Is greening the building envelope economically sustainable? An analysis to evaluate the advantages of economy of scope of vertical greening systems and green roofs

Accepted Manuscript Title: Is greening the building envelope economically sustainable? An analysis to evaluate the advantages of economy of scope of vertical greening systems and green roofs Author: Katia Perini Paolo Rosasco PII: DOI: Reference:

S1618-8667(15)30125-4 http://dx.doi.org/doi:10.1016/j.ufug.2016.08.002 UFUG 25759

To appear in: Received date: Revised date: Accepted date:

24-11-2015 1-7-2016 10-8-2016

Please cite this article as: Perini, Katia, Rosasco, Paolo, Is greening the building envelope economically sustainable? An analysis to evaluate the advantages of economy of scope of vertical greening systems and green roofs.Urban Forestry and Urban Greening http://dx.doi.org/10.1016/j.ufug.2016.08.002 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Is greening the building envelope economically sustainable? An analysis to evaluate the advantages of economy of scope of vertical greening systems and green roofs Katia Perini1, Paolo Rosasco2 1,2

University of Genoa, Department of Architectural Sciences, Stradone S. Agostino, 37, 16123 Genoa, Italy [email protected] [email protected] Highlights    

This paper analyzes the (possible) advantages of economy of scope of green envelopes A cost benefit analysis of vertical greening systems and green roofs was conducted. Combining the installation of green roof and vertical greening systems increases the economic sustainability. Economic incentives (tax reduction) could reduce personal initial cost allowing a wider diffusion of greening systems to reduce environmental issues of dense urban areas.

Abstract A wide replication of green envelopes can be a good opportunity to improve urban environment conditions, as demonstrated by several studies. Greening systems increase also building envelope performances; however their economic sustainability has not been fully investigated. This study evaluates the economic sustainability of two combined greening systems installed on an office building: a vertical greening system and a green roof, and it evaluates the advantages of economy of scope. The Cost-Benefit Analysis on two different combinations of vertical greening and green roofs considers personal benefits and costs over their life cycle. The results demonstrate the advantages of economy of scope, due to the additional benefits coming from the combination of two different systems. The results show that the tax incentives and the combination of green systems can make the installation and the maintenance costs economically sustainable during the life span of a greening system; this could lead to a wider diffusion of greening systems with higher environmental and aesthetic performances.

1. Introduction A wide replication of green envelopes can be a good opportunity to improve urban environment conditions , mitigating urban heat island phenomenon (Fioretti et al., 2010; Onishi et al., 2010; Ottelé et al., 2010; Taha, 1997). The possible integration modalities of green elements in architecture are many, with a major or a minor influence on the project conception and on the formal and functional characteristics (Perini, 2013). Vegetated roofs, traditionally widespread in northern Europe, may use different plant species, for both their influence on architectural aesthetic and the microclimatic improvements obtainable (Dunnett and Kingsbury, 2008; Fioretti et al., 2010). The many products available on the market propose several integrated solutions for proper drainage, waterproofing, and roof protection depending on the vegetation type, such as grass and bigger or smaller shrubs (Bianchini and Hewage, 2012a; Bouvet and Montacchini, 2007). These are commonly classified in: intensive, semi-intensive and extensive solutions and have different uses, stratigraphy and vegetation (Dunnett and Kingsbury, 2008). For every type of green roof substrate thickness (given by the plant species used), maintenance, system weight, obtainable microclimatic benefits, influence on

