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Anne Lanham, Lillian Mwanri

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A Comparative Study on Green Wall Systems

Arash Zia*, Kaveh Zia, Airya Norouzi Larki

Department of Architecture, University of Tehran, Tehran, Iran

Abstract: Greening the building envelope focusing on green facades with vegetation is a good example of a new construction practice. Plants and partly growing materials in case of green wall systems (GWS) have a number of functions that are beneﬁcial, for example: increasing the biodiversity and ecological value, mitigation of urban heat island effect, outdoor and indoor comfort, insulating properties, improve of air quality and of the social and psychological well-being of city dwellers. This paper discusses a comparative life cycle analysis (LCA) for: a conventional built up brick facade, a facade greened directly, a facade greened indirectly (supported by a steel mesh), a facade covered with a green wall system based on planter boxes and a facade covered with a green wall system based on felt layers. Beside the environmental beneﬁts of the above described greening systems, it is eventually not clear if these systems are sustainable, due to the materials used, maintenance, nutrients and water needed. A LCA is used to analyze the similarity and differences in the environmental impacts in relation with beneﬁts estimated for two climate types for building energy saving (reduction of electrical energy used for building cooling and heating).

Keywords: façade greening, green wall systems, LCA, sustainability, environmental benefits

Received October 11, 2013; Revised October 17, 2013; Accepted November 03, 2013

Citation: Arash Zia, Kaveh Zia, Airya Norouzi Larki, A Comparative Study on Green Wall Systems. American Journal of Civil Engineering and Architecture, 1 (6). 143-155. January 2013. doi:10.12691/ajcea-1-6-4

1. Introduction

1.1. Background

Vegetation can be seen as an additive (construction) material to increase the (multi)functionality of facades of buildings. Vertical green, also commonly referred to as a “vertical garden”, is a descriptive term that is used to refer to all forms of vegetated wall surfaces [1]. Vertical green is the result of greening vertical surfaces with plants, either rooted into the ground, in the wall material itself or in modular panels attached to the facade in order to cover buildings with vegetation and can be classiﬁed into facade greening and green walls systems [2,3].

Greening the cities is not a new approach (i.e. hanging gardens of Babylon), but the beneﬁts are rarely quantiﬁed. Greening facades is a good example of combining nature and buildings (linking different functionalities) in order to address environmental issues in dense urban surroundings [4]. The main beneﬁts due to the use of green facades applications are of economic, social and environmental origin such as greenhouse gas (emission) reduction, adaptation to climate change, air quality improvements, energy saving by insulation, habitat provision and improved aesthetics [5,6,7]. Also sound reduction is possible by the use of vegetation [8].

Vertical greening concepts can be divided into categories (green facades and green wall systems) according to their growing method [9]. Green facades are based on the use of climbers (evergreen or deciduous) attached themselves directly to the building surface (as in traditional architecture), or supported by steel cables or trellis. In the ﬁrst case climbers planted in the ground at the base of the building allows to obtain a cheap facade greening but there could be implications for any building works that needed to be carried out (for example damages and maintenance of the facade), besides that climbing plants can only grow to a maximum of 25 m height and it can take several years [2]. Supporting systems are sometimes necessary and planter boxes, such as prefabricated and pre-vegetated systems (green wall systems), attached to walls can require speciﬁc growing substrate to facilitate plant growth. Green wall systems (GWS) consists of modular panels, each of which contains its own soil or other growing medium (soil, felt, perlite, etc.) based on hydroponic culture, that is using balanced nutrient solutions to provide all or a part of the plant’s food and water requirements [2].

1.2. Aim of the Research

The goal of this life cycle analysis (LCA) is to evaluate the actual and potential environmental aspects associated with constructing, maintaining and disposing of 1 m² facade and to determine the impact of the raw material depletion, fabrication, transportation, installation, operation, maintenance and waste for four greening systems compared to a bare facade. In addition, a start was made to transform the positive quantifiable aspects of vertical greened surfaces into a lower environmental impact (due to a reduction of energy savings) during the life span of a greened building.

The presented LCA research, examines a conventional bare built up brick facade, a conventional facade covered with a climber planted at the base of the facade (greened directly), a conventional facade covered with a climber (planted also at the base of the facade) using a stainless steel framework to create a cavity between foliage and facade (indirectly), a conventional facade covered with a green wall system (GWS) based on planter boxes filled with potting soil and a conventional facade covered with a green wall system based on felt layers.

