In addition to cooling in the summer through shading of the facade, vertical greenery systems are also considered an effective means of further insulating a building in creating a stagnant layer of air between the vegetation and facade. Conducted in autumn, very little temperature difference was noted; this was true for another study by Köhler in Germany during a similar time. With warmer temperatures however considerable temperature differences have been recorded of up to 6°C. (31.0 for bare façade, 25.2 for greened façade). This is indication for the intuitive nature of the planting

13 Pérez, G., et. al. Green vertical systems for buildings as passive systems pp. 485614 Gabriel Pérez, Lídia Rincón, Anna Vila, Josep M. González, Luisa F. Cabeza, Green vertical systems for buildings as passive systems for energy savings, Applied Energy, Volume 88, Issue 12, December 2011, pp. 485715 C.Y. Cheng, Ken K.S. Cheung, L.M. Chu, Thermal performance of a vegetated cladding system on facade walls, Building and Environment, Volume 45, Issue 8, August 2010, Pages 178716 Nyuk Hien Wong, Alex Yong Kwang Tan, Yu Chen, Kannagi Sekar, Puay Yok Tan, Derek Chan, Kelly Chiang, Ngian Chung Wong, Thermal evaluation of vertical greenery systems for building walls, Building and Environment, Volume 45, Issue 3, March 2010, Pages 66317 Nyuk Hien Wong, Alex Yong Kwang Tan, Yu Chen, Kannagi Sekar, Puay Yok Tan, Derek Chan, Kelly Chiang, Ngian Chung Wong, Thermal evaluation of vertical greenery systems for building walls, Building and Environment, Volume 45, Issue 3, March 2010, Pages 663

screen though as it is influenced directly by the environment and consequently can condition the building as demanded by the local climate.18

As with the cooling effects of the vegetation on buildings, it has been proven by Holm in 1989 that this is most effective in “low-mass buildings in hot-arid climates” on walls facing towards the equator.19 However even in more temperate climates experiments reveal peak cooling load transfer can be reduced by up to 28% in summer. As the vegetation reduced the incident solar radiation to the facade, it also reduces the exposure of the facade to the immediate external environment during winter, thus significantly lowering the energy transfer. Köhler reported that insulation could provide up to 5°C internal temperature difference compared to an uncovered facade.

wind

The study of the three Dutch facades by perini et. al. Also demonstrated a decrease in wind speed of between 0.43m/s and 0.55 m/s compared to 10 cm in front of the facade. Wind was recorded in the cavity of the indirect climber system and not in the LWS, this is due to the density of the materials (felt, plastics etc.). The drop in wind velocity is still a sign of the potential of green facades as a protective barrier, which can both protect the fabric of the building itself (bioprotection) but also minimise thermal transmittance induced by wind directly passing the building envelope. 20 This was also the conclusion of another report which considered the “prolonged lifespan of building facades” and reduced maintenance to the facade construction itself, but did not balance the argument with maintenance of the vertical greenery system.

VERY DIFFICULT TO ANALYSE DUE TO VAST VARIABLES. For the system is intrinsically based in the context. ...

The same experiment also revealed that the thermal resistance of the wall construction of the direct greened facade was increased 0.09m2K/W, which would reduce energy costs. Interestingly the investigations concluded that the 200mm gap between the foliage and facade of the indirect system was too deep to perform as a stagnant air layer, and thus would be more efficient if this distance was reduced to obtain “optimal thermal qualities”.21 Looking at the example of the LWS it noted that there is potential to determine an optimum air cavity in green facade construction, similar to general wall construction, with an optimum of 40-60mm. The LWS also has the additional benefits over the other systems of the thermal resistance of the support materials (planter boxes, felt etc.). –MISLEADING – read this data, believe better, but must also consider other variables of cost and maintenance, and balance the benefits.

