A NEW DIMENSION IN GREEN INFRASTRUCTURE: THE CASE OF THE GREEN WALL
CAROLYN ANNE FAHEY Ph.D., Newcastle University, 2010
A thesis submitted to the
Faculty of the Graduate School of the University of Colorado in partial fulfillment of the requirements for the degree of Master of Urban and Regional Planning College of Architecture and Planning
This thesis for the Master of Urban and Regional Planning degree by
Carolyn Anne Fahey
has been approved for the
Urban and Regional Planning Program
Austin Troy, Chair
Date: January 4, 2017
Fahey, Carolyn Anne, MURP, Urban and Regional Planning
A NEW DIMENSION IN GREEN INFRASTRUCTURE: THE CASE OF THE GREEN WALL
Thesis directed by Professor Austin Troy
The question as to whether the literal greening of buildings has a measurable environmental benefit is explored in an effort to detail what impact green wall systems have on the environment. It is demonstrated that green walls hold environmental benefits currently not well-known, recognized, or harnessed by urban planners, policymakers, green infrastructure advocates, the building community, and others by looking to the ecosystem services framework. Using the four types of ecosystem services as a framework for analysis, the green wall is shown to have measurable benefits in most areas, and perhaps most substantially in climate regulation. As such, the green wall should not only be actively used as a green infrastructure element, it should be part of a climate mitigation strategy. A look to design strategies as well as applicable urban and regional building and planning codes is provided as a means of suggesting how jurisdictions can concretely harness the benefits of green walls.
The form and content of this abstract are approved. I recommend its publication.
Approved: Austin Troy
To the future and everyone who makes it there.
To Stefan Koller (University of Colorado) for reading various drafts of this work and for his constant support and encouragement. To David Fahey (National Oceanic and Atmospheric Research Administration) for his interest in the work, for comments and references, and for advising me to focus my energies on sustainability and the environmental future of our cities. To Professor Roger Pielke for taking some time to discuss the relationship of land use patterns to broader meteorological trends and what this knowledge might mean in the context of policy making. To Jim Gery and Kirk Moors for taking time to talk through applicable building and fire codes in the City of Boulder. To Jess Andersen for humoring me, for her insight on applicable landscape codes at the City of Boulder, and for being the first I know of to require a green wall when other traditional landscape requirements could not be otherwise met (I). To both Jess and Niki for robinifying this work. To Andreas Christen (University of British Columbia) for numerous emails shared, help early on finding references critical to the climate context section, and for answering research questions. To Shaun McGrath (Environmental Protection Agency) for taking the time discuss indoor air quality issues and the potential role of plants in indoor air quality. To Mehdi Heris (University of Colorado) for showing a great deal of enthusiasm for this project, for helping me to understand this project's significance within green infrastructure research, and for creating Envi-met models illustrating the temperature effect of vertical green wall systems on local microclimate found here in this document. Finally, to Austin Troy (University of Colorado) for giving direction and focus to my initial research topic, and to the entire examination committee for the many insightful and challenging questions posed to earlier versions of this document. That includes Professors Gregory Simon and Julee Flerdt.
TABLE OF CONTENTS
Green infrastructure and the green wall 2
Defining the green wall system 3
ECOSYSTEM SERVICES AS THE BASIS OF A CASE FOR USING GREEN WALLS 15
Regulating services 15
Micro-climate regulation 15
Shade effect 16
Cooling effect 17
Insulation effect 18
Wind barrier effect 20
Heat emissivity effect 21
Micro-climate regulation effects demonstrated 22
Relationship to urban heat island effect 35
Flood and other natural hazard mitigation services 36
A ir p urifi cation 3 7
Water purification 38
Pollination and pest control 38
Habitat services 39
Provisioning services 39
Cultural services 40
Cost effectiveness 42
Urban regional economy 43
DESIGN STRATEGIES 45
Wall selection 45
Environmental lifecycle analysis 45
Cost benefit analysis of design factors 47
Wall area ratio (size) 55
Plant selection 55
A gricultural plan ts 57
Additional design considerations 63
POLICY IMPLICATIONS 67
FINDINGS AND CONCLUSIONS 73
ENVI-MET SEQUENCE i
LIST OF FIGURES
Architectural details of vertical green wall systems 5
Vine growing up a building in Chicago 7
A direct green wall system shown here on the Babylon Hotel by Vo Trong Nghia 7
Front facade of house with stacking direct green wall systems 8
Stacking direct vertical green wall system by architecture firm Vo Trong Nghia 9
One Central Park LWS system with felt layers and the direct green wall 10
King's Road Flats in London by Modece Architects 10
An indirect green wall on the Ambulance-Voorziening in Utrecht by Architectenforum 11
Detail of an indirect green wall system in the Ambulance-Voorziening 11
Musee du Quai Branly, Paris. Photograph taken by Bertrand Garbel 12
Felted green wall system requiring new construction and specialized engineering 13
Patrick Blanc inspects a green wall under construction 14
Architectural details of the four main green wall system types 19
Envi-met model showing the location of two three story buildings 24
10AM, with upper building featuring a green wall 25
Photograph of indirect green wall system or vining plants on metal trellis system in summer 27 Thermal photograph of indirect green wall system on metal trellis in summer 27
Photograph of indirect green wall system or vining plants on metal trellis in winter 28
Thermal photograph of indirect green wall system on metal trellis in winter 29
Photograph of direct green wall system or trellis with vining plants in summer 29
Photograph of direct green wall system or trellis with vining plants in summer 30
Thermal photograph of direct green wall system or vining plant and trellis in summer 31
Thermal photograph of direct green wall system or vining plant on trellis in summer 32
Thermal photograph of trellis in foreground and direct green wall system or vining plants beyond in summer 33
Photograph of direct green wall system or trellis with vining plants in winter 34
Thermal photograph of trellis and direct green wall with vining plants in winter 35
Volumetric illustration of building orientation in relation to a green wall system 54
Demonstration of a direct green wall's location and passive solar design strategy 54
One Central Park could have been planted with vegetable and fruit plants 58
Urban farming 58
Urban farming 59
Urban farming 59
Espalier tree on the Anglesey Abbey in Lode, UK 60
Ornamental Xeriscaped garden located in Boulder, CO with natural landscape beyond 62
Photograph of a failed LWS with planter boxes in England 64
Envi-met model showing the location of two three story buildings i
39 4AM iii
40 5AM iv
41 6AM V
42 7AM vi
43 8AM vii
44 9AM viii
45 10AM ix
46 11AM X
47 12AM xi
48 1PM xii
49 2PM xiii
50 3PM xiv
51 4PM XV
52 5PM xv i
53 6PM xvii
54 7PM xviii
55 8PM xix
56 9PM XX
57 10PM xx i
58 11PM xxii
1 The environmental burdens and benefits of the standard wall systems at different
lifecycle stages 47
LIST OF ABBREVIATIONS
Carbon Dioxide (C02)
Colorado State University (CSU)
Green House Gas (GHG)
Heating, Ventilation, and Air Conditioning (HVAC) Living Wall System (LWS)
The rising social pressure to counteract anthropocentric climate change has led architects and other design professionals to explain their buildings as demonstrating sustainable building practices. Of these interventions, the literal greening of buildings adding such elements as a vegetated roof or wall may be regarded as a particularly explicit or blunt appeal to environmentally friendly buildings practices. Yet is this appeal -the claimed for environmental benefit and friendliness grounded in scientifically verifiable reality, or does it terminate at the level of a purely aesthetic exterior? While the design-based movement surrounding green walls has changed the way we think about and implement green walls, given the elaborate new felt and steel mesh systems allowing for complex plantings, what the precise environmental benefits are if any -is rarely discussed as a significant point of evaluating green walls.
The focus of this thesis is whether the literal greening of buildings has a measurable environmental benefit. In an effort to detail what impact green wall systems have on the environment, this thesis demonstrates that green walls hold benefits currently not well-known, recognized, or harnessed by urban planners, policymakers, green infrastructure advocates, the building community, and others. It illustrates that the vertical green wall facilitates the goals of green infrastructure and therefore should be more actively considered a component in the designing and implementation of green infrastructure plans. It also discusses how the environmental benefits of the green wall are particularly pronounced in applications where space is constrained or costly redevelopment unlikely, thus opening the possibility of using space for greening that is not otherwise usable for greening or much of anything else.
The specific contribution of green wall systems are better understood in the context of
green infrastructure and urban greening. Section 1.2 thus opens by querying these larger notions before turning to green wall systems in particular. Later sections will return to these larger framework notions to (re)emphasize how green walls present an under-utilized means of capitalizing on the ecosystem services of green infrastructure (e.g., section 4.2.2).
1.2 Green infrastructure and the green wall
Green infrastructure as a practice is the harnessing of natural systems to provide services, such as water purification, that are otherwise provided by costly mechanical systems, such as water treatment plants. The European Commission provides a broader and more authoritative definition of green infrastructure here:
a strategically planned network of natural and semi-natural areas with other environmental features designed and managed to deliver a wide range of ecosystem services such as water purification, air quality, space for recreation and climate mitigation and adaptation (Green infrastructure, 2016).
Green infrastructure is an approach to reaching sustainability goals because it is helps to alleviate dependency on man-made systems that typically rely on oil, coal, and natural gas to power cars, homes, manufacturing, etc. It is also an approach to increase resiliency, as it makes areas less prone to natural disasters and more self-sufficient.
Green infrastructure is naturally occurring, as most cities have green river ways, preserved open spaces, shorelines, or other natural features. Green infrastructure is also planned primarily for the aesthetic and cultural benefits, as most cities have parks and most houses have a medium to large tree planted in the yard. Green infrastructure elements are increasingly intentionally
engineered and designed into our cities as a means of solving storm water issues, provide more green space in the "concrete jungles", teach children how to garden, and so on. So there is increased attention on the benefits and potential applications of green infrastructure as we collectively re-examine the urban environment, particularly in terms of its ecological health.
1.2.1 Defining the green wall system
The term green wall itself is also as varied as the wall systems themselves. Some use the term 'green wall system', others use 'green facade' or 'vertical green system' and still others 'hanging gardens', 'living wall system' or 'vertical green wall', and yet still more. Given the variety of terms used to refer to the green wall, for clarity's sake, the term 'green wall' will be used to mean vegetation covering or integrated with the building envelope, either through climbing plants, attached support systems, or hanging baskets or pots(Perini, et al 2013b: 266; Perini, et al 2013b: 257). There is a great degree of variation in wall type, and as more design attention is given to the walls, more variations are developed. Yet there is no standardized green wall typology making discussions of these walls simpler (Perez, et al 2014: 140). Again, for the sake of clarity, a basic wall typology identifying the direct, indirect, Living Wall System with planter boxes, and the Living Wall System with felt layers will be used (see figure 1).