To develop the LCA model an inventory analysis was created. In this phase information has been collected about the materials involved for the bare wall and the different greening systems. The materials needed were obtained from the project construction documents and information provided by the manufactures.

A life cycle analysis for the four vertical greening systems investigated allows evaluating their sustainability in relation with the achievable environmental benefits, as a measure of ecological quality based on our knowledge of the influence on the environment. Sustainability can be defined as a general property of a material or a product that indicates whether and to what extent the prevailing requirements are met in specific application. These requirements, which relate to air, water and soil loading, have influences on well-being and health of living creatures, the use of raw materials and energy, and also consequences for the landscape, the creation of waste and the occurrence of nuisance to surrounding environment [10].

1.3. Ecological and Environmental Benefits

In recent (and older) literature several claims are done about the positive influence of green facades. Even if green facades have been studied from the eighties, especially by older German literature, still most of the benefits have been only estimated and not quantified.

Green wall systems (GWS) and green facades have different characteristics that can have influence on the benefits like cooling and insulating properties. Relevant aspects are the thickness of the foliage (creating a stagnant air layer and shading the facade), water content, material properties and possible air cavities between the different layers.

Between facade and the dense vertical green layer, both rooted in the soil and rooted in artificial pre-vegetated based systems (hydroponic), a stagnant air layer exist. Stagnant air has an insulating effect; green facades can therefore serve as an “extra insulation” of the building envelope [5,6,11]. Also direct sunlight on the facade is filtered by leaves, thanks also to the phototropism effect. 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 [5]. This blocking of the direct sunlight exposure ensures a cooling effect in warmer climates. Secondly, green facades and roofs will cool the heated air through evaporation of water [12]; 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% [2].

In winter, the system works the other way and heat radiation of the exterior walls is insolate by evergreen vegetation. In addition, the dense foliage will reduce the wind flow around the facade and thus also helps to prevent the cooling down of the building.

The greening of vertical surfaces has a beneficial effect on the insulating properties of buildings through exterior temperature regulation [5]. The role of insulation materials and stagnated air layers is to slow down the rate of heat transfer between the inside and outside of a building, which is a function of the difference between the inside and outside temperatures. The insulation value of vertical greened surfaces can be increased in several ways [7]:

•  By covering the building with vegetation, the summer heat is prevented to reach the building skin, and in the winter, the internal heat is prevented to escape.

•  Since wind decreases the energy efficiency of a building by 50%, a plant layer will act as a buffer that keeps wind from moving along a building surface.

•  By the materials and substrates used in the case of green wall concepts.

In the beginning of the eighties Krusche et al. [5] have estimated that the thermal transmittance of a 160 mm plant cover is 2.9 W m⁻² K⁻¹. Minke and Witter [6] also suggested to reduce the exterior coefficient of heat transfer. By reducing the wind speed along a green facade the exterior surface resistance coefficient can be equalized to the interior surface resistance coefficient as demonstrated by Perini et al. [10]. Field measurements on a plant covered wall and a bare wall by Bartfelder and Köhler [13,14] show a temperature reduction at the green facade in a range of 2-6°C compared with a bare wall. Also Eumorfopoulou and Kontoleon [15] have reported a temperature cooling potential of plant covered walls in a Mediterranean climate; the effect was up to 10.8◦C. Another recent study by Wong et al. [12] on a free standing wall in Hortpark (Singapore) with vertical greening types shows a maximum reduction of 11.6°C. Alexandri and Jones [16] simulated a temperature decrease in an urban canyon with greened facades a reduction of air temperature 4.5◦C for the Mediterranean climate and 2.6◦C for the temperate climate. In research done by Eumorfopoulou and Aravantinos [17] they concluded that a planted roof contributes to the thermal protection of a building but that it cannot replace the thermal insulation layer.

The integration of vegetation on buildings, through green roofs and vertical greening, allows to obtain also ecological and environmental benefits and increases biodiversity, besides social and aesthetical benefits [18]. The ecological and environmental benefits regard, as for green roofs, the improvement of air quality [19], energy savings for the building heating and cooling and the reduction of the heat island effect. Greening paved surfaces with vegetation to intercept the radiation can reduce the warming up of hard surfaces, especially in dense urban areas [23]. In the urban area, the impact of evapotranspiration and shading of plants can significantly reduce the amount of heat that would be re-radiated by facades and other hard surfaces. Besides that the green plant layer will also reduce the amount of UV light that will fall on building materials. Since UV light deteriorates the material and mechanical properties of coatings, paints, plastics, etc. plants will also have positive effect on durability aspects and on maintenance costs.