architectural aesthetic, costs, and use are different (Bianchini and Hewage, 2012b; Carter and Keeler, 2008). Vertical greening systems are made by simple climbing plants, supporting structures for their growth or planter boxes placed at several heights with a shading function; other provide the possibility to cultivate species otherwise not suitable for growing on vertical surfaces, thanks to the disposition of pre-vegetated panels, defined as “living wall systems” (Köhler, 2008; Perini et al., 2012). These systems entail very different initial costs (i.e. in a range of 3-315 €/m2), maintenance (i.e. in a range of 3-27 €/m2/year) and disposal (i.e. in a range of 31-220 €/m2) (Perini and Rosasco, 2013). 1.1. Green envelopes performances at building scale Studies conducted on green roofs and vertical greening systems demonstrate that systems provide several benefits, both social and personal (Bianchini and Hewage, 2012b; Perini and Rosasco, 2013). A green roof can reduce the energy demand for heating during winter season thanks to its insulation properties; during summer, vertical and horizontal building surfaces covered by plants improve thermal comfort reducing the energy demand for air conditioning (Alexandri and Jones, 2008; Kosareo and Ries, 2007; Perini et al., 2011). Permpituck and Namprakai (2012) show that green roofs (10 cm and 20 cm substrate thickness) compared to a bare roof reduce the heat transfer (respectively by 59% and 96%) and energy consumption (respectively by 31% and 37%). Similar studies demonstrate that green roofs also reduce the heat flow between 51% and 63% (Morau et al., 2012). During summer a wet green roof can increase the heat dissipation through evapo-transpiratory cooling, reducing the energy demand for air conditioning (Barrio, 1998). With high solar radiation (1,400 W/m2), a surface temperature differs between a bare roof and a soil roof under a dense vegetation layer by up to 31,4 C° (Wong et al., 2003). Studies demonstrate that a vertical green layer can contribute to the building envelope performances by creating an extra stagnant air layer, which has an insulating effect (Perini et al., 2011), and reducing the energy demand for air-conditioning up to 40-60% in Mediterranean climate (Alexandri and Jones, 2008; Mazzali et al., 2012). Another relevant benefit of green roof systems regards stormwater management, with a reduction in stormwater runoff in a range of 60%-100%, depending on system’s characteristics, and climatic conditions (Hashemi et al., 2015; Nawaz et al., 2015; Wong and Jim, 2014), while improving also water quality (Vijayaraghavan and Joshi, 2015; Vijayaraghavan et al., 2012). Vegetation and other layers of green roofs or vertical greening systems can increase the roof longevity from 20 to 40 years (Clark et al., 2005). Studies demonstrate that green roofs and plants along a façade improve also the aesthetic of a building and its real estate value (François et al., 2002; Gao and Asami, 2007; Peck et al., 1999). An intensive or extensive green roof is similar to a green area: Peck et al. (1999) demonstrate that the real estate value of a building can increase from 6 to 15% with the presence of a green roof or green wall. Studies investigated also the relation between worker productivity and the presence of vegetation: employees in office buildings who had view on green area (garden, etc.) increase their productivity (Kaplan et al., 1988). 1.2. Green envelope environmental benefits Greening systems in dense urban areas provide relevant social benefits, mainly related to air quality improvement, also due to lower greenhouse gas output production, mitigation of urban heat island (UHI) effect, improvement of urban wildlife and plant species biodiversity, increasing also the quality of urban space (Dunnett and Kingsbury, 2008; Goddard et al., 2010; Onishi et al., 2010). Vegetation improves air quality: gaseous pollutants can be dissolved or sequestrated through stomata on plants and leaves (McPherson et al., 1994). Tan and Sia (Tan and Sia, 2005) sampled roof temperatures and other air quality parameters both pre and post green roof installation in Singapore; using light sensors, volume aerosol samplers and particle counters they found that acid gaseous pollutants, carbon mass levels and ambient green roof surface temperature dropped significantly after

the installation of green roofs. A green roof located in the urban dense area of Chicago can absorb up to 52% of O3, 27% of NO2, 14% of PM10 and 7% of SO2 (Yang et al., 2008). High levels of pollution in the atmosphere and the “cementification” of urban cause the Urban Heat Island (UHI) phenomenon, resulting in the dramatic two to five degree Celsius temperature difference between cities and their surrounding suburban and rural areas (Taha, 1997). Though the UHI phenomenon has regional-scale impacts on energy demand, air quality, and public health, mitigation strategies, such as urban forestry, living (green) roofs, and light colored surfaces, could be implemented at the community level (Rosenzweig et al., 2006). Although UHI can be mitigated with large amount of surfaces with higher albedo (e.g. green areas) (Rizwan et al., 2008), larger green areas like urban parks may be more effective (Petralli et al., 2006). Akbari (2005) shows that the mitigation of the urban heat island effect with trees, green roofs and green façades can reduce the U.S. national energy consumption for air conditioning up to 20%, saving of more than $10 billion per year in energy costs. 1.3. Economic sustainability of greening systems The economic sustainability of green roofs has been investigated by several authors. Wong et al. (2003) evaluate the economic sustainability of intensive and extensive green roofs and demonstrate that only extensive green roofs are economically sustainable, due to higher installation and maintenance costs for intensive green roofs. Bianchini and Hewage (2012b) evaluated, by means of a Cost Benefit Analysis (CBA), the economic sustainability of intensive and extensive green roofs, demonstrating that both are economically sustainable from social and personal point of view (respectively a Net Present Value –NPV- of 3,606 $/m2 and 5,715 $/m2); in their case study the two authors considered a tax incentive of 48 $/m2; so the financial risk for installing any green roofs type is very low. Other studies (Carter and Keeler, 2008; Clark et al., 2008) show the economic sustainability of green roofs compared to traditional roofs calculating the NPV by means of CBA. In a case study located in Flanders, Claus and Rousseau (2012) discovered that an extensive green roof in Flanders is economically sustainable in two case scenarios (base and best, with public subsidies). In the worst scenario or without subsidies, the systems analysed are not economically sustainable. A study conducted by Perini and Rosasco (2013) demonstrates that in an ordinary scenario of cost and benefit values a direct green façade with a well grown Hedera helix and an indirect green façade with the vegetation supported by a plastic mesh can be economically sustainable, due to the low installation and maintenance costs during a life span of 50 years. For the other systems analysed (indirect green façade with planter box with steel mesh and living wall system) economic indicators show that they are not sustainable even in a best case scenario. The authors suggest that the economic sustainability of such systems can be significantly increased by reducing the initial costs for promoters; this can be achieved through government incentives. For example, the city of New York enhanced installation of green roofs allowing one-time tax abatement of 48,50 $/m2 (up to $100,000