Dust Mitigation

18 Katia Perini, Marc Ottelé, A.L.A. Fraaij, E.M. Haas, Rossana Raiteri, Vertical greening systems and the effect on air flow and temperature on the building envelope, Building and Environment, Volume 46, Issue 11, November 2011, Pages 229119 C.Y. Cheng, Ken K.S. Cheung, L.M. Chu, Thermal performance of a vegetated cladding system on facade walls, Building and Environment, Volume 45, Issue 8, August 2010, Pages 178720 Katia Perini, Marc Ottelé, A.L.A. Fraaij, E.M. Haas, Rossana Raiteri, Vertical greening systems and the effect on air flow and temperature on the building envelope, Building and Environment, Volume 46, Issue 11, November 2011, Pages 229121 Katia Perini, Marc Ottelé, A.L.A. Fraaij, E.M. Haas, Rossana Raiteri, Vertical greening systems and the effect on air flow and temperature on the building envelope, Building and Environment, Volume 46, Issue 11, November 2011, Pages 2293

With increased urbanisation and industrialisation our present towns and cities have become clogged with pollutants. These particles fill the air around us, and can be detrimental not only to our health but that of the urban fabric too. Vegetation is considered a ‘particulate sink’ whereby fine and ultra fine particles adhere to the leaf surfaces, thus mitigating them from the air around us. Greater potential compared to a green roof (area). “vegetation is an efficient sink for particulate matter” 22 particulate capacity depends on plant variety, structure of vegetation, exhibition , location and circumstances (micro climate).23 Quite unbelievable that :“The United Nations states that 600 million urban residents worldwide are exposed to harmful levels of traffic-generated air pollutants, with more premature deaths caused by pollutants in vehicle emissions than by car accidents (WHO, 2000)”

If vegetation acts as a particulate sink there is greater incentive to enhance greenery of urban environments. Studies have shown that leaves certainly do pick up the particulate matter from the air around them. One study into the dust mitigation of Ivy revealed that the under layers of leaves showed considerably less pollution than those of the outer layers and so it can be proven that it could act as a barrier /bioprotective layer: “While much debate about ivy concentrates on its deteriorative role, ivy's high absorption rate of fine and ultrafine particulates points to its pollution reduction role.”24

Bioprotection or biodeterioration? (Ivy) Mediator between the facade and extreme conditions?25

“Roots can break through waterproof seals and penetrate the smallest crevices of brickwork. Tendrils can force their way behind cover strips and window abutments, while leaves and twigs may block rainwater pipes and gutters. Plantings can also impose an additional load on an outer wall and have a negative effect on the structural balance.”26

Insulation of frost in January/heat reduction in summer “spikes”

Ivy is an innately hardy plant: resilient against frost, shade, drought and pollution. Max temp difference recorded in summer of 17°C in summer. Deterioration can be caused through forced exertion of roots into the building materials or “chemical decomposition as they can secrete acids and other compounds”27. The same study revealed (although not conclusive) that ivy will only root

22 Marc Ottelé, Hein D. van Bohemen, Alex L.A. Fraaij, Quantifying the deposition of particulate matter on climber vegetation on living walls, Ecological Engineering, Volume 36, Issue 2, February 2010, Pp. 15523 Troy Sternberg, Heather Viles, Alan Cathersides, Mona Edwards, Dust particulate absorption by ivy (Hedera helix L) on historic walls in urban environments, Science of The Total Environment, Volume 409, Issue 1, 1 December 2010, Pages 162

24 Troy Sternberg, Heather Viles, Alan Cathersides, Mona Edwards, Dust particulate absorption by ivy (Hedera helix L) on historic walls in urban environments, Science of The Total Environment, Volume 409, Issue 1, 1 December 2010, Pages 167

25 Troy Sternberg, Heather Viles, Alan Cathersides, Evaluating the role of ivy (Hedera helix) in moderating wall surface microclimates and contributing to the bioprotection of historic buildings, Building and Environment, Volume 46, Issue 2, February 2011, Pages 293

26 Kaltenbach, F., “Living Walls, Vertical Gardens – from the Flower Pot to the Planted System Facade,” Detail 12 (2008): 1454.27 Ivy on walls seminar oxford university pp.29

into the wall if there is a hole there existing/has been encouraged by attempts to kill the plant. Indeed ivy is invasive, and hardy, and can break through under walls and into cavities. However studies by English Heritage and the University of Oxford demonstrated that aerial roots cause little damage to stone, and their presence is more ‘superficial’.28 Growing plants be may good for some kinds of walls and not for others – this requires further investigation.