2. Materials and Methods

2.1. Basic Approach

In this research, life cycle analysis (LCA) is used to calculate the environmental impact of the production, use, maintenance and waste for four common systems for vertical greening of buildings. This is to compare the environmental burden and benefits of the green system with a bare brick wall (Figure 1):

1. bare wall (brick),

2. direct facade greening system + bare wall,

3. indirect facade greening + bare wall,

4. green wall system (GWS) based on planter boxes filled with soil + bare wall,

5. Green wall system (GWS) based on felt layers + bare wall.

Figure 1. Bare wall with materials layers

Figure 2. Direct greening system with materials layers

As shown in Figure 1, the conventional bare wall (option1), used as well as basis for the greening systems analyzed, is constructed by masonry, air cavity, insulation material and lime stone (Figure 2, Figure 3, Figure 4, Figure 5). The direct facade greening (option 2) consists in a well grown evergreen climber Hedera helix, attached directly to the building surface and planted at the base of the greened facade. The thickness of the Hedera foliage is ±20 cm. The third system analyzed in this study is an indirect facade greenery (option3). This system is constituted by steel frames as support for evergreen climbing plants (H. Helix ±10 cm). The fourth investigated greened facade, a green wall system (option4), is based on plastic modules (HDPE), filled with soil and planted with evergreen species (Pteropsida), working with a system for water and nutrients. The fifth system (option5) is a GWS system based on several felt layers as substrate supported by a PVC sheet and planted as well with ferns (Pteropsida) and working with a system for water and nutrients. A number of factors are considered in the analysis: raw material depletion, fabrication, transportation, installation, operation, maintenance and waste for the facade(s) area.

Figure 3. Indirect greening system with materials layers

Figure 4. GWS based on planter boxes with materials layers

Figure 5. GWS based on felt layers with materials layers

In order to work with the LCA model, a functional unit should be defined to serve as a basis for comparison of the greening alternatives. According to the ISO 14044 standard [24], the functional unit is defined as the reference unit through which a system performance is quantified in a LCA. The chosen functional unit in this research is 1 m². As basis for calculating the materials and products involved in every system a fictitious facade of 100 m² is used.

The results of the analysis are expressed as the accumulation of environmental impact over the service life; therefore the frequency of maintenance activity and the times at which replacements are needed are described. Finally, the assumptions and limitations of analysis and the data will be discussed.

2.2. Tools

The database used to develop the process models for this analysis is based on the Dutch National Environmental database compiled by the Dutch Institute for Building Biology and Ecology (NIBE). The complete set of environmental impact categories is known as the “environmental profile”. The environmental profile is divided in ten categories:

•  abiotic depletion (kg Sb equivalents),

•  global warming (kg CO₂ equivalents),

•  ozone layer depletion (kg CFC-11 equivalents),

•  human toxicity (kg 1.4-DB equivalents),

•  fresh water aquatic eco-toxicity (kg 1.4-DB equivalents),

•  marine water aquatic eco-toxicity (kg 1.4-DB equivalents),

•  terrestrial eco-toxicity (kg 1.4-DB equivalents),

•  photochemical oxidation (kg C2 H₄),

•  acidification (kg SO₂ equivalents),

•  Eutrophication (kg PO₄ equivalents).

The category marine water aquatic eco-toxicity will not be taken into account because of significant problems associated with the calculation of the impact in the method. These problems are related to the time a substance is present in the marine ecosystem and missing data for normalization [20]. The environmental calculation is built up by three main classes: materials, transportation and waste. For every class the environmental burden is calculated according to the ten categories described above.

2.3. Data Inventory

For this life cycle analysis only the components of the building facade directly related to the systems analyzed will be examined. The bare wall is taken as basis for each material inventory; in general a facade is a barrier against environmental conditions to separate the building interior and exterior, nowadays a greening system is added as an extra layer with the previous described multi-functionalities. The difference between the material quantities between the bare facade and the greened ones are the layers involved depending on the greening type: for the direct climber system the layer consists of a single plant layer; the indirect climber system involves also a stainless steel support for the plant; the layer added for the first GWS is based on planter boxes filled with potting soil; the second GWS involves several layers for rooting, waterproofing and supporting.