Perhaps the least commonly discussed green infrastructure element in both contemporary literature and popular conversations on urban greening is the green wall. The particular appeal of the green wall is multifarious, but attracts a great amount of attention for the visual appeal. Yet, this thesis argues the practical value of the green wall is the capturing of space un-utilized by standard approaches to green infrastructure or a "doubling-up" of space as more area is opened up to greening. The direct green wall is a particularly practical solution for building retrofits, or in areas where re-development is unlikely to occur and low-cost retrofits to improve environmental qualities
are desired, as will be argued in the remainder of this thesis. Prior to mounting the argument for
these benefits, however, we have to establish the technical specifics that differentiate green wall systems from other types of green infrastructure.
loon )ni loon IB cm
1. BARE WALL
shrubs Pterop&da (fern) felt layers air cavity
Figure 1: Architectural details of vertical green wall systems. The four main types include the direct green wall, the
indirect green wall, the living wall system with boxes, and the living wall system (LWS) with felt layers (Source: Adapted
from Ottele et al., 2011).
The basic construction details provide a clearer indication of what materials are used in each
system and the relationship of the green wall system to the traditional wall system. Figures 2-12 show examples of each basic wall type. The first green wall type a direct green wall can take on two configurations. The first configuration is a vining plant, such as an ivy that climbs a building facade (see figure 2). The plant's roots are in the soil at the building's foundation or otherwise in a planter box at the building's foundation. The planter box may also be suspended and attached to the exterior wall or placed on balconies. The second approach to the direct green wall is to use hanging or tall growing plants, as opposed to vining plants. The Hanging Gardens of Babylon is an historical example of the hanging direct green wall, and a contemporary play on the design strategy is found in the work of architecture firm Vo Trong Nghia's reinterpretation of the famous hanging gardens (see figure 3). A more commonly seen approach to the hanging green wall is shown in Figures 4 and 5.The architects use a stacking direct green wall system on two facades, that is planted with small trees and large shrub like plants filling the vertical space between planters.
Figure 2: Vine growing up a building in Chicago (Source: www.pintrest.com Accessed January 2, 2017).
Figure 3: A direct green wall system shown here on the Babylon Hotel by Vo Trong Nghia (Source: Dezeen Magazine, 2016).
Figure 4: Front facade of house with stacking direct green wall systems. House is by Vo Trong Nghia (Source: Dezeen
Figure 5: Stacking direct vertical green wall system by architecture firm Vo Trong Nghia (Source: Dezeen Magazine, 2016).
The indirect green wall system is similar to the direct system except that it includes a metal mesh or other screening material intended for a vining plant to vertically grow. Figures 6-9 illustrate different indirect systems, showing how the mesh material is a covering on the existing building facade. The plant grows on the mesh material which allows for a separation between the plant and the building so that the plant is not vining directly on the building as was shown in Figure 2. There are two practical reasons for the indirect approach. One, it allows a plant to vein across surfaces, such as glass, that a plant could not otherwise vein on, and two, it prevents the plant from damaging wall materials by maintaining a separation, however slight, between plant and building.
Figure 6: One Central Park, a high rise residential development in Sydney, incorporates indirect green walls, along with the LWS system with felt layers and the direct green wall (Source: inhabitat.com Accessed January 2, 2017).
Figure 7: The award winning King's Road Flats in London by Modece Architects, utilizes an indirect green wall as part of the design strategy (Source: www.modece.com Accessed January 2, 2017).
Figure 8: An indirect green wall on the Ambulance-Voorziening in Utrecht by Architectenforum (Source:
www.architectenforum.com Accessed January 2, 2017).
Gelamineerd vuren spant Gipsplaat (Rigitone) Stalen dakplaat Steenwol
EPDM dakbedekking Bouwstaalmat 12mm Hemelwaterafvoer Sedumdak
Figure 9: Detail of an indirect green wall system in the Ambulance-Voorziening (Source:www.architectenforum.com
Accessed January 2, 2017).
The Living Wall System (LWS) with planter boxes is one of the more visually striking green
wall types. Because it requires a certain degree of integration with the building's wall system and has more material and installation requirements than the previous wall types, there is a great amount of variation in precisely how this wall type is detailed. There are also some systems, or system components that are patented, serving as the basis of entire businesses. A leader in the practice of the Living Wall System with planter boxes is Patrick Blanc. His green wall on the Musee du Quai Branly facade (see figure 10) is an extreme example of the LWS with planter boxes, as the system is wholly integrated into the wall system. The detailing goes so far as to exaggerate the windows mullions so as to give the illusion that the green wall is the primarily facade material, as opposed to stopping the mullions at the true wall as is traditional. Another interesting detail is the seamless rounding of the wall into the green roof above.
Figure 10: Musee du Quai Branly, Paris. Photograph taken by Bertrand Garbel (Source: Getty Images, 2016).
The last green wall type is the LWS with felt layers. The system stacks felt pocks, in a similar
manner to stacking wood shingles for a roof, to create a similarly striking visual experience to that of the LWS with planter boxes, allowing for high plant density and large scale or scope applications.
The following images show a LWS with felted layers under construction and number of years after construction (see figure 11). Figure 12 shows the amount of manufactured material that is required for the wall's construction. It also shows that the wall is designed into, or integrated with, the building's exterior wall, which requires substantial investment of time, money, and professional expertise to design and implement.
Figure 11: Felted green wall system requiring new construction and specialized engineering demonstrated here at the CaixaForum museum and cultural center in Madrid, Spain. The architecture remodel and expansion was done by Herzog & de Meuron and the vertical green wall system by Patrick Blanc. The work was completed in 2007 (Source: Caixaforum Madrid, 2016).
Figure 12: Patrick Blanc inspects a green wall under construction (Source: eco-publicart.org Accessed January 2, 2017).
Ecosystem services as the basis of a case for using green wall systems
The environmental benefits of green infrastructure are defined in terms of ecosystem services. An ecosystem service is defined as, "the direct and indirect contributions of ecosystems to human well-being. They support directly or indirectly our survival and quality of life" (Green Infrastructure, 2016; Millennium Ecosystem Assessment, 2017). An example of an ecosystem service is water filtration by plants, particularly reeds, in place of energy intensive man-made systems such as industrial scale water filtration plants typical in most American cities and counties. In terms of ecosystem services, this particular example provides the service type referred to as regulating services. Other types of services include provisioning, habitat, and cultural. The following chapter will place the green wall system within the ecosystem services framework with the aim of highlighting the broad and diverse range of benefits. The discussion will kick off with climate regulation, which is arguably the most significant environmental benefit of the direct green wall when considering climate change as a motivating factor.
2.1 Regulating Services
Regulating services are the benefits green infrastructure provides that stabilizes the environment, making human life more comfortable or sustainable. Commonly discussed regulating services include micro-climate regulation, natural hazard mitigation, water and air purification, waste management, pollination and pest control amongst others.
2.1.1 Micro-climate regulation
According to a literature review conducted for this thesis, the impact of green walls on building temperature, green walls are consistently reported as contributing to the stabilization of
indoor temperatures by intercepting solar radiation, providing thermal insulation, providing evaporative cooling through evapotranspiration, providing a screen from the wind, and emitting stored heat back into the atmosphere much faster than hard materials, such as concrete and wood (Wong and Baldwin 2016: 35; Perez, et al 2014: 161). The stabilization of indoor temperatures is also referred to as the effect on the building micro-climate or indoor room temperature. These five direct effects reduce energy consumption by reducing heating and cooling demand, where studies have found that greened facades, "absorb less heat than a non-greened facade" (Perini, et al 2013b: 271) and slow the rate of heat loss (Perini, et al 2013a: lll).The main elements that a plants, including green wall systems provide for micro-climate regulation include shade, cooling, insulation, wind barrier, and heat transfer.
188.8.131.52 Shade effect
The shade effect, or the solar radiation interception provided by plants, is the most significant for the energy savings purpose (Perez, et al 2014: 148). The shade effect is produced by the shade provided by the leaves of the plants. Through the shading effect alone the green wall system provides, "reductions of 28% for peak-cooling loads transferred through the wall in summer days" (Perez, et al 2014: 159).
One of the major variables in the effectiveness of the wall system in providing shade is the density and thickness of the plant foliage. The thickness and orientation were found to be significant factors influencing shade, accounting for variations between 10 and 31% (Perez, et al 2014: 156-160). Relatedly, the plant type and growing conditions impact the available foliage. There is some basic understanding of the types of plants that do well in certain conditions, but there is limited information available about which plant to use in which wall system, on which building facade or building orientation, and in which climate (Perez, et al 2014: 158). It is known that the effectiveness
of a plant to provide full foliage is, "affected by four critical factors, which are (a) soil moisture, (b)
plant type, (c) stage of plant development and (d) local climate which includes solar radiation, wind speed, humidity and temperature" (Wong and Baldwin 2016: 36) or (e) the "microclimatic parameters" (Wong, et al 2010: 671). Essentially, the denser the foliage the greater impact the plant has in providing shade.
Although that shading effect sounds straight forward block the sun's radiation from hitting the building the way in which plants provide this barrier is somewhat more complex. Plants block shade in a variety of ways according to Ottele:
[Of the] 100% of sun light energy that falls on a leaf, 5-30% is reflected, 5-20% is used for photosynthesis, 10-50% is transformed into heat, 20-40% is used for evapotranspiration and 5-30% is passed through the leaf. This blocking of the direct sunlight exposure ensures a cooling effect in warmer climates (2011: 3420).
Additionally, according to Wong and Baldwin, plant foliage captures 60% of solar radiation (2016: 35), with an average reported transpiration heat flux of 42%, where 40% of the heat was lost by thermal convection, and 18% was lost by long-wave radiation (Perez, et al 2014: 152). The added value of plants described in the aforementioned articles begins to describe why plants have been found to provide more effective shade than standard blinding approaches, such as umbrellas or sun sails (Perez, et al 2014: 159).
184.108.40.206 Cooling effect
The cooling effect describes the biological function of the plant as it grows, or the evapotranspiration from the plants and substrates. Phototropism in the plant generates a cooling effect, because the process of evaporation requires energy which is frequently found in the form of
heat energy. The use of that heat energy for evaporation means that there is less heat energy overall in and around the plant, which translates into lower temperatures in and around the plant. Ottele elaborates, stating:
green facades and roofs will cool the heated air through evaporation of water; this process is also known as evapotranspiration. As a consequence, every decrease in the internal air temperature of 0.5 C will reduce the electricity use for air-conditioning up to 8% (2011: 3420).
The effect is difficult to measure and predict, but the effects have been measured and proven in lower area temperatures around both water and plants.
220.127.116.11 Insulation effect
The insulation effect describes the insulating values of the wall system, particularly as pertains to the air gap between the vegetation and physical wall, and the substrate layer in certain wall systems. Looking again to the green wall system wall details, an air gap is clearly shown between the three of the four wall systems, as is highlighted red in Figure 13.
10cm jcm. IQCTB 10cm
1. BARE WALL
2. DIRECT GREEN WALL
climber Hed&ra heJix
3. INDIRECT GREEN WALL
climber Hedora hehx steel mesh air cavity
4. LWS W/PLANTER BOXES
shrubs Pteropsida (fern) felt layers air cavity
5. LWS W/FELT LAYERS
Figure 13: Architectural details of the four main green wall system types (Source: adapted from Ottele et al., 2011).