or the building's tax liability, whichever is less) wherever the green roof covered at least 50% of the total roof area (NYC Energy Efficiency Corporation (NYCEEC), 2015). 1.4. Aim of the study The aim of the study is to evaluate the economic sustainability of two combined greening systems installed on an office building: a vertical greening system and a green roof, taking into account economy of scope. Previous studies evaluate the economic sustainability of single greening system: green roof or vertical greening; none evaluate the economic benefits and costs of combined greening system (green roofs + green façade); in fact it is interesting to evaluate if there are additional benefits coming from the combination of two different systems, e.g. reduced installation and maintenance costs. Moreover, tax incentives are higher, especially for new buildings, as benefits are as well. The present economic evaluation takes into account the social benefits only in qualitative terms; because of the small size of

the greened surface (vertical and horizontal) the social benefits obtainable are very low compared to the personal economic benefits (Perini and Rosasco, 2013). This does not mean that these benefits are insignificant but that they can be taken into account only in a large scale of evaluation (districts or urban areas). 2. Methodology This study develops a CBA for two different combinations of vertical greening and green roof installed on an office building considering personal benefits and costs over their life cycle. The CBA was created to verify the economic sustainability of public projects in USA and has been already used to evaluate different vertical green systems and green roofs (Carter and Keeler, 2008; Clark et al., 2008; Perini and Rosasco, 2013). The combinations of greening systems are the following: 1. System A: extensive green with 5 cm substrate thickness planted with sedum + indirect green façade with a well grown Hedera helix supported by a steel mesh (Fig. 1); 2. System B: intensive green roofs with 20 cm substrate thickness planted with plant and shrubs + indirect green façade combined with steel planter boxes with a well grown Hedera helix supported by a steel mesh (Fig. 2). This study is based in the city of Genoa, a dense city in the northern part of Italy facing on the Mediterranean sea. This location is considered to evaluate costs and benefits (installation costs, energy savings for heating and cooling, real estate value) influenced by a specific location. The two systems analysed are installed on an office building. This was designed for the CBA according the local regulations (Comune di Genova, 2010) and considering the most common characteristics and technologies. The building envelope is made of a double brick wall with an air cavity and mineral wall (thickness of insulation material 8 cm; according to the local regulations; Regione Liguria, 2012). The vertical greening systems are applied to the south façade (total surface without windows of 215 m 2). The green roof total surface is 225 m 2. The economic sustainability of each solution is calculated by means of two indicators: 1. the Net Present Value (NPV), i.e. the discounted value of benefits less costs that occur during the period of life considered; 2. Pay Back Period (PBP), i.e. length of time required to recover the cost of the investment. This indicator is relevant in order to know when the economic benefits occur during the life span of each system. The CBA is founded on the discounting method of costs and benefits during the lifespan in order to calculate their equivalent values referred to the same period (normally, the moment of the evaluation) and the two indicators of economic sustainability. The life cycle of each system is considered to develop this analysis. The life expectancy of the vertical greening systems is assumed to be 50 years (Dunnett and Kingsbury, 2008; Ottelé et al., 2011). The maximum lifespan of a green roof is about 55 years (Acks, 2006); while, the minimum has been estimated as about 40 years (Clark et al., 2008). In this study, a lifespan of 50 years is assumed for both greening systems. Water pipes of the automated watering systems needed for the indirect greening systems combined with planter boxes and intensive green roof are assumed to be replaced every 7.5 years due to crystallizing of salts (Ottelé et al., 2011). The benefits related to the installation of vertical greening systems depend on the plants growing speed. For indirect greening system the full covering of the façade by Hedera helix is estimated after 15 years (Bellomo, 2003) and the benefits are calculated after 10 years from installation. For indirect greening system combined with planter boxed, benefits are calculated after three years to allow a complete coverage. The discounted rate was assumed equal to 3,5%, i.e. an average private discount rate which reflects the capital cost of a private investor in the last four years. This CBA is in part based on published data from other green roof and wall researches and practices to estimate their (positive) effects. This may introduce some bias and indicates that this work is subjected to revision as experience increase with more and better data obtained from researches on

vertical green and green roofs.