The materials used for this inventory were obtained from product forms and information provided by companies (Table 1).

Table 1. Components and materials for bare wall and green system analyzed

2.4. Assumptions

The analysis period to study the environmental aspects and potential impacts is based on a facade’s service life of 50 years. The life expectancy of the conventional bare wall is assumed to be 50 years as well as for the facades covered directly and indirectly with climbing plants [2]. The replacement frequencies of plants for green wall systems are simplified as 10 years (10%replacement/year) for the one based on planter boxes and 3.5 years (30% replacement/year) for the felt layers based system.

The life expectancy for the plastic (HDPE) planter boxes is estimated more than 50 years. With reference to a study performed by Riedmiller and Schneider [21], it is clearly indicated that the service life of PVC layers (structural support for the felt layers) is only 10 years, beside that the whole module (inclusive felt layers) have the same life expectancy. Therefore the whole module is assumed with a life expectancy of 10 years as well.

The automated watering system needed for the GWS concepts (planter boxes and felt layers) have to be replaced every 7.5 years because of crystallizing of salts. The amount of water and nutrients needed for the GWS is controlled by a computer and sensors (due to the complexity of these systems), therefore it is not included in this analysis. Green wall concepts need, besides the watering system, a nutrient solution, which is not taken into account due to the small (1%) influence. The water consumption for the green wall system based on planter boxes is assumed as a quantity of 1 l/day (average value for whole year), for the green wall system based on felt layers 3 l/day (average value for whole year). Watering systems are not taken into account for the direct and indirect greening system due to the fact that the climbing plants are rooted into the ground (Tables 2a-2e).

Table 2a. Bare wall material weight (kg) and service life (years) of components

Table 2b. Direct facade greening system material weight (kg) and service life (years) of components

Table 2c. Indirect facade greening system material weight (kg) and service life (years) of components

Table 2d. GWS based on planter boxes material weight (kg) and service life (years) of components

Table 2e. GWS based on felt layers material weight (kg) and service life (years) of components

3. Analysis of LCA Results

3.1. Environmental Burden Analysis

A LCA was calculated for the entire life cycle for each component of green facade alternatives. The results show that there is a significant difference between the greening systems and the bare wall, except for the direct greening system. This is mainly caused by the materials involved for the supporting systems.

Starting from the full environmental profile, only global warming, human toxicity and fresh water aquatic eco-toxicity are considered for showing the results due to the very small (almost zero) influence of the other six categories.

From Figure 6 it is possible to deduce for global warming that the green wall system based on felt layers is more than double compared to the other greening systems. For the human toxicity the indirect system and both green wall systems have a high impact compared to the bare wall and the direct greening system. The same trend is noticeable for the fresh water aquatic eco-toxicity, except for the GWS based on felt layers which is more than five times of the indirect and GWS based on planter boxes.

Figure 6. Environmental burden profile given for global warming, human toxicity and fresh water aquatic eco-toxicity

The environmental burden for stainless steel in the database is based on 30% of recycled stainless for the production process. This percentage is a common average used in databases worldwide, but the amount of recycled stainless steel could be higher which could lead eventually to a lower environmental burden.

The Figure 7, Figure 8, Figure 9, Figure 10 built up for each system show the influence for the classes material, transportation and waste of the bare wall, supporting systems and vegetation. Also, the transportation factor of this paper is intended to Tehran city (Iran). The highest difference found in the analysis regards the material impact for the supporting systems. Due to this the direct greening system has the lowest environmental burden. For this system as for the indirect one, also the vegetation has a very small impact, since it is only related with transportation (no watering and nutrients system and replacement of plants is needed). For the green wall system based on felt layers the waste class has a major impact due to the impossibility of recycling the entire module involved.