The effect is created by the different layers of a wall system effectively creating a micro-
climate between those layers. The layers help to slow the pace of temperature change thereby contributing to the regulation of the building's overall micro-climate. According to Perini, the insulation effect describes an, "extra stagnant air layer which has an insulating effect and reduces the energy demand for air-conditioning up to 40-60% in Mediterranean climate" (Perini, et al 2013a: 111).
18.104.22.168 Wind barrier effect
The wind barrier effect refers to refers to the barrier created by the green wall system which protects the building from wind chill helping to reduce the building's heat flux otherwise accruing from wind chill. The green wall system's plant and growing membrane serve as a wind barrier adds to the structural strength of the building facade overall (Perez, et al 2014: 163-164). A major factor in the effectiveness of the green wall system to protect from wind is the density of the plant foliage, which, "create[s] an almost stagnant layer of air to reduce the wind strength proportional" (Perini, et al 2013c: 2288).
The density of plant foliage is measured in terms of the leaf area index (LAI) and attenuation coefficient. The attenuation coefficient quantifies the spatial relationship of the leaf density to the wall, providing higher ratings for foliage that is parallel to the wall, thus providing the most effective wind barrier (Perez, et al 2014: 152). The challenge for effective applications of a green wall system is to ensure that the plant species will perform so as to insure the necessary LAI and attenuation coefficient to create the necessary barrier effect. In turn, this means selecting an appropriate plant species for the climate and building orientation.
In cases of successful implementation, the impacts of an effective wind barrier have been
found to lower: "annual heating costs by 8%, but increase^] annual cooling costs by 11%" (Perez, et al 2014: 159). The increase in cooling costs can be attributed to the slowing of the building envelope's heat flux, which in hotter temperatures may translate as increased use of air conditioning. The increase could easily be offset through natural ventilation, and/or more effective implementation strategies in which both the benefits of the heating and cooling costs reductions of green walls is harnessed. Specifically, application of the green wall wind reductions was found to be, "generally beneficial in cold climates, but greenery should not block solar access to south- and eastfacing surfaces" (Perez, et al 2014: 159).
22.214.171.124 Heat emissivity effect
The last direct effect of green walls is the rate of heat transfer or the high emissivity rate. The green wall's stagnant air barrier in combination with the insulation materials, or growing membrane, slows the rate of heat transfer (Ottele, et al 2011: 3420). In one study, it was found that, "Green panels not only reduced solar heat transfer to the wall, but also kept the wall warm at midnight by buffering and delaying the heat transfer through the facade wall" (Perez, et al 2014: 157). The plant material itself also emits heat faster than ordinary solid materials, such as concrete and wood. The higher emission rate helps reduce building temperatures faster. Overall, the slower heat rate helps slow both the building's warming and cooling, and the higher heat emission helps quicken cooling. Thus, the green wall can contribute to a more stable indoor temperature. The more stable indoor temperature translates into reductions in both heating and cooling needs, resulting in less energy consumption (Perez, et al 2014: 158). The impact on energy savings could be as much as a fifty percent reduction, as, "Vegetation can dramatically reduce the maximum temperatures of a building by shading walls from the sun, with daily temperature fluctuation being reduced by as
much as 50%" (Wong, et al 2010: 664). The application of the green wall must be considered in order to optimize the benefits of the rate of heat transfer and minimize any increased cooling costs. It is recommended that exposure to sunlight be maximized, to reduce the building exposure to solar radiation (Liang, et al 2014: 63) and that deciduous plant varieties be used so they do not inhibit solar access in winter when heat gain is desirable.
It is clear looking to climate regulation effects, that some green wall applications can have valuable benefits to stabilizing local microclimate temperatures, both locally (outside) a building and internally (interior) of the building. The temperature stabilization in both cases translates into reduced dependency on mechanical systems (HVAC or heating, ventilation, and air conditioning) to correct the temperature, which in turn means less energy consumed (where a majority of that energy is sourced from fossil fuels, such as coal and crude oil). Given that buildings energy consumption makes up approximately half of the urban emissions, the other half going to vehicle related emissions, substantial reductions in energy demand could have similarly substantial reductions on emission rates.
2.1.2 Micro-climate regulation effects demonstrated
This section provides two different ways of representing the hypothesized impact of vertical green walls on building microclimate. In the first approach, a computer software models the impact based on a series of known factors. The modeling is used to predict the impacts of vegetation on local temperatures by using a micro-climate modeling software called Envi-met. The model has a control and an experiment, as is seen in each of the figures below, which in contrast to one another shows that the presence of vegetation measurably and in some cases substantially reduces heat gain. The model is viewed at hourly increments, and when in viewed in sequence the progression of the day's heat and the evening's cooling off are visualized. The model itself was made using
standard components within the Envi-met software. It uses a building component at three stories,
grass components around the building, and tree components to simulate a wall. The tree components were selected for their tall narrow characteristics and were placed immediately adjacent to a building wall. The walls chosen to place the trees next to the southern and western most wall faces or the faces that would, in North America, receive prolonged direct sun exposure and therefore produce the greatest heat gain. In the second approach, a thermal camera documents the temperature difference between plant and structure, the unearthed data corroborating the empirical findings reviewed in the remainder of thus chapter.
Figures 15 and 16 are taken from a computer model depicting by hour over the course of a day, what the temperatures around two buildings are. One building has a vertical green wall simulation and the other building does not. The images illustrate the most dramatic differences between the two building's external temperatures, but the full hour-by-hour sequence is provided in Appendix I of this document. Figure 15 shows that the top building, the building with a vertical green wall simulation, has maintained a lower temperature than the building without a vertical green wall system by 10AM or after four hours of sun exposure. Figure 16 shows that the top building also has cooler temperatures later in the evening or by 8PM. The sequence shows that the top building does not gain as much heat through the day, and although it seems to lose heat at the same or similar rate to the building without a vertical green wall simulation, the lower building has to lose more heat. This means that the lower building stays warmer longer than the upper building, with the discernable difference being the presence of a vertical green wall system on the western and southern facades.
, f PA LW LW LW LW LW LW LW LW LW PA LW LW LW LW LW
LW LW LW LW LW LW LW LW LW LW LW LW LW LW LW
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
Figure 14: Envi-met model showing the location of two three story buildings. Grey boxes entitled 'PA' represent the simulated green wall system, grey boxes entitled 'LW' represent grass lawn (Source: Mehdi Heris).
GWS Simulation 10:00:0129.06.2014
x/y Sdmitt be k=4 (z=1.8000 m)
liter 25.59 C
25.59 bis 25.64 C
25.64 bis 25.68 C
25.68 bis 25.73 C
25.73 bis 25.78 C
25.78 bis 25.82 C
25.82 bis 25.87 C
25.87 bis 25.92 C
25.92 bis 25.97 C
uber 25.97 C
Mn: 25.54 C Max: 26.01 C
Figure 15: 10AM, with upper building featuring a green wall (Source: Mehdi Heris).
0.00 10.00 20.00 30.00 40.00
GWS Simulation 20:00:0129.06.2014
x/y Sdmitt be k=4 (z=1.8000 m)
inber 28.11 C
28.11 bis 2a 12 C
28.12 bis 28.13 C
28.13 bis 28.15 C
28.15 bis 2ai6C
28.16 bis 2ai7C
28.17 bis 28.18 C
28.18 bis 28.19 C
28.19 bis 28.21 C
Liber 28.21 C
Mn: 28.10 C Max: 28.22 C
Figure 16: 8PM (Source: Mehdi Heris).
The following series of photographs (figures 18-24), features optical and thermal
photographs taken of two different direct vertical green wall systems(respectively featured in Figures 17-19 and Figures 20-25) taken on the University of Colorado at Boulder campus. The thermal camera does not measure air temperature, the images only depict surface temperatures. There are two image sets; one set was taken in early June at mid-afternoon with ambient temperature at approximately 81 degrees Fahrenheit. The second set was taken in early January around 12PM when ambient air temperature was approximately 45 degrees Fahrenheit. The thermal images are juxtaposed with normal color rendered photographs so the reader can clearly identify which objects are depicted in the thermal photographs.
The summertime images clearly show vegetation temperatures between 5-15 degrees cooler than ambient air temperature, and significantly cooler still than the immediately adjacent wood, stone and fencing exposed to direct sunlight. The wood, stone and fencing reached temperatures well above 80 degrees. The temperature difference between the plants and the structure in full sun strongly suggests that the plants have a cooling effect, but in the very least that the plants do maintain a lower temperature themselves despite the direct sunlight and air temperature.
The wintertime images clearly show little to no temperature difference between the dormant plants and the structure they are vining on. That there is a significant difference between summertime temperatures and little difference in wintertime temperatures suggests that potential solar gain in winter would not be lost with the presence of a deciduous plant. And also that there is little to no thermal gain from the plants' presence.
Figure 17: Photograph of indirect green wall system or vining plants on metal trellis system in summer (Source: Author).
Figure 18: Thermal photograph of indirect green wall system on metal trellis in summer (Source: Author).
Figure 19: Photograph of indirect green wall system or vining plants on metal trellis in winter (Source: Author).
Figure 20: Thermal photograph of indirect green wall system on metal trellis in winter (Source: Author).
Figure 21: Photograph of direct green wall system or trellis with vining plants in summer (Source: Author).
Figure 22: Photograph of direct green wall system or trellis with vining plants in summer (Source: Author).
Figure 23: Thermal photograph of direct green wall system or vining plant and trellis in summer (Source: Author).
Figure 24: Thermal photograph of direct green wall system or vining plant on trellis in summer (Source: Author).
Figure 25: Thermal photograph of trellis in foreground and direct green wall system or vining plants beyond in summer (Source: Author).
Figure 26: Photograph of direct green wall system or trellis with vining plants in winter (Source: Author).
Figure 27: Thermal photograph of trellis and direct green wall with vining plants in winter (Source: Author).
2.1.3 Relationship to urban heat island effect
The main implication of the evidence reviewed and shown thus far is that the green wall system likely has a broader impact than just impacting a single building's topo-climate or interior microclimate. It has been found that areas in, "close proximity to the green wall can also experience a reduction in surface temperature" (Liang, et al 2014: 63). Thus, there is a wider role that green wall systems may play in the stabilization of temperatures within a smaller geographic area (Wong, et al 2010: 664). The indirect effect on the area in combination with the direct heating and cooling effects on individual buildings play a role in the stabilization of the urban heat island effect. "[T]he mitigation of the urban heat island effect with trees, green roofs and green facades can reduce the U.S. national energy consumption for air conditioning up to 20%, saving of more than $10B in energy use." (Perini, et al 2013a: 112). It must be noted, however, that whatever impact green walls systems may have on a microclimate, to have a meaningful impact on citywide temperatures,
widespread application is required (Perini, et al 2013a: 118). However, the question of scale and the
current lack of more thorough and systematic scientific research establishing metrics for the impact of green infrastructure on urban climate make these claims difficult to use as arguments for the justification of green wall applications. Even so, the relationship of these individual reductions to the broader urban realm is sufficiently well established to suggest that the implementation of green wall systems should not purely remain the prerogative of individual building owners but has potentially larger implications across urban energy efficiency. There are also strong links to the urban heat island effect, which unambiguously has wide reaching impacts, certainly at the city level but potential at the regional scale also.