3. Data collection and calculation 3.1. Costs For each greening system cost and benefits are estimated within their lifespan (Table 1); some of the installation works as provisional structure (scaffolding) or costs related to technical installation (e.g. irrigation) can be used for both greening system (green façade and green roof). Hence the installation costs of each system (green façade + green roof) are lower compared to the sum of the two different single installations (economies of scope). The costs considered for the vertical greening System A are the following: pot structure at ground level, installation of vertical supporting structure (steel mesh) and Hedera helix plants; no irrigation is needed for System A. For System B: installation of vertical supporting structure (steel mesh), pots and plants of Hedera helix. The same irrigation system can be used for both greening systems, allowing a saving of about of 6,000 €. The installation costs of both systems are calculated considering the use of only one scaffolding structure. During the lifespan of 50 years the annual maintenance costs consists of different elements, depending on the greening system type. For the vertical greening System A, pruning to be carried out every year with costs differing from 3rd to 5th year and compared to the remaining years of service life, while for System B - since a watering system is needed - maintenance includes also the water pipes and water supply. In addition once per year the substitution of some plant species is calculated. For both systems a cladding renovation is considered at the 50th years (instead of the 35th year, because of the protection developed by the vertical green layer). The installation costs of green roofs (extensive - System A and intensive - System B) include different layers (waterproofing and root barrier, drainage layer, growing medium: 5 cm. substrate thickness for extensive green roof and 20 cm substrate thickness for intensive green roof). In addition along the perimeter of both roofs a steel railing is installed while - only for the intensive green roof - a stone pavement path and an irrigation system are installed. For both green roofs, an increase of horizontal roof structure costs related to the higher weight is considered (in total about 2,500 € for extensive green roof and 3,150 € for intensive green roof). For the economic evaluation of the two systems, the different installation and disposal costs of the green roof compared to a traditional roof are considered as a higher additional cost: for the installation the difference considered is about 63 €/m2 for extensive green roof and 138 €/m2 for intensive green roof (comprehensive of design cost) while for disposal cost the difference is 20 €/m 2 and 8 €/m2. The costs during the lifespan are estimated at the time of valuation (2015) and indexed using the annual inflation rate. The disposal costs of each element (vertical supporting system, plants, etc.) are considered at the end of their lifespan. They include the greening systems disposal (removal of plants and structures, transport to landfill, and dump taxes), and the cladding (plaster) and rooftop renewal. These costs for all the greening systems analysed were obtained from product forms and information provided by companies and by the regional price lists (Unioncamere Liguria, 2012). 3.2 Benefits 3.2.1. Property value The presence of vegetation in urban areas can affect the economic value increasing property price or rent due to aesthetic aspects (greening systems can improve the aesthetic quality of buildings) and, in the case of walkable green roofs, providing recreational and liveable spaces: according to Peck et al. (1999) a green wall would yield the same property increase as a "good tree cover" and therefore a

value increase interval for a property of 6-15% with a midpoint of 10.5% can be estimated. Lower values (increase 3.9% of the property value) are estimated by François et al. (2002) using a regression model for hedges or green walls. In the case of green roofs higher values were found with an increase of house property values in New York City of 16.2% compared with houses without green roof (Ichihara and Cohen, 2011). According to Tomalty and Komorowski (2010) the green roof type highly affects the property price increase: 20% for recreational green roofs and 7% for “productive” green roofs (including vegetables and fruit). The location affects the influence of vegetation: Gao and Asami (2007) applied a hedonic pricing of greenery founding that an increase in greenery quality level would increase land price by 1.4% in Tokyo and by 2.7% in Kitakyushu. Since the present study is based in Genoa, it considers the effects on the real estate values of this city in terms of rent variation due to the presence of greening systems on the building. According to Sdino (1998), vertical green and green roofs are relevant to three (of 26) real estate features which contribute in the real estate value of buildings: "building" related to typology and aesthetic of the building; "pollution" related to level of noise and air pollution perceptible inside the building; "green" related to the presence of green/green areas (also not directly accessible) closed to the building. The contribution of each feature in real estate value (estimated in terms of percentage) is related to the building within the real estate market of Genoa (Sdino selects 8 locations: central area, peripheral area, semi peripheral area, historic centre, etc.; Sdino, 1998). For “building” feature, the percentage contribution varies from a minimum value of 3.8 for semi-peripheral location to 8.2 for historic areas; for the acoustic “pollution” feature the percentage contribution varies from a minimum value of 0.3 for historic areas to 12.7 for a semi-peripheral location; for the “green” feature the percentage varies from a minimum value of 0.8 for a peripheral location to 3.5 for semi-peripheral location. In this study, the average percentage feature contribution for a central and a semi peripheral location is considered; due to the installation of vertical greening systems only on a façade of the building (3 sides are no-greened) and to the different type of green systems installed, the estimated average increase of real estate are represented in the second column of table 1. For the estimation of the total incremental value, these percentages are multiplied to the average real estate value for this type of buildings in Genoa collected by the OMI (Real Estate Market Watch Agency Land - Provincial Bureau of Genoa) in the second semester of 2014 within the locations considered; the value estimated for building offices is 2,200 €/m2. In order to estimate the incremental value due to the presence of green systems, the value is multiplied to the percentage feature contribution (3td column - table 1). By applying an annual capitalization rate of 4% (the rate is estimated by the values published by the OMI for this type of buildings) an annual increase in rental income is estimated. In fact, for this type of property (office building), the rental market is more significant. For System A, the annual increases of rent related to the specific locations assumed in this study is 2.20 €/m2 due the presence of the indirect green façade and 1.76 €/m 2 due the presence of the extensive green roof; for System B the annual increase of rent is 3.08 for indirect green façade with pot and 7.48 €/m2 for intensive green roof (as the latter can be used by employees as a small garden) (4th column - table 1). Considering the total offices surface of 384 m2 within the building, the annual increases of building rent due the presence of green installations is about 1,520 € for System A and 4,050 for System B.