Figure 7. Total environmental burden profile for classes' material, transportation and waste for direct greening system

Figure 8. Total environmental burden profile for classes' material, transportation and waste for indirect greening system

Figure 9. Total environmental burden profile for classes' material, transportation and waste for GWS based on planter boxes

Figure 10. Total environmental burden profile for classes' material, transportation and waste for GWS based on felt layers

3.2. Designing Green Facades for Sustainability

This study regards the analysis of four types of greening systems (direct, indirect, GWS based on planter boxes and GWS based on felt layers) and presents an overview of the tendency with respect to the green facades technologies. Since the development in this field is growing rapidly especially the last three to four years, many systems with different materials and characteristics are available. The different systems and materials can have an influence on the environmental burden either positively or negatively. For example for the indirect greening system, based on stainless steel mesh acting as support for climbing plants, other materials, as different types of wood, plastic, aluminum and steel, can be used. Each of the materials enumerated changes the aesthetical and functional properties due to the different weight, profile thickness, durability and cost. Figure 11 shows (for the indirect greening system) the influence on the environmental burden profile of four different materials that can be used as supporting system. The stainless steel support (as used for this LCA) is roughly 10 times higher than supporting systems based on recycled plastic (HDPE), hard wood (FSC certificated) and coated steel.

Figure 11. Environmental burden profile for different supporting materials for indirect greening system

Figure 12. Green wall systems (based on planter boxes and based on felt layers) without the exterior masonry

Figure 13. Comparison between the environmental profiles for two different types of green wall concepts applied to a traditional bare wall and with the omission of the external masonry layer

In the case of green wall systems a sustainable approach can involve, behind the material choice, a higher integration within the building envelope by combining functionalities. In the case of the bare wall analyzed in this study, it is possible for example to skip the outer masonry (Figure 12), since the protection against the environmental parameter can be absolved by a green wall system. Figure 13 shows the possible reduction of the environmental burden thanks this integration.

4. Discussion and Analysis

4.1. Environmental Burden

All the systems studied reveal similar dominating impact categories, though the magnitude of this differs considerably. This difference is mainly caused by the supporting material and the replacement both for plants and material. Due to this the green wall system based on felt layers has the largest impact for the global warming and fresh water aquatic eco-toxicity, since it is necessary to replace the panels five times in a service life of 50 years. The human toxicity profile shows a higher impact for the three green facade types with supporting system for plants (indirect, GWS based on planter boxes and GWS based on felt layers). For the systems studied, vegetation contributes to the environmental burden only because of the transport (which is depending on the different replacement required for every system).

The indirect greening system has a high impact profile for the supporting system due to the use of stainless steel. Since stainless steel is a high quality material it could be possible to use it for a life span longer than 50 years. In this case, due to the foliage package that is weaving through the stainless steel mesh, it is not recommended to reuse the material after the service life. Therefore in this case also other supporting materials could be used (hard wood, HDPE, coated steel). As shown in Figure 11, the choice of one of those materials for the supporting system can reduce the environmental burden profile.

The highest environmental burden that is found in the study regards the green wall system based on felt layers; since it is difficult to recycle the panel, the environmental burden of waste has a similar impact profile as the one for the realization. The environmental profile of the green wall systems analyzed can be decreased by higher integration of the building envelope (omitting the exterior masonry).

4.2. Benefits of Vertical Green Related to the Calculation Method

The benefits that can be derived from the green layer are dependent on the growing rate of the plants (covering of the facade). For the direct and indirect system the full covering of the facade by H. helix is estimated after 20 years (according to Bellomo [22] 0.5 m/year of vertical growing). For both green wall systems, due to the amount of plants and the several material layers involved, it is possible to calculate the benefits after installation of prefab modules.

For calculating the energy savings for heating, due to the increase of the insulating properties with greening systems, the additional thermal resistance is assumed to be 0.09 K m² W⁻¹. This assumption is used for all of the direct and indirect greening systems analyzed due to the fact that there is a stagnant air layer in and behind the foliage [11]. For the green wall systems the thermal transmittance of the substrate and the materials used are added.

Table 3. Energy saving (calculated with Thermo 8.0) for heating, energy saving for cooling and temperature decrease for Mediterranean and temperate climate based on Alexandri and Jones [16]