2.2 Flood and other natural hazard mitigation services
Green walls contribute to the natural hazard mitigation ecosystem service category by increasing the total amount of vegetation in a given area, and as result increases the scale and scope of the ecosystem service. More specifically, the presence of vegetation can contribute to the stabilization of many environmental factors, making them less susceptible to extreme events such as flood, fire, drought, crop failure and extreme heat. In the case of flooding, vegetated land cover intercepts and absorbs water thereby slowing the rate of water flow, which can have meaningful impacts on the rate of water flow and its potential effects. Plants also stabilize the soil making mudslides and erosion less likely to occur or less severe. When vegetation is moist, it can reduce the spread of fire as the plants will not provide combustible material. In arid or desert climates, this is less likely to be a factor as dry grasses and other vegetation provides a plentiful fuel source for fire a large portion of the year. Plants can stave off drought for the same reasons they stave off flood, they slow the rate of water flow. In the case of a drought, the plants slow the rate at which water evaporates or runs off, thereby staving off the onset of a drought. Additionally, plants can provide
additional biodiversity to help maintain crops. They can also provide alternative food sources in the
event of a food shortage. Vegetated cover, as was discussed in the climate regulation section, can help reduce micro-climate temperatures thereby mitigating the potential public health effects of extreme heat. While the role of green walls in the wider gamut of hazard mitigation services is minimal, their mitigating runoff could be substantial and therefore the wall could serve as a meaningful tool in a flood and storm water mitigation strategy.
2.2.1 Air purification
Plants are well known for their positive contribution to air quality by sequestering chemical and particulate pollution, as well as ozone and carbon dioxide. The process of photosynthesis itself takes in carbon dioxide and releases oxygen. Plants are also well known for their ability to remove other air borne pollutants, such as formaldehyde. Plants absorb, "fine dust, particles and [...] gaseous pollutants such as C02, N02 and S02" (Perini, et al 2013b: 269). While it is difficult to quantify exactly what impact plants have on local air pollution, it has been found that in a neighborhood with, "10,000-20,000 dirty particles per litre," a street with greening had, "in the same neighbourhood [had] an air pollution of 3000 dirty particles" (Perini, et al 2013a: 112), which is 15-30% of the air pollution measured in the neighborhood without street greening. Similarly, it was found that, acid gaseous pollutants, ambient green roof surface temperature and carbon mass levels dropped significantly after implementation of green roofs in Chicago (Perini, et al 2013a: 112). In the same study, it was found that there as a 52% reduction of 03, 27% of N02, 14% of PMi0 and 7% of S02 (Perini, et al 2013a: 112). The implementation of green wall systems would increase the amount of plants in built areas, thus increasing the positive impact on air quality. In the case of carbon dioxide sequestration as opposed to air pollution, which is to say particle and chemical pollutants, the scale at which greening would need to take place remains large and uncertain, so it seems
unlikely that even the most aggressive and ambitious urban greening initiative could generate the
amount of carbon sequestration required to combat the effects of climate change. Broader scale collaborative efforts across municipality, state, and federal governments would be required to reach significant sequestration rates.
2.2.2 Water purification
Plants and their soil naturally absorb pathogens, nitrogen, metals, phosphorous, and can be engineered and designed into our built environments so that they can filter water before it reaches the local water supply. Given that preventing water from pollution is easier than cleaning it once polluted, the potential of circulating water through plant-based filtration systems seems practical and highly beneficial. One such way of filtering water, particularly acid rain, is to increase the amount of vegetation in a given area. Utilizing green walls is a means of increasing the vegetative cover, particularly in areas that are already developed meaning spatial reconfiguration is unlikely to occur or in densely built areas where space is a sought after commodity.
2.2.3 Pollination & Pest control
Increasing plant presence and diversity insures both that pollinators have healthy environments to live in and that one insect population does not dominate potentially destroying vegetation. This strategy is called integrated pest management. In the case of bees, the strategy is useful because it eliminates the need for harmful pesticides and herbicides harmful to bees. In the case of managing pests, mixing plants of different varieties can attract birds and insects that eat or compete with the undesirables. Green walls can be used to increase plant presence, particularly in suburban and urban areas.
Habitat services refers to creating habitats that support biodiversity and genetic diversity. The green wall very simply creates more green space in developed areas, where there is little green space in current development. The increased presence of vegetation through implementation of vertical green wall systems in urban areas provides space for wild life within the city. It has been found in areas of previous application, that the presence of vegetation provided a food and breeding source for microorganisms along with insects (Perini, et al 2013a: 112). The presence of insects provides food for birds and the green walls themselves provide nesting areas (Perini, et al 2013:
112). Prior to the installation of the green walls, there was little ecological activity. Given these finds, the green wall systems hold the possibility of creating wild life habits within urban areas, and to generally recapture areas for wild life as urban green field sites are developed. An increase of vegetation in the standard development pattern in any amount would help to increase wildlife habitat, thereby supporting biodiversity and efforts to reforest, but the green wall is a particularly good tool for greening more space without significantly impacting current development patterns. It also lends itself to easy and cost effective retrofitting, as is discussed in more detail in section 4.2.2.
2.4 Provisioning services
The next ecosystem service is known as a provisioning service. This service refers to resources provided, such as food, water, wood, fiber, medicine, and genetic resource. The use of vegetation for food, fiber medicine and genetic resource is typically done through agricultural processes. While agricultural land is usually less preferable to the natural landscape it was, such as forest, wetland, or grasslands, it is preferred to unproductive land such as desert or a concrete and asphalt cityscape. In the case of land developed for human settlement, there is opportunity to use green walls to incorporate vegetation for provisioning purposes. The green walls could become a
part of the urban agriculture movement by planting fruits and vegetables in the walls instead of
ornamental plants. Plants, such as hemp, can provide fibers for rope and other products, and could easily be grown in direct green wall systems properly designed and engineered. The increase in vegetation also provides a local source of genetic material, as the local seed stock will increase and be more readily available to people as they can get seeds directly from the plant as opposed to buying them at stores or from other providers. The greening also helps provide clean air and water through the purification processes discussed above.
2.5 Cultural services
The final ecosystem service is the cultural service. The cultural benefits of greening refers to the non-material benefits to humans. Such benefits may include spiritual enrichment, intellectual development, recreation and aesthetics. Many people refer to forests and their experiences in them as explicitly or implicitly spiritual experiences. In an attempt to better understand what the cultural service is, there are many studies conducted to demonstrate various links between different quality of life aspects and vegetation. Recently, the link physical and mental health benefits and higher quantities of vegetation have been established (de Vries, et al 2013; Joye, et al 2010; Ruijsbroek, et al 2017; Thompson 2011; van Dillen, et al 2011; Van Herzele, et al 2012). Some researchers have found that higher rates of vegetation is correlated with happiness in their neighborhoods (de Vries, et al 2013), that green space plays a positive role in mediating stress and social cohesion (de Vreis, et al 2013), they people have better psychological and cognitive functioning (Joye, et al 2010), and that quality of green space is positively correlated with health (van Dillen, et al 2011). Such positive cultural effects could be strategically designed into an existing urban development with the intention of alleviates social ills such as alienation, aggression, dissatisfaction, and so on. The green wall specifically could alleviate the visual impact of buildings, particularly in urban canyons. In turn
an urban core greening initiative could have substantially positive impacts on area crime rates, community development, unemployment rates, and other factors typically associated with social disintegration.
Another cultural benefit of having green walls, particularly those growing fruits and vegetables as mentioned above, is the potential for educational development. As the walls cans be used to teach kids where their food comes from and how to grow vegetables themselves. The green walls can enhance the recreation experience in developed areas as cycling and walking outdoors would become a more pleasant, cleaner, and dynamic experience. Finally, the aesthetics of greening is a service, one that many have already fully embraced in the field of architectural design. The literal greening of buildings is a design movement that suggests sustainability but whose motivation factor is more strongly linked to beautifying buildings. A similar use of green walls is found in Singapore where contractors are required to use green walls to hide construction sites at the street level in order to maintain a pleasant street experience.
While the economic benefits are not formally defined as an ecosystem service, there are frequently monetary benefits incurred through using ecosystem services that play a major role in creating support for their protection and implementation. Green wall systems are found to in increase property value. For instance, "a green wall [yields] the same property increase as a 'good tree cover,'" (Perini, et al 2013a: 111) where an, "increase from a minimum of 8% [...] to a maximum of 20%" (Perini, et al 2013a: 115) can be expected. The potential economic benefits of the green wall system in terms of increased property value have not been factored into the cost benefit analysis determining whether and which green wall system makes sounds fiscal sense.
2.5.1 Cost effectiveness
The green wall system saves money and resources otherwise allocated to building maintenance by reducing the frequency in which re-painting or recladding must occur. The presence of a green wall system improves building envelope by providing cladding longevity (Perini, et al 2013a: 113). The building material that would otherwise be left fully exposed to rain, snow, and wind now has a vegetative barrier protecting the building material from full exposure. In other words, "Vertical greening systems protection delays the decay of the underlying wall caused by UV rays, temperature changes, acid rain, ice, and air pollution reducing the deterioration" (Perini, et al 2013a: 115).
The use of a green wall system may appear to make the building more vulnerable to maintenance issues, particularly given the presence of moisture so close to the wall system. However, "With respect to the building facade maintenance, the adoption of vertical greening systems does not increase it; it reduces the frequency of intervention thanks to a protective action given by the leaves' (and to the other layers involved) shading effect" (Perini, et al 2013a: 115).
The cost benefit analysis looks at whether the initial costs and maintenance costs of implementing and running a green wall system outweighs the potential monies saved through increased energy efficiency. The analysis must show that the cost of implementing the optimum green wall system has the potential for high economic savings, beyond ecological and environmental concerns. The major component of the test of economic efficiency is whether or not the potential energy savings outweigh the costs of implementation and maintenance (Perini, et al 2013a: 115). Specifically, "the two most relevant items are the energy savings for summer air conditioning (because of the location of the building: Genoa, Italy) and the increase in rental income, both around 1000/year" (Perini, et al 2013a: 120). From the abstract: "Net Present Value (NPV), the
Internal Rate of Return (IRR) and the Pay Back Period (PBP). The CBA demonstrated that some of the
vertical greening systems analyzed are economically sustainable. Economic incentives and subsidies could be also to internalize or reflect the societal benefits described above which in turn benefits the city as a whole. Such incentives and subsidies could reduce personal initial cost allowing a wider diffusion of greening systems into urban cores that would reduce environmental issues common in densely built out areas, such as urban heat island phenomenon and air pollution" (Perini, et al 2013a: 110).