3.2.2. Energy saving for heating and air conditioning The economic benefits deriving from to the installation of greening systems are also related to the reduction of energy demand for air conditioning and heating. The annual energy demand for air conditioning has been calculated thanks to a simulation model (Termo Microsoftware) of the virtual building used in this analysis. A literature review allowed the calculation of the input data used for the simulations.

Kotsiris et al. (2012) examined on a test cell located in Athens (Greece) different green roof systems scenarios. The estimated thermal transmittance of the green roof component with a 20 cm deep perlite mix substrate were 0.525, 0.492, 0.489, 0.435 and 0.441 for moisture contents of 49.26%, 48.54%, 47.35% 41.56% and 40.18% respectively. The estimated U-values for the substrate with 10 cm deep perlite mix were 0.651, 0.55 and 0.548 Wm -2 K-1 for moisture contents 36.61%, 33.28% and 32.95%, respectively. In each case, a linear relation seems to exist between the determined thermal transmittances of the green roof substrates and their moisture contents. According to these results, the substrates' thermal conductivities increase when the water content of the substrate ranges from 0.05 to 0.7 Wm-1 K-1 (Ouldboukhitine et al., 2012). This study calculates the potential energy saving due to the installation of the green roofs (A, B), to this end, assumptions were made starting from the results obtained by Kotsiris et al. (2012), and considering the worst case scenario in terms of moisture content (thermal transmittance with highest moisture content assumed). For the intensive green roof (20 cm substrate) 0.525 Wm-2.W -1 was assumed (moisture content 49.26%); for the extensive green roof (5 cm substrate), 0.909 Wm-2.W -1 (moisture content 37.5%). Considering the different thermal transmittance and insulation properties of the two green roof systems analysed in this study, the estimated total annual economic benefit for heating is 41.47 € for extensive green roof and 63.71 € for intensive green roof (table 2). Green roofs improve the energy savings for cooling if installed on a well-insulated roof because of the additional cooling effect of evapotranspiration on the building shell in Greece (Kotsiris et al., 2012). The energy saving due to a 20 cm substrate thickness, considering an insulated roof according Greek EPBR, is in a range of 11.89-15.45% depending on the green roof type and substrate (perlite, pumice, rock wood). Simulations for a one-storey office building show that in summer, no advantage is achievable adopting a short-sedum vegetation, while good performances are achieved by tall gramineous vegetation (8.2% cooling demand in Rome, 8.5% in Amsterdam (Ascione et al., 2013). The estimated annual cost for cooling for the whole building without greening systems is 9,031 € (only for the last floor is 4,846 €); the percentage of saving for cooling indicated from literature for the presence of an extensive green roof ranges from 5 to 10%, while for an intensive green roof from 10 to 15%. In this study average values are assumed; so the annual saving for cooling is 363.47 € for extensive green roof and 605.78 € for intensive green roof (table 2). To calculate the energy savings for heating, due to the increase of the insulating properties with vertical greening systems, the additional thermal resistance is assumed to be 0.09 Km 2W -1. This value is used for both systems analysed due to stagnant air layer in and behind the foliage (Perini et al., 2011). The annual economic benefit is 5.80 € (table 2). The potential energy saving for air conditioning which can be obtained with vertical greening systems depends on the climate. Insulation material moderates the prevailing temperature difference between the outside and inside (Mazzali et al., 2012; Ottelé, 2011; Scarpa et al., 2014): in this study, due the presence of vertical façade and considering the location used for this study (Genoa, Italy) and the exterior wall stratigraphy, in System A and B an energy saving of 10% for air conditioning is considered; the annual benefit for both vertical green is 903.14 € (table 2).