Acoustics

It was reported in 2000 that 44% of the population in the EU was exposed to road traffic noise levels of more than 55 dB.29 Research on the effectiveness of vertical greenery in sound mitigation is limited, however the effects of ground and roofing vegetation have been written about more extensively. Indeed, Wong et. Al. in referring to Schroeder noted a 5-20dB sound reduction from the presence of a roof garden.

“The WHO reported “burden of disease by environmental noise” quantified the many health-related effects by long-term exposure to environmental noise.30 Our cityscapes are composed of hard materials, of brick, masonry, concrete and glass which are unforgiving, amplifying high sound pressure levels. Experiments have been conducted to ascertain the performance of green facades as a means of mitigating sound levels; the wall is essentially a buffer (as with the thermal insulative properties) and can considerably reduce the noise of our towns and cities, mainly emitted by traffic.

Studies have demonstrated effective use of green walls in street canyons, and interestingly it has been noted that a more considerable difference in sound levels was recorded in streets adjacent to the street canyons implemented with the green façade as much of the direct sound still reaches the receiver in the greened street. 31

It is very difficult to analyse noise transmission in urban canyons due to the variables of a variety of reflective surfaces, of varying heights and shapes etc.32 In a study investigating the noise in courtyards when using green walls as a source of sound mitigation, it was found that greening the upper halves of the facades was more effective – it was the edges which were most important in sound reduction, and also when used with harder materials such as concrete or masonry compared to softer bricks.33 Greening of the courtyard sides was proven more effective than of the street canyon side in reducing sound levels in the court. In the same study however green roofs were proven more effective, or even a combination of green roofing and wall strategies.

28 Ivy on walls seminar oxford university pp.3029 Nyuk Hien Wong, Alex Yong Kwang Tan, Puay Yok Tan, Kelly Chiang, Ngian Chung Wong, Acoustics evaluation of vertical greenery systems for building walls, Building and Environment, Volume 45, Issue 2, February 2010, Pages 41130 Timothy Van Renterghem, Maarten Hornikx, Jens Forssen, Dick Botteldooren, The potential of building envelope greening to achieve quietness, Building and Environment, Volume 61, March 2013, Pages 3431 Timothy Van Renterghem, Maarten Hornikx, Jens Forssen, Dick Botteldooren, The potential of building envelope greening to achieve quietness, Building and Environment, Volume 61, March 2013, Pages 3432 Timothy Van Renterghem, Maarten Hornikx, Jens Forssen, Dick Botteldooren, The potential of building envelope greening to achieve quietness, Building and Environment, Volume 61, March 2013, Pages 3533 Timothy Van Renterghem, Maarten Hornikx, Jens Forssen, Dick Botteldooren, The potential of building envelope greening to achieve quietness, Building and Environment, Volume 61, March 2013, Pages 40

A study conducted in Singapore in 2010 recorded that vegetated walls are most effective for higher frequencies of sound (between 200Hzto 1kHz), however the substrates of living walls performed better with lower frequencies, as it absorbed the acoustic energy reducing the reverberation time. The same study also recorded however that course concrete block outperformed the vegetated walls for low frequencies. 34

It is important to consider the type of vegetated wall system utilised if for the purpose of mitigating noise pollution, as not all systems are effective. It is important to consider and test the effects with more conclusive results to that which is currently available, and test the variables of structure, materials, panel dimensions, type, compositions, depth and moisture content of substrate, and the type of plant.

IRRIGATION!!!!

LIMITATION OF HEIGH OF PLANTS- GROWTH

TYPES OF PLANTS

BIODIVERSITY

CASE STUDIES

Patrick Blanc and his ‘Mur Végetal’

Patrick Blanc, a French botanist, comes from a scientific background, yet channels his passion for art, botany and the urban environment through his patented “invention” of the vertical garden from 1988 (which as we mentioned before was possibly first patented by Stanley Hart White). Nevertheless, today it is Patrick Blanc with whom we strongly associate the vision of LWS with.

