تجزیه و تحلیل هزینه و منفعت برای نماهای سبز و سیستم های دیوار زندگی

Vertical greening systems can be used as a mean to improve the environmental conditions of dense urban areas. Several researches have proved the environmental benefits of green envelopes. It is still not clear if vertical greening systems are economically sustainable, differently several Life Cycle Cost Analysis and Cost–Benefit Analysis determined economic costs and benefits of green roofs. This paper presents a Cost–Benefit Analysis of different vertical greening systems – green façades and living wall systems – considering personal and social benefits and costs over their life cycle. Installation, maintenance, and disposal costs of each analysed system are compared with the related private and social benefits (increase of real estate value, savings for heating and air conditioning, cladding longevity, air quality improvement, etc.), determining three indicators: the 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 analysed are economically sustainable. Economic incentives (tax reduction) could reduce personal initial cost allowing a wider diffusion of greening systems to reduce environmental issues in dense urban areas, such as urban heat island phenomenon and air pollution.

Greening the building envelope can be an opportunity to restore the environmental quality of dense urban areas by combining nature and built areas [1]. In general terms the main benefits connected to a green building envelope regard environmental practices, economics, and social aspects, as the greenhouse gases output reduction, climate change adaptation, air quality and indoor and outdoor comfort conditions improvement, urban wildlife (biodiversity), etc.; these benefits concern several fields, which are all related and operate on a scale range; some only work if a large surface in the same area is greened and their benefits are only apparent at a neighbourhood or city scale, others operate directly on the building scale [2], [3], [4] and [5]. Greening horizontal surfaces with intensive and extensive green roofs are widespread especially in the northern part of Europe and several studies investigate their economic benefits [6], [7], [8] and [9]. Wong et al. [8] compare the cost of traditional roof solutions with green roof solutions. Bianchini and Hewage [9] perform a probabilistic analysis to estimate personal and social NPV and pay back period of green roofs. Claus and Rousseau [10] perform a private and a social CBA for green roofs. Vertical greening systems can be classified into façade greenings and living walls systems according to their growing method [2] and [3]. Green façades are based on the use of climbers attached directly to the building surface, as in traditional architecture, or supported by cables or trellis. In the case of an indirect greening system, where cables or meshes support vegetation, many materials can be used as a support for climbing plants such as steel (coated steel, stainless steel, galvanized steel), different types of wood, plastic or aluminium. Indirect greening systems can be combined with planter boxes at different heights of the façade. In this case the system requires nutrients and a watering system, if the rooting space is not sufficient. If nutrients and a watering system are needed, it can be defined as a living wall system [2] and [3]. Living wall systems (LWS), also known as green walls and vertical gardens, are constructed through the use of modular panels, each of which contains its own soil or other artificial growing mediums, for example foam, felt, perlite and mineral wool, based on hydroponic culture, using balanced nutrient solutions to provide to the whole or part of the plant food and water requirements [2].

Vertical greening systems provide personal and social benefits. The CBA determined that installation and maintenance services costs play a fundamental role on the economic sustainability. For some of the systems analysed in this study the benefits can not pay back these costs. The most relevant benefits calculated in this CBA are connected with the real estate and the energy savings for summer air conditioning. While all the benefits could be considered for studies based in other locations, the latter benefit is connected with the specific location of this study characterized by Mediterranean climate. The social benefits calculated in this study have a very small influence on the CBA, since only the benefits connected to air quality improvement and carbon reduction have been quantified. However also the benefits mainly related to macro scale can be considered as additional social benefits. From the presented CBA it can be concluded that: • The Direct green façade (1) is economically sustainable for all the scenarios assumed in this study with an average pay back period of 20 years; • The Indirect green façade (2A–B) can be (almost) sustainable depending on the material used for the supporting system: in the case of HDPE mesh the NPV are positive for middle and best scenario with a pay back period of 16 years (best scenario); the higher initial costs for the indirect greening system base on steel mesh determine a positive NPV only for the best scenario; • Indirect green façade combined with planter boxes (3A–3B) presents a (minimum) economic sustainability only for the best scenario due to its installation and maintenance costs (supporting system + watering system); • The living wall system analysed in this study can not be considered economically sustainable due to high (compared with the other greening systems analysed in this study) installation and maintenance costs. Beyond the systems examined in this study, some vertical greening systems can be very expensive and difficult to maintain; this is an important aspect, since it is mandatory to minimize the economical impact to enable a wide use of greening systems to improve dense cities' environmental conditions. Installing vertical greening systems would be an attractive business if social benefits were (more) taken into account by governments; considering that ecological and environmental benefits' financial support (government incentives) could be provided for new and existing buildings. By reducing initial costs for the installation of vertical greening systems the economic sustainability of such systems could be significantly increased, enhancing a wider diffusion.