Comparing the cost of a street tree to a green wall is a large economic disparity. A street tree, which typically is about 15 to 20 feet in height, costs approximately $500 dollars to buy and install. Most urban street trees also have a grate over the root ball which costs another $1000. Street tree maintenance costs are added on top of this and are usually absorbed by the local municipality. When comparing a single street tree at $1500 plus maintenance costs to a green wall at $100 per square foot, it looks like green walls are more expensive. A 20 foot tall tree provides approximately a 20 foot by 20 foot area of coverage which would cost approximately $8,800-60,000 to replicate in green wall.
2.5.2 Urban regional economy
Is greening a means of increasing an area's resource, and therefore a means of becoming resource rich? Looking to the ecosystem services framework, a city could reduce its costs and increase its supply of each of the ecosystem services harnessed. The value proposition for green walls is based on the fact that green walls allow us to deploy green infrastructure, and all the services provided by it, in highly space-constrained urban areas where there is not enough room to place the vegetation horizontally. The greening could increase a city's self-sufficiency in terms of providing a reliable food source, a means of recycling products (compost and water), and for
filtering pollutants from water and air, thereby making its economy less dependent on trade for
resources. The increased greening resource could also increase the resiliency in terms of making it more flood resistant, stabilize the local climate, reduce dependency on centralized systems for food, heating, and cooling, etc. The resources may not have to be viewed in terms of physical holdings. In the case of the climate regulatory function of the green wall, a city could utilize an extensive green wall network to carbon finance; it could use the green wall to produce urban crops such as hemp; it could become an attractive place to live because of the clean air and water; and so on.
In light of the above, how do we act on this knowledge of green walls ecosystem services? Basic design strategies for how to best harness the ecosystem benefits of a green wall are discussed in this chapter. The aim is to optimize the green wall application by selecting most appropriate wall type and planting scheme so as to insure a successful and productive wall, insuring feasibility or the practical ability to install and maintain the wall, affordability, and net environmental gain. Additional factors impacting the performance of a green wall system such as wall orientation are discussed as well.
3.1 Wall selection
In terms of wall selection, there are a number of different considerations that must weigh in. The research here relies on an environmental lifecycle analysis and a cost benefit analysis to isolate the direct wall system as the preferred wall type.
3.1.1 Environmental life cycle analysis
Although many green wall systems shown in design magazines promoted on its perceived environmental benefits, few wall systems are able to achieve a net positive environmental benefit when a comprehensive life cycle analysis is conducted. A key point is that the passive energy savings of the green wall system alone does not account for the environmental or economic benefits of the system in sum.
Each wall type has a different set of implications in terms of sustainability, cost, passive effectiveness, etc. Each system uses different resources, such as metals, felts, plastics, etc. that
affect both the environmental and economic impact of a green wall system application. In the case
of felt, both the cost of manufacturing and installing the wall and the negative environmental impact due to the manufacturing, sourcing of materials, installation, and degradation of the wall system increases. The main factor in the life-cycle analysis separating each wall system type was not the strength of its passive energy savings, but rather the materials used to build the system in the first instance (Ottele, et al 2011: 3420). The more intensive the manufacturing process is to produce the materials for the green wall, the higher the environmental burden given the associated emissions, and the more difficult it is for any potential benefit to outweigh the costs. There are risks associated with the potential gains as well, as the wall may not successfully harness the benefits intended, it may not perform as well as necessary, or it may simply fail to perform at all.
According to Ottele, the optimal green wall system currently available is the "direct green wall" as depicted in Table 1 below. The table illustrates the environmental cost, juxtaposed with the environmental benefits in terms of heating and cooling (see section 2.1.1 for a more detailed analysis of the heating and cooling benefits of the green wall). What is clearly demonstrated here is that not all green walls have a net positive environmental benefit, because the environmental costs of manufacturing materials far outweighs any potential gain.
On the other hand, the environmental gain of each wall system is comparable, with all producing relatively similar results in terms of net benefit, except for the LWS with planter boxes because it provides a heating benefit that the other walls do not.
Some exploration into materials could be useful to reduce the environmental burden of the other wall types. If, for example, the indirect wall system were built with a natural and locally sourced material such as thatch instead of metal, the environmental burden would be reduced so as to be comparable with the direct wall system. Similarly, if the LWS with felt layers were redesigned
to use a natural and locally sourced material such as burlap instead of felt, its environmental burden may be reduced to a level comparable to that of the direct green wall.
3 Jdrw syst. n I njrKt tyst. (3 4-IW5 bcnos a 5.LWS fe*t layers
Table 1: The environmental burdens and benefits of the standard wall systems at different lifecycle stages (Source: Ottele, et al 2011).
3.1.2 Cost benefit analysis of design factors
A major consideration when selecting a green wall system is the up from cost of the wall and installation in comparison to the long term maintenance costs and any returns, where returns would be secured in the form of lower energy bills and increased property value. Walls are reported as costing between 22-83 Euros per square meter in Europe, and between 80-150 dollars per square foot in the United States (Perini and Rosasco 2013). Rates will vary widely depending on the wall type (any wall with felt or steel will be more expensive), complexity (an irrigation system is a costly add on for instance), and location (due to the local costs and availability of materials and labor). Perini and Rosasco have analyzed many of the variables associated with arriving at a cost for a green wall. They examine the direct, indirect, indirect with planter boxes, and the living wall system in
terms of the Net Present Value, Internal Rate of Return, and Pay Back Period and identifies the
indirect green wall as the best choice to insure eventually profitability of the wall (Perini and Rosasco 2013).
When considering the standard wall systems, researchers isolated the direct wall system as the most economically efficient system due to the lower cost of materials for manufacturing and installation (Perini, et al 2013a: 119). Effectively, the higher economic cost is directly proportional to a higher environmental burden, as is demonstrated in Table 1. However, a potential limitation of the direct green wall system is that, "climbing plants can only grow to a maximum of 25 m height and it can take several years" (Ottele, et al 2011: 3419), meaning it may take several years until the passive solar energy savings needed to justify implementation are achievable thereby making the wall a more risky investment. Additional research is needed in order to identify where the threshold between cost and wall performance lies in order to decipher whether there are applications in which a higher cost wall pans out in terms of economic benefits provided due to higher wall performance.
The energy savings a green wall system can provide is calculated not just on the passive savings associated with heating and cooling, but on a full life cycle analysis and cost benefit analysis of environmental and economic impacts pre and post installation. If the intent of utilizing a green wall system is indeed to mitigate the impacts of climate change by reducing our energy dependency, we must also take a look at whether implementing a green wall system is not simply displacing energy usage to another stage in the life of the green wall system. If the wall does not pass the lifecycle analysis, it undermines one of the most significant motivating factors in seeking to implement the system in the first place. Additionally, if the wall system creates too strong an economic barrier to implementation, the wall system is a not a feasible or viable means of seeking
climate change mitigation. Therefore, the preferred wall system should both reach our
environmental and economic goals, with the view of creating strong economic incentive to implement the system instead of relying solely on moral appeals to environmental concerns as has proven widely ineffective. The direct wall system is the preferred system in terms of both environmental and economic benefits.
3.2 Wall placement
A major factor in a green wall's success and productivity, as well as its feasibility in terms of practical installation and maintenance, relates to the wall's physical location on a building. The main factors to consider are the wall's orientation to the sun and the wall area ratio. But before delving into the specifics of the green wall placement, a broader argument for the spatial advantage of the green over other forms of green infrastructure, namely the tree and green roof, which requires that a large amount of horizontal space be provided.
3.2.1 Site suitability for green walls
The green wall has a twofold advantage over other types of green infrastructure when considering the context of implementation. The green wall primarily utilizes a building's facade which is usually only otherwise used for fenestration. Other forms of green infrastructure are not applicable on vertical surfaces. A tree planted close to a building wall can have comparable effects, but only for buildings less than three stories. Buildings taller than three stories in height would not be feasible, which is to say the majority of urban cores or of Manhattan Island. As such, these building surfaces are typically left un-used or as exposed concrete and stone. The other aspect of the green wall's spatial advantage is its applicability in small or awkward spaces. Space for trees is not always available, particularly in densely built areas but also in existing developments where reconfiguration is not a practical solution. The green wall's applicability in under utilized and
awkward spaces and its ease of use in retrofitting projects makes it not just a practical greening
solution, but can be the preferred solution in some contexts.
According to Simpson and McPherson, the physical placement of a tree significantly impacts the effectiveness of the tree to provide its climate regulation ecosystem services. They recommend that trees:
be sited no farther than 35ft from the house; trees cannot be sited where they interfere with power lines; trees of smaller mature size must be sited a minimum of 8ft away from the house, while those with larger mature size must be 15ft away from the house; trees must be sited at least 6ft from sidewalks, patios, driveways, or any other concrete surface; and smaller trees must be spaced a minimum of 8ft apart, while larger trees must be 15ft apart (Simpson and McPherson, 1997: 70).
Given these parameters, it remains that, "the magnitude of the effect depended on the tree's location" (Donovan and Butry, 2009: 663). For the effect of providing needed shade in the summer but not blocking sun in the winter, shading on the west of a building is recommended (Donovan and Butry, 2009: 662).
A significant advantage of green walls over other forms of green infrastructure is the ease of implementation on existing buildings, because implementation does not require retrofitting or significant modification of the building or its systems. The system has a greater application range than trees or other planting schemes because it can be applied in cases where there is limited space, such as dense urban environments, in retrofitted buildings in which pre-planned space for the green wall system would be difficult to source, or new construction as the system does not significantly impact programmatic requirements, is easily integrated into standard wall systems, and depending on system type is a low cost add-on for long term energy savings (see section 4.1.2 for an
explanation of the a green wall system's cost benefit analysis). However, there is some skepticism as
to how simple it would be to implement a green wall system, given that, "from the architectural point of view, it is probably easier to use a flat space. [...] But on the other hand, greening the walls of a building has potentially more effect on the building environment than greening roofs, as the surface area of the walls of buildings is always greater than the area of the roof" (Perez et al., 2014: 140). One of the system's great advantages over a standard tree in the front yard or a new green roof is the area a green wall is capable of capturing. The, "high wall to roof ratio can offer larger areas for planting" (Wong and Baldwin 2016: 35), which allows for nearly the full extent of a building facade for greening, depending on the wall system. The large area a building facade creates makes up a large portion of built environment and this space remains virtually uncontested. If the vertical space were to be harnessed across a city, significant reductions in the energy required to regulate buildings' internal microclimates could be achieved.
Another advantage of the green wall over other forms of green infrastructure is lifespan of the plants. A tree can take decades to reach full maturity, which leaves many years where the full benefits of the trees cannot be harnessed. In contrast, green walls typically take a few years to reach full maturity. The short growing time allows property owners to harness the benefits of the green walls much faster than other alternatives.
