Greening the built environment is one of the most feasible and cost effective mitigation options for building sectors in rural and low density urban areas. Simple techniques, such as providing a garden and a pond, can be found in traditional houses in many countries. Taking a traditional house setting in Vietnam for example, plants in the garden provide vegetables and fruit, absorb carbon dioxide, offer shade and cool the ambient temperature. The pond collects rainwater run-off, supplies water for garden irrigation, and can be used to grow fish, and create a pleasant microclimate through evaporative cooling.
Building integrated greenery systems allows the provision of greenery beyond the conventional garden and courtyard, to the building itself (such as roof and façade) and even becoming part of a building component (such as a sky terrace). These technologies are relevant in high density urban settings, where land is scarce. They provide multiple benefits, such as lowering the ambient temperature, acting as additional insulation to roof and wall surfaces, thus reducing cooling load and saving energy. Plants also absorb carbon dioxide, help cleanse the air, and provide visual amenity.
Green roofs are covered extensively with vegetation, such as grass or shrubs using an integrated support system. This system often includes substrate, filter, irrigation, water storage and drainage systems as well as water proofing of a roof surface/structure. In-situ installation is a conventional green roof application. It involves assembling the green roof layer by layer directly on the roof. The size and shape of the layers are configured to suit the roof design. Green roofs are designed to be lightweight, and typically cannot support heavy activities, just maintenance.
Roof gardens, balcony gardens and sky terraces are gardens with plants located on rooftops, balconies and terraces of buildings with accessibility for outdoor activities. Plants on these gardens can be more diverse and often include trees in addition to grass and shrubs. Depending on the type of plants it supports, soil depth typically ranges from 0.2m to over 1m (NParks, 2002). Integrated irrigation, drainage and waterproofing of the roof surface are common components of a rooftop garden.
Green façades/wall sallow plants to grow on building façades/wall surfaces through various means i.e., creepers with self clinging roots on wall surfaces, twining plants on mesh or cable support, and carrier panels with pre-grown plants fixed vertically on walls (NParks, 2010). Lightweight supporting structures can be made of polypropylene-based or synthetic fabric materials, while lightweight growing mediums consist mainly of volcanic stones and pumice.
Although building integrated greenery systems are not a new concept, their application has been picked up in recent years offering opportunities for further research and development, innovation and improvement.
An important ongoing research and development area is plant selection for various climatic regions and greenery systems. For green roof and green façades/wall applications, the selected vegetation must be able to thrive under intense sunlight and be drought-resistant. Selecting plants with shallow roots is a criterion to meet the light-weight and low maintenance nature of green roof systems. Other criteria in plant selection include:
- Plants with thicker and denser coverage of leaves for better shading effect and better thermal performance
- Use of native plants to nurture local biodiversity.
In the technological aspect, the performance of building integrated greenery systems has been improved, thanks to the development of new substrate system, built-in automatic irrigation systems with rain sensors, and built-in drainage systems. Such technologies help to make the greenery systems more lightweight, more water efficient, less maintenance intensive, and to eliminate potential water leakage problems.
The application of green roofs and green façades/walls is also shifting from in-situ application (i.e., assembling the green roof layer by layer directly on the roof) to modular based. Such application provides shorter installation time, minimum risk of damaging building materials, flexibility in design (in terms of mixing and matching various type of plants to create interesting design patterns), and ease of maintenance and replacement.
In green roofs, modules are small trays with sizes ranging from 0.25 to 2m2. Each tray is equipped with drainage, drip irrigation (optional), filter layer, substrate, media layer and grass/shrubs. In green façades/walls, modulisation is applicable for carrier system types. Each carrier panel is a module with a depth ranging from 100mm to 250mm. The modules can be lined up on a metal frame, which is fixed onto façade/wall surface. Irrigation and drainage pipes are interconnected between the modules and hidden within or behind the frame.
Building integrated greenery systems are most useful and feasible in cities and densely populated areas. To counterbalance the densely built-up environment, such greenery areas create alternative spaces for gardens, leisure activities, open spaces and a pleasant urban living environment. Building integrated greenery systems are most appreciated in tropical regions and in temperate regions in summer months. Under such climatic conditions, plants thrive and thus optimise their environmental benefits. However, these systems may not be suitable for application in hot or arid climatic regions, where most plants may not survive the heat.
The various systems described above have the same objective of integrating greenery into buildings, and thus share several issues that require technical attention and solutions. These issues include:
- Building structure must be able to support additional loads on the roof and/or on walls, depending on the greenery systems installed.
- Roof, sky terrace/balcony floor surface and façade areas must have proper waterproofing and measures to prevent root penetration and structural damage.