Blanc challenged the traditional methods of vertical greenery in the 1980s, which were hindered by the logistics of suspending the sheer weight of the planter boxes containing earth soaked in water. Having travelled young to Malaysia he investigated how plants thrived on rock with little or no soil. He suggested that plants would be able to grow sufficiently even in temperate climates when only the roots were covered with adequate water and nutrients – and that this could be stored within substrates. This hydroponic cultivation, without soil, however is not simple, and the substrate and structure is complicated.35

Le Mur Vegetal consists three elements: metal frame hung from or standing separate to the wall, 10mm thick PVC sheet for rigidity and waterproofing, and a layer of polyamide felt: the substrate for the vegetation which allows even distribution of water.36 Irrigation is fed from the top into the substrate through drip, in which nutrients are also provided. The entire system is <30kg/m2 which allows it to be integrated into facades with no limitation to size or height.

34 Nyuk Hien Wong, Alex Yong Kwang Tan, Puay Yok Tan, Kelly Chiang, Ngian Chung Wong, Acoustics evaluation of vertical greenery systems for building walls, Building and Environment, Volume 45, Issue 2, February 2010, Pages 41835 Kaltenbach, F., “Living Walls, Vertical Gardens – from the Flower Pot to the Planted System Facade,” Detail 12 (2008): 145736 ‘A Scientific and Artistic approach by Patrick Blanc’ from http://www.verticalgardenpatrickblanc.com/ accessed: 13/10/13.

Blanc assures that the LWS presents no threat to biodeterioration of the existing facades, as roots are embedded in the felted substrate, an element separate from the façade structure, a second skin. However this does not mean that they are not invasive beings – the living wall is different to all other “cladding” systems because it is living. This means that it demands maintenance and care to ensure its success, as failure would result in the ‘death of the wall’.

Blanc is an artist at heart – the building envelop is his canvas. Although he makes claims of the ecological effect of the living wall systems, this is not the emphasis of his work – it has a different ambition, for uniting nature and architecture, and improving the appearance of the urban environment.

However the initial and life-cycle costs of these Living Wall Systems is very much glazed over. While LWS may present (in some cases) the most efficient ecological system in terms of sound absorption or thermal conditioning of the buildings, they are currently not economically viable solutions. Indeed this aspect of the design is criticised as “play[ing] no useful ecological role” and quite harshly as “a commercially driven synthesis of aesthetic and morphological elements removed from their original context”.37 Is this not true of many building materials too though – Mie’s marble in the Barcelona pavilion? Or what of the use of timber imported from tropical regions? Criticisms such as this fail to acknowledge the variety of benefits that façade greenery can perform, for urban sustainability should not only address the reductions of energy but also the psychological bearings of our urban fabric on society. Blanc is an artist and botanist, not an architect.

Studies in cost-benefit analysis reveal that LWS such as those designed by Patrick Blanc can cost up to £1000, depending on the façade surface, height, location and connections. However this does not end with the initial set-up costs, as the LWS require more maintenance than other forms of vertical greenery systems.

4. Living wall system Initial Plant species One time 27.49Panels and transportation One time 176.23Irrigation system One time 27.61Installation One time 83.50Maintenance Pruning and panels adjustment Annual 14.41Irrigation (H2O) Annual 0.96Panels replacement (5%) Annual 6.05Plant species replacement (10%) Annual 2.75Pipes replacement (irrigation system) Annual 2.85Cladding renovation One time e 50th year 486.96Disposal Green layer disposal One time e 50th year 218.56

1. Direct green façade Initial Plant species and installation One time 21.78a/m length of baseDig þ pot One time 519.92aMaintenance Pruning Annual e after 4th year 2.81Cladding renovation One time e 50th year 1224.35Disposal Green layer disposal One time e 50th year 31.1038

37 Gandy, M. (2010), The Ecological Facades of Patrick Blanc. Architectural Design, 80: pp.3338 Katia Perini, Paolo Rosasco, Cost–benefit analysis for green façades and living wall systems, Building and Environment, Volume 70, December 2013, Pages 114

The same study concludes that the living wall system does not present a viable system for sustainability. It is by far the most expensive installation, in initial set up, and maintenance (and replacement etc.) which does not repay sufficient benefits (both economical and social) to justify the use of the system. In fact is it calculated that economic benefit comes in the 50 th year – the year of disposal/ replacement.

DIRECT GREEN FAÇADE PBP 20years Indirect - HDPE better than steel mesh – for the former PBP 16 years (best)39

39 Katia Perini, Paolo Rosasco, Cost–benefit analysis for green façades and living wall systems, Building and Environment, Volume 70, December 2013, Pages 120