In cases where the retrofitting concerns above are not at issue, the full gambit of green infrastructure elements are at our disposal. In these cases, adding trees to the periphery of a building are unambiguous and direct, and will often be a suitable and cost effective option for harnessing the energy related benefits of green infrastructure. Yet, in application the use of trees is severely restricted by spatial limitations, particularly urban environments that are densely built or where land is an inaccessible commodity. In these instances, vertical green walls are typically a good
option as the building has little to no horizontal or ground space for other greening options. The
walls are also primarily utilizing the building's facade, which is more often than not punctuated only by fenestration, leaving the vast majority of the building face un- or under utilized. Additionally, space for trees is not always available even in suburban or rural locations given other site planning requirements such as parking and storm water, irregularities in parceling, and local zoning and building codes requiring priority use of the land for other purposes such as setbacks, traffic visibility, signage, etc. Given these additional development burdens, the goal of increasing tree coverage may not be a practical reality, particularly on sites with existing development. Vertical green walls (and green roofs) on the other hand, do not require a large amount of volumetric space fitting well in planar space and are largely applicable along surfaces that are not otherwise used. In a sense, the vertical green walls harness another dimension of green space, particularly given the fact that potential green space is typically planned and reviewed using site plans, not elevations, sections or 3D drawings. Thus, the green wall allows for capturing space un-utilized by standard approaches to green infrastructure, or a "doubling-up" of space utilization, and another means of capitalizing on the ecosystem services of green infrastructure.
A lingering question for those skeptical of the green wall as a tree substitute, is whether green walls are sufficiently comparable to trees? Solar gain is calculated based on the, "percentage of each wall and roof surface shaded for each hour based on building and tree dimensions, orientation and distance of trees and adjacent structures from buildings, local time zone, latitude and longitude, and time of year" (Simpson and McPherson, 1997: 70). Most studies substantiating the potential benefits of green walls and roofs actually focus on trees. It is assumed that green walls and roofs have comparable benefits, because the primary benefit of trees is their ability to shade and evapotranspire, which green walls would do arguably as effectively as trees. If this point is granted, the ease of installation of green walls and roofs is far superior to trees in urban settings
where space is restricted and development patterns established. The practical benefits of green
walls and roofs in this regard may be strong enough to argue for a focus on their application over more general tree planting programs in urban settings and in cases of retrofitting existing development.
3.3 Wall orientation
The orientation of the wall system toward the sun is a strong factor in the performance of a green wall system. It is known that, "the influence of a green layer on the wall surface is more pronounced for east or west oriented surfaces" (Jaafa, et al 2013: 560; Perez, et al 2014: 153), which is to say that the greatest energy savings are achieved with walls on the east and west facing building facades (see figure 28). There is some variation on this seeming consensus, as it was found that the, "largest reductions in external surface temperature took place in South-West and East facade orientations" (Perez, et al 2014: 164). In some cases, the southwest and east facing facades may not be the best choice. A building may have obstructed solar access that limits growing areas, or shifts other factors such as plant selection (see figure 29 for a passive solar design strategy utilizing an indirect green wall and which diagrams the relationship between solar access and the green wall). Similarly, geographic location effects the preferred orientation, where regions near artic latitude would benefit from primarily south facing orientation, and locations in the Southern Hemisphere would require northwest and east facing orientations. Given this information, and assuming the wall is implemented in the Northern Hemisphere, any guidelines or regulations should require that green wall systems be implemented on the westernmost and easternmost facing facades in order to insure that the wall is able to provide the passive energy savings desired (see Figure 28 for an illustration of building orientation in relation to green wall system placement).
Scheme of the building used for the analysis (no green situation).
Green south facade of the building used for the analysis.
Figure 28: Volumetric illustration of building orientation in relation to a green wall system (Source: Perini and Rosasco, 2013).
Figure 29: Demonstration of a direct green wall's location (left) relative to a broader passive solar design strategy (Source: www.architectenforum.com Accessed January 2, 2017).
An added complication to building orientation itself is the street grid orientation. Ideally, the street grid would be oriented so as to allow for maximized solar access, and individual building lots configured so as to allow not only for appropriate access but also for the building to be oriented so
as to optimize solar gain. Given that street grids are often not laid out with these concerns in mind,
the green wall becomes particularly advantageous for its flexible application allowing for easy correction despite poor street grid and property layout.
3.3.1 Wall area ratio (size)
Depending on the type of ecosystem service sought after, the area of cover to reap the benefit will change. Generally though, the greatest wall coverage possible will provide the largest benefit, except in the case of the passive solar energy gains where multiple factors are at play. It was stated in the wall orientation section (3.3) that the west and south facing walls are the most advantageous for green walls. By extension it is not advantageous to cover north and east facing walls as the added cooling benefit not gained on these facades, however, if other ecosystem services are sought these facades may still be beneficial.
3.4 Plant selection
A determining factor in the size of a wall is the plant type selected, as the plant's growing height and density is a major factor in the design of a wall, it also a critical aspect of the success of the wall. The placement, type and age of a plant significantly impacts the plant's potential ability to provide the fullest range of ecosystem services. The plant's performance will determine the degree to which the climate regulation functions shade, cooling, insulation, wind barrier, and heat transfer are effective, whether habitat is provided for local wildlife, whether they can be used as a reliable source of food and fiber, or even at what rate they are absorbing pollutants from the air.
The plant palette selected presents a number of constraints and opportunities related to the local climate and seasonality. While not all associated variables can be accounted for, a major factor in plant selection is climate. A tropical or sub tropical climate will have a year round growing season
and a large and diverse plant palette to select form, making plant selection most green wall
applications straight forward. In other climates, the climate and growing season will make the design of success green walls more challenging, say in continental United States, and in others such as the Artie, wholly impractical. In areas where there are four seasons and a limited plant palette more consideration will have to be given (see the Colorado-ization section below for more analysis on plant selection for arid climates). The seasonality constraint in the continental United States climates is basically the winter season when plants are dormant. The plants cannot provide the passive cooling benefits when their leaves have fallen. Yet, during the winter solar heat gain is beneficial, not passing cooling. The solar heat gain can help reduce heating costs through the winter and interference with the heat gain would not be beneficial. The challenge then is to plant a green wall for full vegetation at the times of year when passing cooling is beneficial, or generally speaking, during the late spring, summer, and early fall.
In order to insure the most productive plant for the longest period of time, the plant should be selected based on climate, wall orientation, level of maintenance and water, and planting medium. The climate will impact whether the plant will survive and perform at the necessary levels. The wall orientation can negatively impact the survival and performance if the plant is located in an area with too much or too little light, in a wind tunnel, or in a hot or cold pocket. The level of maintenance and water required to insure the plant survives and performs must be thought through, as a plant without the appropriate maintenance and water will not perform and may not survive. Similarly, the planting medium determines the area in which the plant can grow. Plants with large root balls or those that need deep soil to perform well are likely not good candidates for the wall system, unless the plant is planted directly into the ground or a substantially deep planter.
The plant type is also important. Deciduous trees are preferred because they lose their
leaves, when increased solar gain is desirable. Despite this, deciduous trees still cause some solar energy loss by intercepting solar energy that would otherwise heat the shaded structure in winter although the net benefit of remains positive (Simpson and McPherson 1997: 69). The plant type used in single green wall systems may vary, such flexibility in type allowing for the selection of the most appropriate plant variety for shade function, climate, and available space.
Secondly, green wall systems are easily designed into a new building and about as easily retrofitted onto an existing building. Green roofs are not, as they require sufficient load bearing capacity, water drainage, and roof replacement in order to properly install. And finally, although there is energy efficiency to be gained in the urban, suburban and peri-urban settings, the suburban setting appears best poised for green wall implementation because the energy inefficiency of buildings is at its highest in this building type. However, the green wall lends itself to the urban setting, where other green infrastructure options are impractical.
3.4.1 Agricultural plants
There is an opportunity to use the new green space for agricultural purposes, planting tall growing or vining fruit and vegetable plants. Looking to the One Central Park building (Figure 30), the same green wall design strategy using an indirect green wall with planter boxes at the outer edge of the unit's balcony and some cabling to provide plant supports, tomatoes, green beans, peppers, eggplant, and other foods could be planted with fruits and vegetables instead of the ornamental vines shown in the photograph. The easy access food source provides food security and reduces travel distances for food production and delivery (cf. Figures 31-32). The food security aspect can help those unable to afford fresh produce on a regular basis, for those that live in so-
called (urban) food deserts, and provide a food source during emergencies such as a natural
Figure 30: One Central Park could have been planted with vegetable and fruit plants instead of ornamental vines (Source: inhabitat.com Accessed January 2, 2017).
Figure 31: Urban farming (Source: gardenwithpassion.com Accessed January 2, 2017).
Figure 32: Urban farming (Source: growveg.com.au Accessed January 2, 2017).
Figure 33: Urban farming (Source: oca-testbed.blogspot.com Accessed January 2, 2017).
Some varieties of pears, for instance, perform well when heavily pruned into a flattened version of themselves resembling vining plants. Figure 34 shows a pruned pear tree covering the wall of an old church. The tree's roots are planted near the building's foundation, and the branches follow the wall, creating a similar effect to a vining ivy except the plant also provides food.
Figure 34: Espalier tree on the Anglesey Abbey in Lode, UK (Source: www.flickr.com Accessed January 2, 2017).
Given that this research has been conducted in Colorado, an arid climate, there is interest in how a green wall system can be implemented in harsher climates, particularly those where fresh water is a scarce commodity. In Colorado, planting can begin as early as late February to early March, and by April planting is in full swing. Frosts can occur late in April, and heavy wet snow is typical through April and into May. Some awareness of late frost and snow is necessary, but can be worked around by keeping vulnerable annual plants covered. Only perennials that tolerate frost can survive the winter. Otherwise the growing season continues into mid October when the first frosts and snows typically occur. In mid-summer temperatures can reach over 100 degrees Fahrenheit for
days at a time, and with little to no moisture. Typically the heat is punctuated by a temperature
drop and some moisture, making Colorado distinct from desert climates. A way of maximizing the growing season of approximately 6-9 months is to use perennial plants. Additionally, the times at which plants are not performing their ecosystem services coincides with the time of year where heat gain is beneficial, or winter.
In terms of planting a green wall for the Colorado climate it is highly recommended that native plants be used. While there have been successful green roof installations in Colorado using succulents and other draught resistant, sun loving plants, it is likely that natives will ultimately perform better. Natives will perform well with little water, high and low temperatures, and the wide temperature swings jumping from 70 to 30 and vice versa that periodically occur in Colorado. A concern is simply that the green wall system be designed so as to prevent freezing temperatures. Preventing freezing temperatures could entail insulated planters. Walls on building facades may not reach freezing temperatures as the building itself will not freeze. While a great deal more research and prototyping would need to be done to test the green wall system and native plant palettes that work best, generally speaking, the current difficulty with looking to use a native plant palette is that true natives are not widely cultivated and sold by nurseries, and thus are not as readily available for mainstream commercial activity. More development in this sub-industry is needed before native plants become a readily available option.