- The risk of plants or tree branches falling from the buildings must be prevented. Measures include additional securing of trees/plants and regular maintenance procedures.
- Irrigation, water storage and drainage systems need to be designed, installed and maintained appropriately to match local climatic conditions.
- Substrate and media for plants to grow on should be lightweight and designed to allow secure root penetration by plants.
Although there are common application requirements, different building integrated greenery systems have distinct application requirements:
Green roofs are most suitable for existing buildings in urban areas. This is because their lightweight system and public inaccessibility do not add significant additional dead-loads on existing roofs, and do not pose any security concerns. To be suitable for a green roof, an existing building roof needs to be relatively flat, with access for installation and periodic maintenance.
Roof gardens, balcony gardens and sky terraces require early design decisions for spatial and structural provisions to allow for additional dead-loads and accessibility for leisure activities. Therefore, these systems are mostly installed in new buildings. Design of sky terraces should also allow for an appropriate height-todepth ratio for sunlight penetration. The appropriate ratio varies from region to region, depending on the latitude and orientation of the sky terrace. In tropical regions, a ratio of 1:1 is considered to be sufficient regardless of orientation. However, in Eastern and north-eastern Europe, it is unadvisable to place sky terraces or balcony gardens on the northern side of a building, as plants may not grow well with limited sunlight accessibility. Likewise, in far southern regions of South America and Africa, sky terraces and balcony gardens should be placed on the northern side of the building for sunlight accessibility.
Green façade/walls can be implemented on both new and existing buildings. They can be very effective in terms of reducing heat gain, if installed on a building’s west facing façade. Furthermore, they can be strategically placed to hide undesirable elements/components of buildings, such as mechanical and electrical plant rooms. For carrier systems, the plants are pre-grown on panels with a lead time of about 3 to 8 months depending on the plant type. When installed onsite, a carrier system can provide an instant lush effect. However, for support systems, it should be noted that climber plants can take up to 3 to 12 months to grow on site (Chiang and Tan, 2009).
Feasibility for implementation
Building integrated greenery systems are more feasible for implementation in urban settings, especially in densely populated cities where land available for gardens and greenery is scarce. High land prices make it less feasible for developers and building owners to set aside sufficient land area for conventional on-grade gardens, open spaces and public spaces. It makes more economic sense to provide alternative green spaces integrated in buildings to provide leisure and, to a certain extent, communal activities. The cost to install and maintain building integrated greenery systems are easily offset by the high land prices and increase in property value.
Without government intervention,implementation of building integrated greenery systems will only be implemented in an ad-hoc manner by a small group of socially- and environment-friendly conscious building developers. Scientific based policy tools can facilitate the widespread application of building integrated greenery systems. For example, in Singapore, the concept of the Green Plot Ratio is developed to be a tool to quantify the environmental benefits of integrating greenery into buildings three dimensionally. Instead of measuring the greenery provision of a building site in terms of a two-dimensional area, e.g., percentage of green coverage in China, Green Plot Ratio measures the total leaf area index over a building site using a volumetric approach, taking into consideration green walls, green roofs, sky gardens, etc. (Ong et al., 2003).The Green Plot Ratio has been adopted in building regulation, i.e., Singapore’s Code for Environmental Sustainability of Buildings.
Government incentives are also necessary for the large-scale implementation of building integrated greenery systems. In several countries, governments incentivise developers and building owners through cost sharing methods. In Singapore for example, the National Parks Board administers the Green Roof Incentive Scheme, in which the government shares up to half of the cost of green roof installation capped at S$75 per square metre for buildings in the city centre areas (NParks, 2010).
Incentivising policies for one greenery system can also be a catalyst for the widespread implementation of other greenery systems. In Tokyo, for example, the city government targeted rooftop gardens, setting up the programme to support the creation of at least 12 km2 of rooftop gardens by 2011, many types of building integrated greenery systems have benefited from the programme. Green façades have also attracted more interest and have been marketed to architects, contractors and developers (Dunnett and Kingsbury, 2008).
In regions where building sector professionals and related trades are not familiar with building integrated greenery systems, capacity building is needed prior to large-scale implementation of the technologies. Capacity building should be in the following areas:
- Planning, designing skills and plant selection, so that building integrated greenery systems can contribute positively to the local biodiversity and eco-system
- Installation techniques (for trade technicians), including water proofing and irrigation systems
- Maintenance procedures (building’s owners and facility management personnel)
- Manufacturing and supplying light-weight components for green roofs and green façades/walls modules.
