The first category of living walls is simply a vertical garden with all the benefits customary to a garden. Green walls, both indoors and out, decrease local CO2, increase local humidity, trap dust, reduce noise and create a habitat for urban wildlife. Interior green walls can boost morale and productivity. The fresher indoor atmosphere keeps building occupants more alert and a feeling of overall health. Exterior green wall installations reduce solar gain (the entrapment of heat by passive solar gain on the building surface) and, by extension, building energy costs; provide protection from the effects of UV radiation and acid rain; and help lessen the building’s contribution to the heat island effect (when forest is replaced with concrete and asphalt, causing urban centres to become warmer than nature areas).
The second major category of living wall goes beyond these basic benefits by employing microbes within the planting substrate as a bio-filtering mechanism that removes potentially harmful hydrocarbons from a building’s ambient air. The bio-filtration system is structurally similar to the first type but is a closed loop system, controlling the circulation of the indoor air through the building’s mechanical equipment. Airborne, potentially harmful volatile organic compounds (VOCs) are drawn through the vegetative wall and then metabolized by microbes that naturally exist within the plant roots and planting medium before being re-circulated throughout the building. Plant species are specifically chosen for their VOC metabolic prowess and CO2 and water are the by-products of the decomposition process.