For the benefits related to energy savings (increase of R-value) in this study a simulation model (Termo 8.0 Microsoftware) is used to calculate the influence on the environmental profile for two different types of climates, namely temperate climate and Mediterranean climate, since a green layer affects more the insulation properties for the temperate climate and the cooling potential for the Mediterranean climate. The simulated buildings used for both locations have a ground floor area of 75 m², with a volume of 296 m³ (three floors) freestanding and situated in an urban context. The whole building envelope is 100 m² (fictitious facade used for the basis LCA calculations and includes north, south, west and east orientation) and consists of the same materials and layers analyzed in the LCA (regular brick wall, direct greening system, indirect greening system, GWS based on planter boxes, GWS based on felt layers). The energy savings due to the cooling potential of the four greening systems is based on the research conducted by Alexandri and Jones [16], regarding the temperature decrease in an urban canyon with green facades and the percentage of reduction that is reached for the air-conditioning (Table 3). For the temperate climate the energy saving for air conditioning in not taken into account, for the Mediterranean climate the calculation is based on the consumption of energy for air conditioning (energy class B). Energy savings from the additional insulation provided by the green facade types and for the cooling potential is found for the building and converted into unit savings to be applied across the LCA calculation. The calculations described are used to quantifying the indoor effects only, since a lack of data at the moment restricts to evaluate the macro scale environmental benefits.

Also the use of biomass produced (capturing of CO₂) by pruning the H. helix and by the replacement of plants (Pteropsida) from the green wall systems can be converted in energy (kWh). The calculation of this benefit shows a very small impact on the total environmental benefits.

4.3. Environmental Benefits

The calculation of the savings for the environmental impact as above described is based on the benefits thanks to the energy saving for heating (insulation) and cooling (only for Mediterranean climate). The energy saving thanks to the thermal properties of the systems is calculated through subtraction of the amount of energy that can be saved due to the “extra” insulation layer. For the direct and indirect greening systems the energy saving for heating is estimated as 1.2% of the annual consumption. For the green wall systems based on planter boxes and felt layers the saving was respectively 6.3% and 4%. The temperature decrease thanks to a green layer is estimated to be 4.5◦C (43% energy saving for air conditioning) for the Mediterranean climate and 2.6 for the temperate climate according to Alexandri and Jones [16].

The categories considered for the environmental benefits thanks to energy saving for heating and air conditioning are, as for the environmental burden, global warming, human toxicity and fresh water aquatic eco-toxicity. From Figure 14 and Figure 15 (environmental benefits profile for Mediterranean and temperate climate) it can be derived that the benefits for heating for the green wall systems are more than three times the direct and indirect greening system. This is mainly caused by the contribution for the insulation properties of the materials involved. The figs show a similar benefit profile for both direct and indirect such as for the green wall systems; since for the direct and indirect system only the vegetation layer (H. Helix for both systems) has an influence on the insulating properties. The profiles for the green wall systems have both major benefits. It is a little bit higher for the one based on planter boxes thanks to resistance of the soil package.

Figure 14. Environmental benefits profile (heating and cooling) for Mediterranean climate given for global warming, human toxicity and fresh water aquatic eco-toxicity

Figure 15. Environmental benefits profile (heating) for temperate climate given for global warming, human toxicity and fresh water aquatic eco-toxicity

Thanks to the energy saving for air conditioning the environmental benefits profile for Mediterranean climate is almost double than the one for temperate climate. The cooling potential and the extra insulation property due to a green layer have a similar influence on the environmental profile for the green wall systems analyzed. For the direct and indirect greening systems the energy saving for air conditioning has a higher benefit than the energy saving for heating.

4.4. Overview

The life cycle analysis presented shows the difference between the greening system analyzed for the environmental burden and for the environmental benefits related to two types of climate (Mediterranean and temperate).

Looking to the environmental burden profiles the indirect greening system and the green wall systems analyzed show a major impact (due to the materials used and the life span) even if, as shown, the environmental profile can be reduced by more sustainable material choice and an integrated envelope design. The environmental burden profile for the green wall system based on felt layer appears to be higher even with a different envelope design since the durability aspect plays an important role. For the indirect greening system the outcome can be changed thanks to a different material for the mesh, since the stainless steel has a high contribution on the profile. In general the direct and indirect greening systems have a low contribution to the energy savings for heating but, for the Mediterranean climate, a higher influence was noted for the cooling properties of the plants. The materials involved for the green wall system based on planter boxes affect the insulation properties and cause the highest energy saving for heating.

A difference is found in the benefits calculation. For the Mediterranean climate less annual energy consumption for heating is needed, so the energy saving thanks to the greening systems has a lower impact on the positive environmental profile; on the other hand the cooling potential of vegetation plays an important role for the indoor comfort (energy savings for air-conditioning).