Green roofs: Due to their limited benefit – i.e., inaccessibility for leisure activities and maintenance requirements, green roofs have not been widely implemented. Their application is mainly for retrofitting existing buildings that have flat concrete roofs. This building configuration constitutes only a small segment of the existing building stock. Therefore, the market potential is limited.
Roof gardens, sky terraces and balcony gardens. Because the additional cost incurred to make provision for additional structural supports and maintenance requirements,these building integrated greenery systems are mainly implemented in newly constructed high-end buildings. However, the potential market for roof gardens is high in the tropical bell regions of China and India, where both population growth and urbanisation rates are high. In these countries’ high density cities, with high land cost and land scarcity, roof gardens, sky terraces and balcony gardens provide alternative spaces for leisure outdoor activities and improve biodiversity.
Green façades/walls: Due to the difficult and frequent maintenance requirements, green façades/walls are not widely implemented and have limited market penetration. They are mainly implemented for their aesthetic benefit in institutional buildings and in retail and entertainment complexes. The environmental benefits are often considered as a secondary objective. However, given the large area of building surfaces in an urban setting, green façades/walls have enormous potential to be implemented at a much larger scale to provide a positive environmental change in densely populated cities (GRHC, 2008).
Integrating greenery systems into buildings brings many benefits for the environmental, social and economic development of cities and dense urbanised areas. The environmental benefits include:
- Reducing heat gains for buildings in hot climatic regions. Research findings show that green roofs can reduce roof surface temperatures by 300C (Wong et al., 2003). Similarly, green façades can reduce the temperature immediately outside the façade by 5.50C, creating a 50-70% reduction in energy demand for air-conditioning (Peck et al., 1999).
- Reducing the heat island effect in urbanised area by shading heat-absorbing building surfaces, such as concrete, masonry, metals, etc. Green roofs can reduce immediate ambient air temperature by about 40C in tropical regions (Wong et al., 2003).
- Absorbing airborne particles and improving ambient air quality in urban settings. Green façades/walls located near busy roads can break down and absorb volatile organic compounds and unburnt hydrocarbons from vehicle exhaust (Chiang and Tan, 2009). Creepers also have a well-developed capability to trap and filter dust in their tissues (Johnston and Newton, 1993).
- Nurturing and enhancing urban biodiversity, especially when selecting indigenous vegetation species and coordinating building integrated greenery systems in a larger urban greenery network.
- Reducing rainwater run-off during downpours through rainwater retention by greenery and water storage in building integrated greenery systems.
- Absorbing carbon dioxide for photosynthesis and, as such, acting as carbon sinks.
The benefits related to social development include:
- Creating biophilic value to building occupants and city dwellers, and encouraging them to lead environment-friendly lifestyles.
- Providing alternative public places for leisure activities and fostering community ties through the opportunities of interaction in a high-rise urban setting.
The economic benefits of building integrated greenery systems include:
- Reducing building cooling load, leading to lower energy consumption and thus cost savings to building owners/tenants.
- Enhancing the marketability for buildings and increasing property value, thanks to their increased aesthetic appeal and biophilic value (Chiang and Tan, 2009).
- Reducing the diurnal temperature fluctuation of the building roofs and façades, leading to a reduction in materials’ contraction and expansion, thereby prolonging the lifespan of building roofs and façades. Research findings from the tropical region show that the change in temperature from day to night on a typical concrete wall is approximately 10°C, whereas the temperature change on a similar concrete wall equipped with a carrier greenery system is as low as 1°C (Wong et al., 2009).
- Nurturing the prosperity of new supply chains and new job creation to support a green economy.
Financial requirements for building integrated greenery systems include the investment cost of the products and their installation, and ongoing maintenance costs. These costs vary from system to system and from region to region. The following are indicative costs and considerations.
Green roofs. The investment cost for a lightweight, modular green roof system in Singapore ranges from S$150 to S$400 per square metre (DLS, 2009). In China, the indicative initial investment cost for green roofs ranges from 200-1,000 Yuan per square metre (China Real Estate News, 2010).
Rooftop gardens, sky terraces and balcony gardens. The investment costs vary depending on how elaborate the gardens are. These costs are similar to the cost of building a conventional on-grade garden plus additional costs for the stronger building structure and waterproofing measures and additional drainage system. Maintenance costs are also higher compared to that of an on-grade garden.
Green façades/walls. The investment costs for green façades/walls vary depending on the system. The cost of the support system is lower than that of the carrier system, which is in the range of S$300-S$2,000 per square metre. This cost range does not include the structural steel frames and drip irrigation. It is suggested that a budget be allowed for replanting in 1-2 years time (DLS, 2009).
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