Green walls and roofs [Better Homes and Buildings]

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Green walls and roofs insulate buildings, clean the air, absorb carbon dioxide, reduce noise pollution and can provide habitat for insects. They are increasingly used in cities to help buildings avoid overheating, to cool the air and absorb air pollutants.

Buildings are greened by supporting the growth of plants. This is either done by encouraging climber plants, or through the construction of planter boxes or fixing substrate for plants to grow. Numerous plants, including herbs, wildlife flowers or even edible fruit may be grown on green walls or roofs.

Green Walls

Green walls improve the aesthetics of a building as well as offering numerous environmental and wellbeing benefits. They can increasingly be found internally as businesses are realising that these natural features improve working environments. They are also now being installed as stand-alone and on-building features outdoors in cities to tackle air pollution as well as the overheating experienced during increasingly hot summers.

At the most basic level, green walls are achieved by encouraging climbers, such as ivy or wisteria. Vertical gardens are another way to make a green wall, whereby a loose growing media is contained within a shelf or a bag attached to the wall. The more complex systems result in a polyurethane sheet, a mat or structure media being attached to a wall, often provided and maintained professionally. All systems require some form of irrigation and replacement of growing media. Some integrate greywater systems into this to allow for natural cleaning of water from the building.

Matt media is used mostly in interiors. It is achieved by using felt or coir fibre matting attached to a wall, with plants rooted within this. This matt is then kept damp through near constant cyclical irrigation. These systems need to be replaced every two to five years as roots restrict irrigation potential. This is done by cutting out sections and replacing them.

Sheet media systems use two layers of polyurethane sheeting. The first is structured similar to the bottom half of an egg box, for the plants to be set in, with the second layer containing the system. Plants roots (usually without soil) are inserted into the individual container areas by perforating the second layer. These systems are hung against another sheet to prevent damage to the wall. These systems are also cyclically irrigated, but due to the holding capacity of the system, they are not as demanding as Matt media and do not need to be replaced until the system deteriorates, with their estimated lifespans being 20 years. These systems can be used outdoors on walls or flat roofs or indoors.

Structured media are systems built-in blocks allowing for sections to be replaced periodically. The media used is a mixture of the best aspects of sheet and matt media. The benefits of these systems are their ability to create mosaic green walls with different shades of green or other colours.

Commonly used plants on green walls include heuchera, thyme, sedum, carex and ajuga. Edible plants that may be grown include tomatoes, draft cabbage, English spinach, basil, coriander, dill, lavender, lemongrass, marjoram, mint, oregano, parsley, rosemary and sage.

Green roofs

Green roofs increase the energy efficiency of a building, they capture and clean water as well as capturing carbon, and provide habitat for insects and birds. They are increasingly being used in cities to support wildlife and due to their properties of absorbing heat and water, and they are being used to mitigate issues associated with climate change, whereby they can absorb water as well as heat. Ponds are also being built on roofs to clean greywater.

A green roof is constructed by laying a polyurethane sheet across a roof, with walls built around the sides to contain the medium. Varying depths and types of medium may be used, which determine the types of plants that may be planted. Green roofs may survive without an irrigation system depending on rainfall, the type of vegetation planted and the depth of the medium.

Green roofs need to be replaced every 10-15 years as the polyurethane sheet decays, with mediums also needing top-ups. Green roofs require maintenance as determined by the vegetation planted.

The cost of Green roofs are double that of an ordinary roof and it is recommended that they should not be installed on any roof with a rise of more than 10 degrees in angle.