3.2.3 Longevity Both green roofs and vertical greening systems reduce the frequency of intervention for maintenance thanks to a protective action: leaves and other layers involved delay the decay of the underlying surface caused by UV rays, temperature changes, acid rain, ice, and air pollution reducing the deterioration (Peck et al., 1999; Wong et al., 2010, 2003). According to several studies, the lifespan of green roof is in a range of 40-55 years (Bianchini and Hewage, 2012b; Clark et al., 2008; Kosareo and Ries, 2007; Saiz et al., 2006). Conventional roofs need maintenance every 25-30 years (Di Giulio, 2003). Therefore the presence of green roofs allows a reduction of maintenance costs compared to a conventional roof, resulting in an economic benefit. In this study, a life span for intensive and extensive green roofs of 50 years is assumed instead of 30 years assumed for an

ordinary roof. The saving for longevity roof in System A - discounted to the first year – amounts to 13,894 € while in System B is 13,246 € (table 3). For vertical surfaces, the frequency of maintenance of a plastered façade (complete remaking of the coating layers and painting) depends on the quality of the plaster and the environmental conditions (pollution, precipitation frequency, etc. (Di Giulio, 2003): from 25 and 30 years or more. In this study it is estimated that without a green layer the renovation of the façade would have to be realized in the 35th year while green lengthens the coating lifetime by 15 years (incremental value assumed in the current examination for all solutions of considered). Therefore, at the end of the 50 th year (the end of the life cycle assumed for this study), a maintenance of the plaster façade must be done. The longevity benefit is equal to the cost recovery that should have been borne at 35 th year and the lower cost at 50th year due to a better condition of the plaster. The vertical building surface requires at that point removal of all of plaster facade (1st layer, 2nd layer and surface finishing) in the case of System A (indirect green façade), remaking and subsequent painting, while for System B building surface requires for most of the area only the 2nd layer and surface finishing. The saving for plaster longevity - discounted to the first year - amounts to 14,196 € for System A and 20,896 € for System B.

3.2.4. Tax reduction In Italy, about 328 local municipalities regulate the installation of green roofs; 22 oblige to cover part of new roof with a green roof while 23 incentive green roof installation (Cresme and Legambiente, 2013). For new public buildings some local municipalities establish also a minimum surface to be covered by vegetation instead of realizing an equivalent green area on the ground (Cresme and Legambiente, 2013). Building owners can exploit a tax relief between 50% and 65% of the initial costs related to energy saving interventions (e.g. thermal insulation) spread over a time period of 10 years (Decree Law n. 63). Since greening the building envelope ensures a substantial energy saving, as shown in the present study, the Decree Law n. 63/2013 can be applied to obtain the 65% tax incentives (applied on the installation costs) for green roof installation. For extensive green roof, the annual tax relief (from 1st year to 10th) year is 3,347 € while for intensive green roof is 4,513 € (table 2). In the case of green façades tax relief cannot be applied, although such systems can significantly increase performances of the building envelope.

4. Discussion and analysis 4.1. System A: extensive green roof and indirect green façade The values obtained from the economic indicators show that System A is economically sustainable. The NPV is positive and equal to 57.40 €/m 2 (Graph 1). The PBP (i.e. number of years needed for the discounted economic benefits to reach the costs) is 14 years (Graph 2). Both values assume the use of tax incentives only for the green roof (65% of installation costs). If tax incentives were assumed for both greening systems, the NPV would be 72% higher (99,38 €/m2); the PBP 8 years lower. These values highlight the importance of tax incentives for the economic sustainability of such systems.

4. 2. System B: intensive green roof and indirect green façade with planter boxes The System B analysed in this study shows positive NPV values, although 50% lower of System A, due to the higher installation costs and the higher maintenance needs (irrigation system, plants

substitution, etc). The value is equal to 29.27 €/m2 (Graph 1). A similar trend results for the PBP with a value of 20 years (Graph 2). Also for this system, the economic sustainability significantly improves in the case of tax incentives considered also for the vertical greening system: the NPV results amount to 85.80 €/m2 while the PBP decreases from 20 to 9 years. Comparing this result with System A, a major decrease in PBP can be noticed, due to the higher installation costs of the system and, consequently, the higher tax incentives.