Figure 16. Total environmental burden for the four greening systems (supporting systems + vegetation), benefits for heating and cooling for Mediterranean climate and benefits for heating for temperate climate

Table 4. Percentage of reduction given for the environmental burden profile with respect to global warming, human toxicity and fresh water aquatic eco-toxicity due to the energy savings for heating and cooling for Mediterranean and temperate climate

For the temperate climate the environmental burden profile is higher than the energy savings for heating for all the greening systems (supporting system + vegetation), except for the direct greening system that is sustainable, considering a system sustainable when the environmental burden is lower than the environmental benefit profile (Figure 16). For the Mediterranean climate, thanks to the energy savings related to air conditioning, the direct greening system is sustainable and also the green wall system based on planter boxes is almost sustainable. For the green wall system based on felt layers in both climate types the environmental burden profile is higher than the benefits gained for heating and cooling. The environmental burden and the benefits for heating and cooling are calculated both for the service life of the greening systems studied.

As was shown above, the choice of a different material (hard wood, HDPE, coated steel), as supporting system for the indirect greening system, can lead to a sustainable option for the Mediterranean climate (Table 4).

In order to carry out this analysis the study relies in part on published data from other green roof and wall researches and practice for estimating these effects. This may introduce some bias, and indicated that this work is subjected to revision as increasing experience with more and better data obtained from green facade researches.

4.5. Unquantifiable Categories

Other categories may be relevant in green facade applications, but were not included in this analysis either because of a lack of reliable data or incompatibility of the benefit with the tools and categories used in this study. Those categories are mainly related to macro-scale ecological and environmental benefits such as:

•  increased biodiversity,

•  human health,

•  improvement of air quality mainly related to reduction of fine dust levels,

•  Reduction of the heat island effect with regard the lower amount of heat re-radiated by greened facades and the humidity affected by the evapotranspiration caused by plants.

In this study the micro-scale benefits are only taken into account such as the energy savings for air conditioning and heating. However for quantifying the cooling potential several environmental parameters are involved (humidity, temperature, wind, etc.) and due to a lack of reliable data the calculation is based on an estimation for all the greening systems and it does not take into account the specific characteristics per system (green wall systems evaporate a larger amount of water than the systems based on climbers).

5. Conclusions

The results from the conducted life cycle analysis provide insight in the environmental impact of different greening systems. The energy benefits provided by the greening options make a noteworthy impact in the LCA and are calculated for Mediterranean and temperate climate; for the Mediterranean climate the benefits calculated are roughly two times higher thanks to the energy savings related to the cooling potential. The materials needed to build up the (green) facades are important (environmental impact) when the energy demand of a building can be reduced or when the multifunctional of the construction due to the integration of vegetation can be increased. From the presented LCA research it can be concluded that:

•  The direct greening system has a very small influence on the total environmental burden, for this reason this type of greening, without any additional material involved, is always a sustainable choice for the examined cases.

•  The indirect greening system analyzed based on a stainless steel supporting system has an high influence on the total environmental burden; the choice of another material for the supporting system can lead to a sustainable option for the Mediterranean climate (thanks to the energy saving for heating and air conditioning).

•  The GWS based on planter boxes has not a major footprint due to the materials involved, since the materials affect positively the thermal resistance of the system. The environmental burden profile could be further improved by a higher integration within the building envelope (combining functionalities).

•  The GWS based on felt layers has a high environmental burden due to the durability aspect and the materials used.

•  Greening the building envelope considering the materials involved, which, as shown, can have a high influence on the environmental profile, and taking into account all the (unquantifiable) benefits is a sustainable option.

Despite the need of additional resources initially, the direct greening system, the indirect greening system with a supporting system based on hard wood, coated steel or HDPE and the green wall system based on planter boxes are the environmentally preferable choice when constructing and retrofitting a building, due to the reduction in energy demand for heating and cooling (this study can be easily applied to other construction types). However, it should be noted that this case study is limited to the facade type, climate, and location of the study but is also depending on the assumptions that are made inside the analysis. Further research is essential for improving the analysis to confirm or refute the assumptions made in this study, especially for the unquantifiable categories (increased biodiversity, human health, the improvement of air quality, mitigation of urban heat effect).

Beside this, economic benefits are involved and could be estimated in a life cost analysis thanks to durability, aesthetical value and social factor. Furthermore, lots of systems now available on the market have different characteristics that could influence the environmental profile.

Acknowledgement

The authors wish to appreciate the support of the University of Tehran, Department of Landscape Architecture, Tehran, Iran.

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