Living walls are also known as green-, bio-, vegetated-, living- or eco-walls. At their simplest, they are vertical gardens and can include any type of vegetative covering of a standard wall, such as hanging gardens and climbing vines. The term has recently come to include specialized and engineered envelope systems where vegetation is planted, irrigated and grown in modular elements which are secured to or integrated with the wall of a building.
In these latter systems, plants typically grow without soil between layers of fibrous material (such as felt or plastic mesh), or in pre-vegetated panels, that are suspended in front of a building wall. They are not planted in the ground or in planter boxes. Based on the principles of hydroponics, water with added nutrients drips slowly to the bottom of the wall where any excess is pumped up and re-circulated. Some living walls incorporate a pool at the base of the structure which can include fish and small animals such as amphibians.
Structural weight, moisture retention, nutrient supply and water distribution are important design considerations in all living walls.
Types of Living Walls
These feature vertical structural systems that support climbing plants on the building exterior. Climbers and vines are supported by stainless steel cables, webbing or metal grids and grow up from grade or planters.
Active walls such as the one at Queen’s University in Ontario, are indoor features joined to the building’s air circulation system where fans draw air through the living wall before being circulated through the building for increased oxygen and reduced pollutant levels.
These are also indoor features, but rely instead on passive open design for free air circulation rather than on mechanical air systems.
Outdoor Living Walls
Outdoor living walls are the engineered building envelope systems that allow a screen or layer of living plant material to be suspended at some distance from the outside wall of a building. Specialized membranes and drainage layers support the growth of a range of mosses, vines and perennial plants. Most of the information on this fact page relates to this type of living wall, along with the indoor walls, rather than green facade types.
Advantages of Living Walls
Though there is little evidence that stormwater is a design consideration, living walls can be designed to slowly use up stormwater which lands on the roof or other hard surfaces of a building site. Plants in a soil-less design need a relatively constant supply of water. This could be provided with the aid of a cistern placed higher than the top of the growing medium. Some cleansing would be provided by the plants and soils, and by the bacteria which would eventually inhabit the growing medium and root surfaces. Indoor and outdoor living walls could both take advantage of stormwater for re-use.
Living walls trap many airborne pollutants and particulates on the plant surfaces. In addition, plants take up Carbon Dioxide. A three store high living wall inside the Queen’s University Faculty of Applied Sciences, Live Building Integrated Learning Centre, is designed to remove Volatile Organic Compounds (VOCs) and Carbon Dioxide from the indoor air and its performance is constantly being monitored. A Toronto company, Quality Air Solutions, markets their ‘bio-wall’ as designed for bio-filtration of interior environments, including the removal of volatile organic compounds (VOCs).
Living walls make excellent use of vertical space within cities, providing micro-habitat, aesthetic benefits and air cleansing where none would have typically existed before. The high ratio of wall to roof area in urban spaces means the potential to generate positive environmental changes via green walls versus green roofs is also much higher.
Living walls add thermal mass to a building. They also provide shade and an insulating dead air space on the surface of the building wall. Vegetation also lowers adjacent air temperatures by evaporating enormous amounts of water from leaf surfaces. All of these processes help moderate indoor and outdoor building temperatures. One Canadian study found the reduction of summer cooling load by living walls was even more dramatic than for green roofs. The same study showed that significant reductions in the urban heat island effect could be attained if living wall technology was used extensively.
Despite being constantly wet, the engineered soil-less systems (PVC layer, felt & metal frame for example) can weigh less than 30 kg/m2 so are considered fairly light-weight for adding on to existing walls.
Living walls could meet some of the habitat requirements of small wildlife species, such as birds and insects, especially if suitable native plants are included.
Many of the new living walls, both indoor and outdoor projects, small and large, are designed for artistic effect and to enhance livability by providing calming greenery in very urban spaces. Some of the best known and most dramatic large scale examples in Europe, Asia and the US were designed by botanist Patrick Blanc of France.
Green walls can help reduce sound transmission into buildings due to the layer of plants, growing medium and, depending upon the design, the dead air space between the living and conventional walls.
While it is hard to find an example of a living wall designed for food growing, some proponents suggest homemade versions for greenhouse walls and vegetable gardens.
Limitations of Living Walls
Some sources note that, at least for indoor projects, monthly maintenance programs would be similar to other indoor garden requirements. Other sources suggest that vegetated walls require a much higher level of maintenance than climbers on a vertical frame. Practically speaking, the sheer height of some outdoor living walls will likely pose challenges in terms of maintenance access.
Energy and Water Use
The more complicated engineered systems have been criticized for using energy to supply light (indoor applications only) and pump water and nutrients through the system, and for using embodied energy in the building components. Living walls can be water-use intensive depending on their exposure. Irrigation systems are usually required to supplement rainfall.
Proper air flow and water movement must be established to help ensure harmful moulds do not grow, particularly in indoor applications. In addition, the constant presence of moisture means that the walls must be well separated from the adjacent structure.
Designers should consider pollen generation when choosing plants, especially for indoor applications or beside operating windows.
Future Potential for Living Walls
The use of living walls for stormwater retention and cleansing seems to be mostly theoretical at this point although the potential is good. In addition, some designers and writers are thinking about, and experimenting with, how to use vertical gardens for greywater treatment.