Why green wall?

Green wall

A green wall is comprised of plants grown in supported vertical systems that are generally attached to an internal or external wall, although in some cases can be freestanding. Like many green roofs, green walls incorporate vegetation, growing medium, irrigation and drainage into a single system. Green walls differ from green facades in that they incorporate multiple ‘containerised’ plantings to create the vegetation cover rather than being reliant on fewer numbers of plants that climb and spread to provide cover. They are also known as ‘living walls’, ‘bio-walls’ or ‘vertical gardens’.

Green walls provide an attractive design feature, but also add to building insulation by direct shading of the wall surface. They create cooler microclimates and improve local air quality, and provide the possibility of growing plants in locations that would not normally support vegetation. A wide range of plants is used on green walls, usually herbaceous, though some small shrubs can also be suitable.

The provision of adequate light is an important consideration, particularly when planning an interior green wall, where artificial lighting may be necessary.

Many different proprietary green wall systems are available. Some are hydroponic and others use a growing substrate. Green wall structures vary from modular systems to sheet or boardbased structures with felt pockets to contain and support plant life. All green walls require irrigation, often inclusive of fertiliser (fertigation). Fertigation solution can be re-used, but requires careful monitoring and management to ensure nutrients do not build up over multiple re-circulations to damaging levels.

A well-designed green wall system will fulfil both design and functional aims by providing growing conditions suitable for the selected species, have a long lifespan, require minimal component replacement, and have achievable demands for maintenance.

Benefits of green roofs, walls & facades

Stormwater management

Green roofs absorb and retain rainwater and can be used to manage stormwater run-off in urban environments. They can also filter particulates and pollutants. Stormwater run-off can be reduced or slowed because it is stored in the substrate, used by or stored in the foliage, stems and roots of plants, and also evaporates directly from the substrate. Additional water storage capacity in green roof systems can be provided through incorporation of a water retentive layer or drainage layer at the base of the green roof.

Improved thermal performance

A significant benefit of green roofs, walls and facades is the potential for reducing building heating and cooling requirements. Green walls and facades can reduce heat gain in summer by directly shading the building surface. Green roofs reduce heat transfer through the roof and ambient temperatures on the roof surface, improving the performance of heating, ventilation and air conditioning systems.

Cooling a city – urban heat island effect

Hard surfaces in urban environments, such as concrete, brick, glass, asphalt and roofing, have a high thermal mass; collecting the sun’s heat during the day and re-radiating it slowly back into the atmosphere. This contributes to a rise in ambient temperature in cities, creating large, stable masses of hot air (urban heat islands), especially during periods of calm, still weather.

Temperatures can be reduced by covering a roof or wall with a layer of vegetation that shades building materials which would otherwise absorb heat. Evapotranspiration provides cooling effects, as water is evaporated from the soil and plants and plants transpire by taking water in through roots and releasing it through leaves. Energy from the sun that would otherwise heat the roof or wall surface and increase ambient air temperatures is instead used in the evapotranspiration process, resulting in latent heat loss that lowers surrounding air temperatures. When green wall and facade plants are grown on a support system that leaves a gap between the wall and the planting, hot air moves up by convection through the space between the wall and the vegetation, providing passive cooling.

A city-wide strategy to implement green roofs, walls and facades could help mitigate some of the negative consequences of urban heat islands, and consideration should be given to appropriate plant selection and substrate depth to maximise cooling potential.

Creation & preservation of habitat & ecological biodiversity

Green roofs can contribute to and enhance biodiversity by providing new urban habitats and specific habitats for rare or important species of plants or animals. Green roofs can also provide a link or corridor across urban ‘ecological deserts’ and assist in migration of invertebrates and birds. Designing for biodiversity requires consideration early in concept development with regard to plant species, food sources, habitat values, access points and building heights.

Aesthetics, open space and urban food production

The liveability of cities is increasingly dependent on the availability of and access to green open space. Green roofs, walls and facades can increase amenity and provide opportunities for food production, recreation, relaxation or commercial ventures.

Cleaning the air

Green roofs, walls and facades can contribute to the removal of gaseous pollutants from the air, although their effectiveness varies with plant species and area of cover. Plants with a high foliage density or with textured leaf surfaces that trap small particles also assist in removing particulate pollution, through dry deposition on the foliage or through rain wash. On a larger scale, green roofs, walls and facades can help to reduce overall environmental heat gain (re-radiation of heat from building materials with high thermal mass), in turn improving air quality as less photochemical pollutants are produced at lower air temperatures.

In interior environments, plants have been shown to have a significant capacity to reduce volatile organic compounds from the air. Carpets and other soft furnishings and office equipment are common sources of these gaseous pollutants; inclusion of vegetation, such as a green wall, can help to improve the air quality of the indoor environment.

Reduce noise level

One of the lesser known benefits of living green walls, the structures can reduce noise levels in buildings. Plants have been used, throughout the world, to reduce noise along roads and highways. Living green walls expand on this concept as vegetation ‘naturally’ blocks high frequency sounds while the supporting structure can help to diminish low frequency noise.

Living green walls act as extra insulation with a layer of air between the plants and the wall. They also reduce noise levels by reflecting, refracting as well as absorbing acoustic energy.

References: Growing Green Guide

(http://www.growinggreenguide.org/wp-content/uploads/2014/02/growing_green_guide_ebook_130214.pdf)