A green roof could reduce annual energy demand by more than 30 per cent when compared to a standard BCA compliant metal deck roof.
In the Australian climate, the cost of cooling buildings during summer is by far the greatest draw on annual energy demand as the energy consumption of a building in most of our capital cities is predominantly cooling dominated. This is because most Australian cities have hot summers and relatively mild winters.
In commercial buildings that are mostly occupied during the day, it is the heat loads that put the highest demand on mechanical air conditioning costs over the year. These heat loads are not only produced by the outside temperature (from the sun), but also by internal sources such as occupants, lighting, computers and other equipment. Because of this, over the year commercial buildings typically have a higher building energy demand for cooling than heating in maintaining occupant thermal comfort.
The traditional way to counter this has been to include higher levels of thermal insulation in the building fabric (walls and roof). This is required under Section J of the Building Code of Australia (BCA) in order to achieve a minimum regulatory thermal performance.
This insulation although relatively inexpensive, in some instances does very little (especially in Northern Australia) and can cause issues when installed below the roof membrane as it takes up valuable ceiling space.
An alternative approach is to reduce, or not use any insulation and replace it with a green roof, or roof terrace garden.
Green roofs, in general consists of a few main components:
- Gravel ballast/mulch
- Growing media (soil) and water reservoir
- Drainage and filter layers
- Water proof membrane
- Roof deck (e.g. concrete slab or roof sheeting).
A green roof, if designed properly and integrated into the overall building project, may not only provide the required thermal performance (thereby reducing energy costs) but it will also have many other benefits making it more sustainable and responsive to the effects of climate change.
The roof need not be expensive, as it can be lightweight (around 1.5 kPa) and relatively low maintenance, requiring only intermittent low intensity inspections, weeding, fertilising and adjustments to irrigation.
A green roof can also manage stormwater, reducing the size of any detention tank that may be required, and can contribute to acoustic performance by dampening noise from outside and reducing noise impacts from within in the case of industrial and manufacturing buildings.
Further to this, green roofs look fantastic when viewed from afar, blending the building into its surroundings and providing quality visual amenity. They also provide for biodiversity and contribute to the overall cooling of the city reducing urban heat island impacts. These factors contribute to overall human well-being.
The potential energy benefits of green roofs in terms of building thermal performance have mainly been studied from a general qualitative perspective, and primarily in the context of the Northern hemisphere.
The unique qualities of green roofs in the Australian context with our climate, vegetation, and construction methods, codes and standards have not been studied until recently.
However a study has been completed as is soon to be published in the Australian Institute of Architects’ Environment Design Guide.
This study models the thermal performance of a green roof compared to a BCA compliant traditional roof of both concrete and metal deck roof structures in eight of the nine climate zones in Australia. It estimates the energy benefits and whole-of-life costs of a green roof.
The indications are that green roofs are a good investment as an energy saving option, especially for commercial buildings in the warmer parts of Australia, including Darwin, Brisbane, Perth, Adelaide and Sydney. In Melbourne they are still sound, providing a 10 per cent annual energy saving with a payback period of 20 years (compared to a 31 per cent saving and 13-year payback for Sydney).
One of the study's co-authors is mechanical/ESD engineer Arjun Adhikari of Thermal Environmental. Arjun has used a sophisticated thermal simulation computer model developed by David Sailor (Sailor’s Theory).
This simulation accounts for several interrelated phenomena and variables involving heat transfer, mass transfer and plant physiology.
Arjun said it is quite a complex task to accurately model a green roof this way. He noted that “based on our research and findings the dominant variables that have major impact on green roof thermal behaviour are:
- shading effects of foliage;
- growing media moisture content (thermal mass);
- solar absorptance, transmittance and reflectance of the leaf surface area and leaf reflectivity and emissivity; and
- plant physiology.
These are dynamic and time based variables associated with a green roof that influence the thermal conductivity into the building. These variables are fluctuations throughout the day and night in soil moisture content, and the direct solar influences on the building due to foliage variability and the dynamic nature of solar radiation on to the roof surfaces over time and throughout the year.”
Notwithstanding this complexity, Arjun’s simulations of the thermal performance of green roofs for eight of the nine climate zones in Australia concludes that “green roofs provide great passive cooling and could not only reduce the size of air conditioning plant required but also help the air conditioning plant to operate more efficiently due to the relatively stable fabric heat loads with a resultant saving in energy costs.”
Arjun’s results are summarised in the following table which highlights the energy benefits of a green roof in some major cities of most climate zones of Australia.
It well may be the case that green roofs provide one of the pieces to solve the puzzle of a sustainable built environment for the future in Australia.