4.3. Overview The results of the CBA show that the economic sustainability of greening systems depends on the installation and maintenance costs: for System A the economic indicator show the full sustainability of the system; for System B the economic benefits (energy savings for heating and air conditioning, and longevity of plaster façade) balance the higher costs of installation and maintenance during the life span. However the combination of two systems obtains higher economic performance, compared to a single green installation (green façade and green roof). Assuming the installation of both a green roof and a green façade, the combined use of some elements (scaffolding, irrigation) and unique design determine some economies of scope, with a total cost saving of 7.1% for System A and 8.2% for System B (table 3). Beyond the specificity of the case study examined, the CBA shows that the integration of two greening systems (i.e. green roof + vertical greening system) improves the value of two indicators of economic sustainability compared to those obtained considering the two green systems separately (Graph. 3). For System A, the NPV of the two green systems separately and only with tax incentives for green roof is 49.68 €/m2, while considering a system as a whole it is equal to 57.40 €/m2; for System B the values are respectively 17.02 €/m2 and 29.27 €/m2 (Graph 3). The combination of two systems reduces also the PBP: for System A, it is 14 instead of 17 while for System B 20 years instead of 27 years (Graph 4). The sensitivity analysis shows how the variables influence the NPV unitary value, considering the personal costs and benefits (with tax reduction only for green roof). For System A the non-economic factors which negatively influence the NPV value are installation and maintenance costs (Graph 5); the same trend is noticed for System B, although the irrigation system and the maintenance cost of the indirect green façade with planter boxes have a major negative influence (an increase of 20% reduce about 100% the NPV) (Graph 6). For both systems the most relevant economic factor that positively influenced the NPV is tax reduction: with a 20% increase (from 65% to 80%) the unitary NPV increases up to 46% for System B and about 100% for System B (from 33.78 €/m 2 to 69.85 €/m2 because of the higher installation cost used to calculate the tax reduction). In Italy this could be possible with the use of tax incentives also for vertical greening systems. Worth mentioning that although social benefits are not considered in this study (due to the difficulty of estimating the effects of a single façade), a wider diffusion of greening systems can play an important role for improving environmental quality of dense urban areas, therefore tax incentives should be considered as social costs. Economic saving for heating and cooling, which can be increased with a careful design and material choice, is relevant as well. Discount and inflation rates are the less relevant (positively or negatively) factors. Even if the tax incentives have a negative social economic impact, this is the key factor for a wider diffusion of green envelopes with positive social effects. Table 4 shows the percentage variation for each variable annulling the NPV value; the results underline that for both systems the most critics variables are installation and maintenance cost: for

System A, if the installation cost increase about 45%, the NPV is equal to zero. For system B, the same effect occurs with a 20% increase of the maintenance cost. This happens due to the high relevance of such cost within during the life spam. For System B also tax reduction plays an important role: with a reduction of 23% the economic sustainability is not verified (Table 4). Otherwise the NPV diminish but is never equal to zero even if some variable don’t explain their positive effect: five in System A (saving heating and cooling, longevity façade, longevity roof and inflation rate) and three in System B (saving heating, longevity roof and property value).

5. Conclusion The present study evaluates the economic sustainability of two combined greening systems installed on an office building: a vertical greening system and a green roof. The CBA compares the personal benefits and costs over the life cycle of two combined solutions (green roof + vertical greening system) evaluating also separately installation costs and benefits. The results highlight two important aspects:  tax incentives play an important role for the economic sustainability of greening systems: tax incentives not only for green roofs installations but also for vertical greening systems have positive effects on the global economic results (in Italy as in other countries such systems are not considered for tax incentives).  additional benefits derive from the combination of two different systems due to lower installation and maintenance costs, demonstrating the advantages of economy of scope. This is relevant especially for System B because of its higher installation and maintenance costs. Although all the single greening systems analyzed - extensive green roof, indirect green façade, intensive green roof, and indirect green façade with planter boxes - can be economically sustainable, for the case of intensive green roofs and vertical green façade with planter box the economic sustainability is at the edge of acceptability; otherwise, the combination of two systems significantly improves NPV and PBP, especially in the case of System B. The results show that the tax incentives and the combination of green systems can make the installation and the maintenance costs economically sustainable during the life span of a greening system; this may result in a wider diffusion of greening systems with higher environmental and aesthetic performances.

Acknowledgements Luca Capanna, Ing. Dr. Enrica Cattaneo and Umberto Valle are acknowledged for their fundamental help for the energy saving calculations. The vertical greening systems companies and Peirano Vivai (Genoa) that provided the data needed for this analysis are also acknowledged.

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Fig. 1. System A: extensive green roof and indirect green façade

Fig. 2. System B: intensive green roof and indirect green façade with planter boxes

NPV (€/m2)

Graph 1. System A and B: NPV for two cases: with tax incentives only for green roof and with tax incentives for both greening systems. 110 100 90 80 70 60 50 40 30 20 10 0

tax incentives only for green roof tax incentives for both greening systems

System A

System B

Graph 2. System A and B: PBP for two cases: with tax incentives only for green roof and with tax incentives for both greening systems.

PBP (number of years)

30

tax incentives only for green roof

25 20 15

tax incentives for both greening systems

10 5 0

System A

System B

NPV (€/m2)

Graph 3. System A and B: NPV obtained considering single green system as a whole (with tax incentives only for green roof) 110 100 90 80 70 60 50 40 30 20 10 0

Green system as a whole

Single green systems

System A

System B

Graph 4. System A and B: PBP obtained considering single green system as a whole (with tax incentives only for green roof)

PBP (number of years)

30 25 Green system as a whole

20 15 10

Single green systems

5 0 System A

System B

NPV (€/m2) variation (%)

Graph 5. System A - Sensitivity analysis for the personal economic factors 60%

Saving heating

40%

Saving cooling Tax reduction

20%

Longevity facade 0%

Longevity roof

-20%

Property value

-40%

Inflation rate Discount rate

-60%

-20% -15% -10% -5%

0%

Variation

5%

10% 15% 20%

Installation cost Maitenance cost

Graph 6. System B - Sensitivity analysis for the personal economic factors 100%

Saving heating

80%

Saving cooling

NPV (€/mq.) variation (%)