Colorado State University (CSU) has an extensive archive of local plants species, however neither the fact of the archive's existence nor information on how to germinate and care for the species in the archive are commonly known, nor is information regarding either of these readily available at present. Hence, a significant amount of communication initiatives such as the CSU Extension Program which teaches a Natives Masters course on Colorado and regional native plants -
directed at the general public needs to take place in order for the state to tap into this resource. We
further require initiatives to test local plants in a variety of different applications, in terms of wall system type, orientation, watering and climate, etc., to determine the preferred species, mixture, and planting density for native plants when used in urban farming and green wall systems.
Figure 35: Ornamental Xeriscaped garden located in Boulder, CO with natural landscape beyond (Source: http://www.viriditas5.com/ Accessed January 2, 2017).
Alternatively, all of the plants shown in the xeriscaped garden plan (Figure 36) could be used in a green wall system application that does not require a vining plant. Such applications include direct and indirect systems, as shown in the stacking planters incorporated into the building facade in the building by Vo Trong Nghia (Figure 3.).
No significant amount of knowledge is published at this point that provides standards of best practice, or that addresses utilizing green strategies in climates where greening generally is an obstacle, such as the arid climate of Colorado. In order to probe the boundaries of knowledge and further the practical solutions additional modeling is required. In the meantime, a series of prototypes to begin to work out the problem more concretely, to develop a model that can be used to set standards for cities across the country and potentially beyond, and point to areas of future improvement, is badly needed.
Additional design considerations
There are many additional design considerations, certainly more (in sum) than can be accounted for within the confines of the present thesis. Each well-executed green wall must be designed with a great amount of attention to all of the factors mentioned so far, in addition to water, maintenance, retrofitting and space requirements, and many other building and context specific concerns.
Each green wall must consider its source of water and plan accordingly. In many cases an irrigation system is designed in, but there are numerous problems associated with this approach. The irrigation system typically translates as higher up front costs, higher environmental burden due to product manufacturing and shipping, higher water usage and thereby higher environmental burden in areas where water is scarce, and higher risk to the building for water damage, as well as higher risk of green wall failure as irrigation systems have a particularly high failure rate (see figure 36).
Alternative approaches exist to all these problems. One method is to rely on natural weather patterns for water, thereby eliminating the costs and risks associated with irrigation systems. To insure successful plant growth, the wall's plants must do well in the local climate and the planter system engineered so as to provide the necessary soil and moisture conditions preferred. In cases where rainwater collection is permitted, a planter system can be designed to catch and move water around the green wall system to insure appropriate moisture levels but by relying on natural means to do so. The exact amount of water a green wall requires depends on the climate, plant palette, depth of planting membrane, and how quickly the water drains out of the wall.
In Colorado, for instance, harvesting rainwater is restricted. Individuals are permitted two
55 gallon barrels to collect rainwater, and site development proposals must demonstrate that water is not retained for more than 72 hours. Within the 72 hour period water must be returned to the general water supply. This normally occurs as water drains from a site into a street gutter and storm drain. Sometimes sites are designed with onsite retention. The retention ponds must drain into area storm water infrastructure within the 72 hour period. Colorado's water restrictions can make successful green wall installations more challenging when trying to avoid elaborate irrigation systems for economic or environmental reasons. However, it seems that some of the difficulty can be overcome through the wall's design and the plant palette used.
Figure 36: Photograph of a failed LWS with planter boxes in England (Source: www.ugl.sg Accessed January 2, 2017).
The green wall must be maintained just like any other vegetation, such as the tree or lawn. Trimming will need to be done, fertilizing to some degree, pollinating, aeration and other forms of root maintenance. Some pest control may be required as well, but if planting schemes are planned intelligently, pests can be designed out of the scheme. In some case, watering may need to be done if the irrigation system fails or if natural weather patterns have not provided enough moisture to sustain the plant life in the wall. Plant maintenance would also increase dramatically with agricultural food. In the very least plants will need to be harvested in the fall and cut back for the winter. Additionally, the level of maintenance a wall requires will have varying cost implications. A wall that requires frequent waterings, seasonal replantings, weeding, pruning, or other plant maintenance will substantially impact the cost benefit analysis discussed earlier.
Given the impacts of orientation and facade area ratio, an effective implementation should take into consideration these factors prior to installation, but ideally at the design and policymaking stages. The known factors affecting the success of the wall, in the sense of both plant success and the ability to harness the fullest capacity and range of ecosystems services possible, relate heavily to all factors discussed in this chapter. The green wall failure rate, like the street tree failure rate, is a risk that effective design and maintenance can successfully overcome. Highlighted areas in need of further exploration are appropriate plant types, substrates or growing mediums, and maintainability to insure that the walls do not fail and that they effectively harness the ecosystem services to the greatest degree and variety. In doing so, the practical advantage of the green wall over other forms of green infrastructure, such as the street tree is revealed.
As green walls receive more attention from the development community, green wall design is bound to develop beyond just the risk of wall failure. Other performance criteria will improve so
as to yield better results in all the ecosystem service areas. For example, the lifecycle analysis of the
indirect green wall could be improved by using a locally sourced natural material, such as bamboo, hemp rope, or other material, in place of metal mesh. The use of a local natural material would reduce the environmental cost of material production, likely making the wall nearly as environmentally productive as the direct green wall system. The major advantage of seeking such a design solution is to avoid potential damage to the building, increase growing area, and increase ease of maintenance. Many more ideas are sure to come as more collective and sustained attention is given to improving the green wall from its current iterations.
This chapter explores the realizability of the green wall in today's development context by discussing practical building code limitations, design guidelines, and planning and landscape codes realities.
The 2015 Paris Agreement's emphasis on policies and procedures as a means of achieving greenhouse gas (GHG) mitigation supports the notion that a constructive next step would be to look to governance, specifically in the form of policy drafting for the successful implementation of green wall systems. The use of policies as a means of effectively reaching environmental goals is commonly reinforced, as is evidenced here, "sustainability assessment tools are considered to be reliable in achieving the aim of sustainability and have gained interest from authorities" (Yoon and Park 2015: 14451) and, "the pursuit of sustainability [...] provoked neighborhood sustainability assessment tools, [where] major transitions in the thinking and practice of urban design guidelines were required" (Yoon and Park 2015: 14452). These types of collective initiatives are perhaps best realized utilizing municipal, state, or federal regulations for building development. In this case, municipal regulations are the focus.
Despite the increasing evidence that clear policy documents are required for successful implementation, "Few urban jurisdictions have explored or simulated mitigation options and implications at the scale of a neighborhood," and for that we require, "spatialized urban form attributes and data at sufficient specificity and scale" (Kellett, et al, 2013). Factors needing consideration for the implementation of the green wall system by policy makers such as urban planners are building type, orientation, and density, plant type and growing season, wall type and installation method, and potentially more factors (including all performance factors discussed in the
preceding chapter). These factors will require additional research and prototyping of green walls to
measure and test the impacts, but at this point, available research has overall supported notions that the vertical green wall is a viable passive solar strategy, as well as provider of a range of other ecosystem services as discussed. There are strong indicators that the green wall can play a positive role in local economics, raising questions as to whether green walls can be considered part of an economic strategy.
Whatever the motivations for utilizing green walls, performance indicators to measure impact and standards of minimum performance for new and existing development should be amongst the first steps for regulatory bodies. The standard metric for greenery remains land cover (Liang, et al 2014: 54), yet with vertical green walls this metric does not capture or otherwise represent the vertical space available for capture. Other metrics to consider include those currently being used by the urban greenery initiative. They are as follows: Green Plot Ratio (GPR), a concept that is developed by combining the concepts of Leaf Area Index (LAI) and Building Plot Ratio (BPR). The Urban Neighborhood Green Index (UNGI) can be used by planners to quantify the proximity to greenery for each neighborhood and the park provision ratio and per capita green cover are urban planning benchmarks that have been used in countries such as Singapore (Liang, et al 2014: 54).
If such metrics were imported to quantify green wall performance (or performance targets), urban planners could act on that basis by passing ordinances to allow for green walls, removing any barriers to implementation such as fire and building code restrictions, create economic incentives for implementation, or otherwise formulating development requirements mandating the use of green walls to meet certain ecosystem service objectives. With the metrics currently available, planners can move to add landscaping requirements as part of municipal or county site development requirements that encourage or explicitly require the use of the vertical green wall to
meet landscaping requirements. For example, the City of Boulder allows for vertical green walls and
green roofs to meet overall site greening requirements. Another avenue would be to implement wall to floor area ratio requirements, which instead of restricting building height or size are intended to set minimum requirements for wall greening. Currently, the established site development criteria at the City of Boulder do not sufficiently take into account the implementation of vertical greening, which would require dealing with vertical space and occupying space beyond the plan.
Moving beyond simply identifying that planners can play a meaningful role in codifying, crafting policies, and creating incentives to encourage the implementation of green wall systems, there are of course many potential approaches. Site development criteria could: increase the greening requirement so as to require vertical greening components to meet minimum requirements; incorporate a wall height and wall coverage ratio simply requiring a certain percentage to be covered with green walls; instate a passive solar strategy requirement that requires all south and west facing facades to demonstrate passive solar strategy, etc.
Although most codes and other regulatory policy documents do not call out green walls as they are such a new trend in building, it is possible that green walls are not permitted on a building in the first instance. Design guidelines sometimes call out materials. If the green wall is not called out explicitly as an acceptable material, it seems the wall would not be permitted in areas with this level of regulation. The City of Boulder's Form-Based Code only permits a list of specifically called out materials as acceptable. Materials not on this list are not permitted, or otherwise would require additional discretionary review and approvals. The direct green wall system, which is a covering over a base material, would not be against the design guidelines. It is possible that some municipalities have created more restrictive regulations than the norm described, or otherwise have worded their
code in such a way as to disallow green walls unintentionally. Given that people seem
overwhelmingly to like the idea of greening, and specifically of the green wall, it seems prudent to allow for green walls by right. Such allowances would entail code amendments particularly in urban interface areas, writing design guidelines in such a way as to allow for the possibility of green walls, granting exceptions for variances, or even better, requiring the use of green walls.
Design guidelines could play an important part in the selection of plants for a wall. The guidelines can be prescriptive in terms of material, calling out specific allowed materials. Assuming that design guidelines will begin to develop to deal with urban greening as a design issue, they will likely start specifying acceptable or preferred plant species, as well as disallowed or discouraged species. This initiative could similarly take place in many other areas of the standard code, but is mostly like to appear in the landscape development requirements.
For cities that do have robust landscape codes, few directly acknowledge green walls as a landscape feature, but several appear to feature regulations that indirectly impact the implementation and placement of green walls. The City of Boulder, for instance, has provisions for green walls to count toward greening quality, but not to the foundational landscape requirement. City landscape codes could allow for green walls to meet landscape requirements, especially in areas where there is no other means of reaching such a requirement. In existing areas where the buildings are built to maximum bulk and density, and where buildings are for the most part built to property lines, there is little room for additional landscaping. These are excellent opportunities for landscape requirements to apply to properties with such restrictive space conditions, whilst maintaining a citywide, regional, or statewide initiate to reach a specified percent green.
