By Robin Mellon, Green Building Council of Australia
19 November 2010 – The discussion about green roofs and their benefits has been gaining volume in Australia; Green Star certified buildings such as 30 The Bond in Sydney or the Pixel Building in Melbourne have used green roof technologies to different effect.
In the US, where everything is larger – by reputation, anyway – there are already some incredible green roofs. The Ford Dearborn plant in Michigan has one of the world’s largest planted roof areas, and even the roof of Chicago’s City Hall building has been retrofitted and ‘greened’ to reduce the building’s environmental impacts.
However one of the most visible, and widely-respected, green roofs of recent years can be found atop the California Academy of Sciences. Located in San Francisco’s Golden Gate Park, the Academy’s one hectare (2.5 acres) living roof was inspired by the concept of lifting up a piece of the park and sliding the museum underneath.
The sinuous design is a salute to San Francisco’s hilly terrain – architect Lorenzo Piano wanted the building’s seven rooftop hills to reflect San Francisco’s reputation as “The City of the Seven Hills” – and this makes the building one of the most complex living roofs ever built. The two largest hills not only house the Academy’s rainforest and planetarium but also help hot air escape from the building through the skylights in warmer weather.
The many weather stations spread across the roof are part of the natural ventilation system. Each station packs five different instruments that measure rainfall, solar radiation, wind direction, wind speed, temperature and humidity. Data is fed into a computer that opens and closes the building’s many windows and skylights. This automated system uses the outside environment to cool interior spaces. The rooftop skylights vent hot air and allow natural sunlight to reach the living rainforest and coral reef.
Sixty thousand photovoltaic cells in the roof canopy convert sunlight to electricity to supply the Academy with 250,000 kilowatt hours of clean energy per year, which is put towards the building’s daily energy needs, preventing the release of more than 20.5 metric tons of greenhouse gas each year. These photovoltaic cells, when installed, represented some of the most energy-efficient on the market, achieving at least 18 per cent efficiency.
The roof canopy also shields the building interior from hot sunlight in summer but lets in warming rays during the winter months, acting as a giant layer of insulation spread right across the roof helping to stabilise building temperatures.
In cities, the air is usually several degrees warmer that surrounding areas, and rooftop temperatures can be scorching. This “urban heat island effect” caused by amassing dark, solid surfaces increases both air conditioning demands and pollution levels. According to the Academy, 16 per cent of all electricity consumed in the US goes to cool buildings; more green roofs would save billions of dollars.
The concept of the living roof has been around a long time; green roofs have helped to keep buildings either warm or cool for thousands of years. Ancient builders used rooftop gardens to keep buildings cool in extremely hot climates. The concept seems to have begun in Mesopotamia, and then spread to Greece, Rome, Persia and beyond. In Iceland, sod roofs have long been used to insulate homes from the cold. Modern living roofs were developed in 1960s Germany and spread to other parts of Europe, the US and Asia. The Hanging Gardens of Babylon are the most familiar early living roofs, although their existence is still disputed.
Unlike normal roofs, living roofs absorb most of the rainwater falling on them. They reduce runoff pollutants and reduce flooding and stress on stormwater systems. The Academy retains 98 per cent of the rainwater that falls upon it; almost 13.63 million litres per year. During heaving rains, a system filters overflow water back into the ground.
The Academy’s living roof represents a carefully-designed substitute for a natural soil system, with a number of complex layers that combine to support healthy plant life, manage moisture and protect the building. The soil mix must be lightweight and porous, retain oxygen, water and nutrients for the plants, provide anchorage for root systems, and be less than 15.25 centimetres in depth.
To keep the plants and soil from sliding off the roof in heaving rains, the researchers created biodegradable planter trays made from tree sap and coconut husk fibre. Above the building’s concrete structure sits thermal plastic waterproofing to keep the moisture out. Above that sits polystyrene insulation to prevent heat loss and any UV damage. Above that is a vinyl protection layer, above that the drainage layer which prevents the plant roots from rotting.
Above that is the polypropylene filter sheet, above that the thin layer of additional soil, and above that the biodegradable tray with soil and plants.
Green roods are not rewarded directly within the Green Star rating tools; however, their benefits are rewarded in different credits around each tool. The insulation, heating and cooling benefits are rewarded within the Energy category, the plant life and biodiversity is rewarded within the Land Use and Ecology category. The slowing and filtration of rainwater flow is rewarded within the Emissions category and, depending upon the approach, design and construction, there may be further points rewarded in the Innovation category.
We can certainly hope to see more green roofs – and living roofs – in Australia, as the benefits to the building, the occupants and the community are more fully understood. Green Roofs Australia has been working hard in this area – more information can be seen on their website at www.greenroofs.wordpress.com – and the Green Building Council of Australia is looking forward to more discussion about living roof advantages as part of the Green Star Communities tool development and evolution of the Green Star rating tools.
How long might it be before every roof on every building can help the building’s performance, become an enjoyable space in which to spend time, and generate the ultimate in ‘locally-grown produce’? With projects such as the California Academy of Sciences’ living roof leading the way, perhaps not that long.
Robin Mellon, is executive director – advocacy and international at the Green Building Council of Australia. He has been attending Greenbuild, the US Green Building Council’s annual conference, in Chicago.
Some information kindly provided by the California Academy of Sciences.