60% 40%

Tax reduction

20%

Longevity facade

0%

Longevity roof

-20% -40%

Property value

-60%

Inflation rate

-80%

Discount rate

-100% -20% -15% -10% -5%

0%

Variation

5%

10% 15% 20%

Installation cost Maitenance cost

Table 1 – Greening system and percentage increase in real estate value R. E. value increment (%)

System

A Indirect green facade

R.E. value increment (€/m2)

R.E rent increment (€/m2/year)

2.5

55.00

2.20

2.0

44.0

1.76

4.5

99.0

3.96

B Indirect green façade with planter boxes

3.5

77.0

3.08

Intensive green roof

8.5

187.0

7.48

12.0

264.0

10.56

Extensive green roof Total

Total

Table 2. Personal costs and benefits of Systems A and B (included value added tax)

System A

Cost (€/m2 )

Category

Cost

Time frame

Initial

Dig + pot

One time

425,09

Supporting system + transport

One time

Installation of steel mesh Plant species and installation

Time frame

Energy saving for heating

Annual - 11-50th year

5,80

104,63

Energy saving for cooling

Annual - 11-50th year

903,14

One time

76,98

Increase income property

Annual - 11-50th year

843,48

One time

1,54

Annual - 3-5th year

0,52

Cladding longevity

One time 35th year

14.196,06

Tax reduction

Annual - 1-10th year

3.285,04

*

Steel indirect green facade

Mainteinance

Pruning Annual - 6-49th year

Extensive green roof

Benefit (€/year)

Benefit

2,59

Cladding renovation

One time - 50th year

258,31

Disposal

Green layer disposal

One time - 50th year

50,04

Initial

All layers

One time

134,24

Design

One time

4,39

Energy saving for heating

Annual - 1-50th year

41,71

Pruning

Annual - 2-49th year

2,09

Energy saving for cooling

Annual - 1-50th year

363,47

Green layer and membranes

One time - 50th year

38,40

Increase income property

Annual - 1-50th year

674,78

Roof longevity

One time - 25th year

13.894,51

Mainteinance Disposal

Other costs

Design

One time

4.016

Scaffold

One time

18.220

Municipality fees

One time

1.260

Plant species

One time

26,15

Energy saving for heating

Annual - 4-50th year

5,80

Supporting system + transport

One time

104,63

Energy saving for cooling

Annual - 4-50th year

903,14

Installation of steel mesh

One time

76,98

Increase income property

Annual - 4-50th year

1.180,87

Planter boxes

One time

26,15

Design

One time

19,65

Pruning

Annual until 49th year

1,31

Irrigation (H2O)

Annual

1,60

Plant species replacement (5%)

Annual until 49th year

5,19

Cladding renovation

One time - 50th year

240,25

Cladding longevity

One time 35th year

20.826,25

Disposal

Green layer disposal

One time - 50th year

59,45

Initial

All layers

One time

209,20

Tax reduction

Annual - 1-10th year

4.457,94

Irrigation system

One time

33,64

Energy saving for heating

Annual - 1-50th year

63,71

**

**

**

System B Initial Steel indirect green facade with planter boxes with planter boxes

Mainteinance

Intensive green roof

Mainteinance

Disposal

Other costs

* €/ml of pot lenght ** € (total cost) *** € (value discounted to first year)

Design

One time

7,85

Energy saving for cooling

Annual - 1-50th year

605,78

Plants

Annual until 49th year

4,65

Increase income property

Annual - 1-50th year

2.867,83

Irrigation (H2O)

Annual

1,28

Roof longevity

One time - 25th year

13.246,10

Irrigation system

Annual - 3-49th years

2,18

Green layer and membranes

One time - 50th year

48,00

Design

One time

5.932

Scaffold

One time

18.220

Municipality fees

One time

1.260

**

**

**

Table 3. Cost savings of Systems A and B compared to single green façade + green roof. Item

Saving (€/mq.)

System

Installation

A B

Irrigation system maintenance

A B

Design

A B

7.47 11.20

Saving* (€/year)

Saving* * (%) -

6.6 7.7

0.11 0.52 0.89

-

0.5 0.5

* Annual cost saving (calculated at 1st year of life span) ** On installation total cost

Table 4. Percentage variation of the variables that annul the NPV Variable

Percentage variation (%) System A

System B

En. saving for heating

-

-

En. saving for cooling

-

-41,1

-77,7

-23,3

-

-73,5

Longevity roof

-

-

Property value

-90,7

-

Tax reduction Longevity facade

Inflation rate

-

321

Discount rate

+171

+168,0

Installation cost

+45,0

+26,4

Maintenance cost

+66,0

+19,8

25