Fire codes can also limit the use of green walls, by either restricting their location or disallowing them as a wall material altogether. Urban areas will have the building related fire code
alone to respond to where, other areas will need to respond to both the building related fire code
and the International Wildlife Urban Interface Code (IWUIC). In the urban areas where the IWUIC does not apply (this includes the City of Boulder), combustible materials cannot be within 30 feet of a property line for fear of a fire spreading from one building to another. In the case of the green wall, there is some difficulty in determining whether or not the green wall would be considered combustible under the code. Given that the code does not directly speak to green walls and their use is fairly infrequently, particularly in permit review, whether or not a green wall is considered a combustible material is open to interpretation and therefore will vary. Yet, a particularly strong reading runs as follows: If the vegetation is simply covering an acceptable material, there is no fire danger. If however, the vegetation is the wall material, the wall would likely be considered combustible and therefore would not be permissible. There is a possibility that the wall's substrate could be (re)classified as non-combustible through extensive material testing, but additional funds and research would need to be dedicated to determining the parameters. Assuming that the point of issue is the direct green wall system, given the highest lifecycle rating of the green wall typology discussed earlier, the wall would be considered a covering and therefore would be permissible in urban areas. This means that there is no setback, height, or other requirement facing the green wall in the urban areas insofar as the wall remaining on the individual property and the building meets all other applicable requirements. The use of direct green walls in urban areas falling outside of the remit of the IWUIC is wide open.
In the case of areas where the IWUIC applies, the scope of potential application is considerably smaller, as there are minimum separation requirements from structure to structure. The intention of setting such requirements is to prevent the spread of fire, as the code applies to most suburban and peri-urban areas in addition to fairly rural settlements. The use of the IWUIC in suburban areas means the typical strip mall or single family home will face additional difficulty
greening their buildings as there will be added setback or separation requirements that will limit the
scale and scope of potential green wall application.
Wind load on the wall could mean that vegetation be easily ripped off by high winds, and regulations to insure this does not happen will set the parameters on wall construction and likely create some inbuilt limitations as to where walls can be placed. For instance, if the code were to require a certain amount of structural tie in to the existing structure, the wall placement would be determined by the location of the existing structure.
Water law may come into play if there are a series of planters seen as redistributing water through the site or if the planters are retaining water for a period of time deemed too long. Natural or self-watering systems are options to reduce water use in arid and desert climates such as Colorado or Arizona, however Colorado State Law currently prohibited the reuse of grey water as it seen as a violation of retention laws. These water restrictions can limit the scale of a planting effort, the location of planters so as to avoid water retention or claims of recirculation.
Then there is the bigger picture question as to whether or not the installation of a direct green wall would require a building permit, which is to say, would a direct green wall require municipal review. In the case of new building, if an applicant does not show the direct green wall on the plans submitted, would the approved plans be brought into question for a difference as minor as a planting system? And in the case of retrofits, would a property owner be required to submit their direct green wall system plans to the local municipality for review? If so, in which cases it is required and why? At first blush, if there is any change to the building's structure particularly in terms of additional load as planters are quite heavy, any possibility of the structure being subjected to wind load, or any other perceived health and safety risk.
FINDINGS AND CONCLUSIONS
Although the research presented here overwhelmingly supports the positive impacts of direct vertical green wall systems on energy demand and on the provision of wider ecosystem services, there is ample room for further research, analysis, and testing. We need more research on the impacts of the vertical green wall system on building temperature, its cumulative benefits for urban emissions, and the link between vertical green wall systems on the urban heat island effect. Yet, "the avoidance of air conditioning by improving the urban microclimate is a key factor" (Steemers 2003: 10). Yang and Li discuss the empirical relationship between building morphology and street temperature thus linking external microclimate with the building's internal microclimate (2015).
We also require more research or prototyping of vertical green walls themselves, so that we can develop a set of best practices that would be easily communicated and delivered to practitioners across municipalities, states and other borders. One aspect specific to our current location is the need to explore how to reduce or eliminate the need for water to sustain a vertical green wall system, which likely means exploration into the exact construction method, place of implementation, and plant type(s) utilized.
What we do know however is that vertical green walls are more than mere aesthetic symbols of environmentally friendly urbanization. That, depending on the wall type, installation, and plant type, vertical green walls can have substantially positive impacts on energy consumption by reducing the dependency on HVAC systems to regulate internal microclimates, as well as their many other environmental benefits. The practical advantage of the green wall system over other green infrastructure elements such as trees and green roofs is that it lends itself to a much wider range of
applications making the lofty implementation goal accessible and realistically achievable in practice.
This knowledge leaves the possibility of capturing the vertical dimension for greening purposes tantalizing and open to a seemingly infinite variety of design-based solutions, and the practical simplicity of its installation lends itself to rapid uptake and normalization in building practice. Given the practical and aesthetic appeal of vertical green wall systems, we could be on the cusp of a major shift in urban greening.
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The following images (Figures 30-50] show the temperatures surrounding two buildings from 3AM in the morning to 11PM at night. The images shown here were generated in the Envi-Met program. The temperatures are shown in relation to a horizontal point at 1.8 meters high, or the average height of a person. The building shown to the top of the model has a simulated green wall on the west and south facing building facades, along with a grass lawn to the west (see Figure 29 below]. The building shown on the bottom of the model does not have a green wall on the west and south facades, and does have a grass lawn to the west.
, r PA LW LW LW LW LW LW LW LW LW PA LW LW LW LW LW
LW LW LW LW LW LW LW LW LW LW LW LW LW LW LW
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
Figure 37: Envi-met model showing the location of two three story buildings. The grey boxes with PA written in them represent the green wall system simulation and the grey boxes with LW written in them are grass lawn (Source: Mehdi Heris).
The hour-by-hour snapshots show the temperatur rises as the sun comes up in the morning, that the temperatures continue to rise throughout the day with prolonged sun exposure and therefore solar gain, and then begin to slowly drop off after the sun goes down (at approximately 6PM], The snapshots show that the building without the green wall simulation gains heat faster and retains heat longer than the building with the green wall system in place The model also shows the impact of a westerly wind, which pushes heat around the building, creating warm pockets on to the east of the building.
0.00 10.00 20.00 30.00 40.00
GWS Simulation 03:00:0129.06.2014
x/y Schnitt bei k=4 (z=1.8000 m)
inter 16.14C 16.14 bis 16.26 C 16.26 bis 16.38 C 16.38 bis 16.50 C 16.50 bis 16.62 C 16.62 bis 16.74 C 16.74 bis 16.86 C 16.86 bis 16.98 C 16.98 bis 17.10 C Cber 17.10 C
Mn: 16.02 C Max: 17.22 C
Figure 38: 3AM (Source: Medhi Heris).
0.00 10.00 20.00 30.00 40.00
x/y Sdinitt bei k=4 (z=1.8000 m)
inter 15.57 C
15.57 bis 15.69 C
15.69 bis 15.80 C
15.80 bis 15.92 C
15.92 bis 16.04 C
16.04 bis 16.16 C
16.16 bis 16.27 C
16.27 bis 16.39 C
16.39 bis 16.51 C
iter 16.51 C
Mn: 15.45 C Max: 16.63 C
Figure 39: 4AM (Source: Mehdi Heris)
x/y Sdinitt bei k=4 (z=1.8000 m)
inter 15.06 C
15.06 bis 15.18 C
15.18 bis 15.29 C
15.29 bis 15.41 C
15.41 bis 15.53 C
15.53 bis 15.65 C
15.65 bis 15.76 C
15.76 bis 15.88 C
15.88 bis 16.00 C
Mn: 14.94 C Max: 16.11 C
Figure 40: 5AM (Source: Mehdi Heris)
0.00 10.00 20.00 30.00 40.00
x/y Sdinitt bei k=4 (z=1.8000 m)
inter 14.54 C
14.54 bis 14.67 C
14.67 bis 14.79 C
14.79 bis 14.92 C
14.92 bis 15.04 C
15.04 bis 15.17 C
15.17 bis 15.30 C
15.30 bis 15.42 C
15.42 bis 15.55 C
iter 15.55 C
Mn: 14.41 C fvbx: 15.68 C
Figure 41: 6AM (Source: Mehdi Heris).
0.00 10.00 20.00 30.00 40.00
x/y Sdinitt bei k=4 (z=1.8000 m)
inter 18.20 C
18.20 bis 18.27 C
18.27 bis 18.34 C
18.34 bis 18.41 C
18.41 bis 18.48 C
18.48 bis 18.55 C
18.55 bis 18.62 C
18.62 bis 18.69 C
18.69 bis 18.76 C
Lber 18.76 C
Mn: 18.13 C Max: 18.83 C
Figure 42: 7AM (Source: Mehdi Heris)
0.00 10.00 20.00 30.00 40.00
x/y Sdinitt bei k=4 (z=1.8000 m)
inter 20.96 C
20.96 bis 21.02 C
21.02 bis 21.08 C
21.08 bis 21.14 C
21.14 bis 21.20 C
21.20 bis 21.25 C
21.25 bis 21.31 C
21.31 bis 21.37 C
21.37 bis 21.43 C
Cber 21.43 C
Mn: 20.91 C fvfex: 21.49 C
Figure 43: 8AM (Source: Mehdi Heris)
0.00 10.00 20.00 30.00 40.00
x/y Sdinitt bei k=4 (z=1.8000 m)
inter 23.58 C
23.58 bis 23.63 C
23.63 bis 23.68 C
23.68 bis 23.73 C
23.73 bis 23.77 C
23.77 bis 23.82 C
23.82 bis 23.87 C
23.87 bis 23.92 C
23.92 bis 23.96 C
Lba- 23.96 C
Mn: 23.54 C Max: 24.01 C
Figure 44: 9AM (Source: Mehdi Heris).
x/y Sdinitt bei k=4 (z=1.8000 m)
inter 25.59 C
25.59 bis 25.64 C
25.64 bis 25.68 C
25.68 bis 25.73 C
25.73 bis 25.78 C
25.78 bis 25.82 C
25.82 bis 25.87 C
25.87 bis 25.92 C
25.92 bis 25.97 C
Cber 25.97 C
Mn: 25.54 C Max: 25.01 C
Figure 45:10AM (Source: Mehdi Heris)
x/y Sdinitt bei k=4 (z=1.8000 m)
inter 27.38 C
27.38 bis 27.43 C
27.43 bis 27.48 C
27.48 bis 27.53 C
27.53 bis 27.58 C
27.58 bis 27.63 C
27.63 bis 27.68 C
27.68 bis 27.73 C
27.73 bis 27.78 C
Lber 27.78 C
Mn: 27.33 C Max: 27.83 C
Figure 46:11AM (Source: Mehdi Heris).