The combination of soil, plants and trapped layers of air within green roof systems can act as a sound insulation barrier. Sound waves are absorbed, reflected or deflected. The growing medium tends to block lower sound frequencies while the plants block higher frequencies. The amount of sound insulation is dependent on the system used and the substrate depth. A green roof with a 12 cm substrate layer can reduce sound by 40dB and one of 20 cm by 46-50dB. A study by Kalzip [www.kalzip.co.uk] compared sound insulation of their standard unvegetated roof system with that of the Kalzip vegetated ‘Nature Roof’:

1. Standard Unvegetated 33dB
2. Vegetated [dry] 41dB
3. Vegetated [wet] 51dB
4. 100mm Concrete Wall 43dB

This suggests that a green roof can reduce sound by 8dB compared with a conventional roof system. This could be particularly important in areas of high noise pollution such in the approaches to airports, as these levels are sufficient to provide noise insulation to buildings under aircraft flight paths.

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4. Green space

The value of green spaces to people living and working in towns and cities has increasingly been recognised by Government. The work of the Urban Green Spaces Taskforce (Green Spaces, Better Places, 2002) demonstrated the various benefits that green space provide, such as ecological function, visually softening the built environment, supporting biodiversity, aiding people’s mental and physical health, and providing a communal focus and sense of place. Government has subsequently launched a raft of new policies, initiatives and funding to promote the good design and management of green spaces.

English Nature has published research that suggests that an accessible natural green space should be no more than 300 metres from where anyone lives in order to meet people’s needs for contact with nature. Evidence suggests that regular direct contact with natural green space (and elements of the natural world such as birdsong and seasonal colour change) is good for people’s health (see below).

There is a need for increased densities in urban residential development (>30 dwellings per hectare), which could result in terrestrial green space being reduced or lost. In the urban core the provision of green space is usually already severely limited, partly through historical circumstances, and more recently very high land values; this makes the creation of new green space both important and difficult. Given the nature and pressures of urban regeneration, the creation of new spaces has to meet a number of interests; these generally result in highly formal spaces with little ecological benefit. Creating low-maintenance, terrestrial, naturalistic green spaces in the urban core is not popular; green roofs may provide one solution. Green roofs can provide both visually accessible and physically accessible green space. Roofs are largely visually ‘dead’ and unappealing and their appearance to those overlooking them can be softened by vegetation. There are instances where the sole justification of a green roof installation is for visual aesthetics. Areas of green roofs can also provide accessible space for people to enjoy, and some can be landscaped to extend existing green space, for example at Canary Wharf station on the Isle of Dogs, London. Roof gardens and terraces are options for smaller buildings and have some historical ancestry. The Berlin roof gardens of the 19th century, have been adopted on similar housing blocks in Britain (for example, Peabody Trust’s Balderton Flats in Mayfair) and were one of the inspirations for the first modern green roof in the UK, at Derry & Toms, Kensington, 1938, which still serves as a garden, albeit with limited public access. Roof gardens are increasingly being proposed for new office and housing developments. Large areas of accessible green roof space can be created if the building is large enough, for example above Cannon Street Station in the City of London [intensive], and at Chicago City Hall [extensive]. More ‘extreme’ examples include a golf course on a roof in the USA. The key issues that need to be considered in providing accessible open space are health & safety (the need for a external rail or fence), over-looking neighboring properties (a material planning consideration), access to and from the roof-space, load-bearing (if the proposed numbers of people are to be more than a few), and management. The existence of green roofs that provide this function suggest that these issues can usually be easily addressed.

3. Energy Conservation improved thermal performance

Green roof systems are recognized as providing greater thermal performance and roof insulation for the buildings they are laid on. This can vary depending on the time of the year, and the amount of water held within the system.

Cooling [summer] Poorly protected and insulated roofs can lead to substantial overheating of spaces beneath them. This can lead to the need for increased air-conditioning. A green roof not only acts as an insulation barrier, but the combination of plant processes [photosynthesis and evapotranspiration] and soil processes [evapo-transmission] reduces the amount of solar energy absorbed by the roof membrane, thus leading to cooler temperatures beneath the surface.

Research by Nottingham Trent University has shown the following:

Mean daily temperature 18.4C (65 F)

Temperature beneath membrane of normal roof 32C (89.6 F)

Temperature beneath membrane of green roof 17.1C (62.7 F)

A study conducted in Chicago, USA, recently estimated that building energy savings to the value of $100,000,000 could be saved each year if all roofs were greened, as the need for air conditioning would be reduced.

Thermal Insulation [winter] Green roofs can help to reduce heat loss from buildings during the winter when root activity of plants, air layers and the totality of the specific system create heat and thereby provide an insulation membrane. However the efficiency of green roofs as thermal barriers is dependent on the amount of water held within the system. Water retention can increase the amount of heat lost through the system and therefore any efficiency gains are dependent on daily conditions. It is therefore difficult to provide accurate figures on the net effect of green roofs on energy efficiency during the winter months.

The study at Trent University on the temperatures under membranes of standard roofs and those under green roofs also showed that green roofs appear to have a positive effect in winter.

Mean Temperature 0C (32 F)

Temperature under membrane standard roof 0.2C  (32.36)

Temperature under membrane green roof 4.7C  (40.6 F)

This shows that green roofs do have the ability to affect the temperature and insulation properties of roofs, though this is variable due to the daily conditions of the green roof. The potential for the cooling and thermal insulation properties of green roofs can have costs benefits for building owners/managers.