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    Green roofs: functional gardens

    Peta Earley

    Compared to the uptake in countries of the Northern Hemisphere, Green Roofs in Australia and New Zealand are an underutilised building feature.  However, despite the fact they can provide a place of respite in our hectic world, there are a number of other benefits they provide to the building and the surrounding environment.

    Before exploring the benefits of a Green Roof, let’s look at the types of Green roofs and what is required to establish a Green Roof.  A Green Roof (also known as a Living Roof) consists of multiple layers:

    1. The roof structure.  The roof structure must be able to support the weight of the green roof.  If retrofitting an existing roof, structural analysis must be undertaken to ensure the structure is suitable.
    2. Protection Layer – this is the waterproofing layer and must be included to protect the roof structure.
    3. Drainage layer – this is a crucial component as it drains excess water and hence reduces the overall weight on the structure.
    4. Filter fabric (cloth layer) – to prevent the substrate from entering the drainage layer.
    5. Substrate or Vegetation layer.  This is the vegetation support layer or growing medium for plants.  Things that must be considered are roof loading capacity, mass and depth, purpose and availability.  When deciding on the type of substrate to use consideration must be given to the balance of air filled porosity (AFP) and water holding capacity (WHC) of the product.
    6. Plant Layer. It is important that plants are selected that will be able to grow and thrive in the site conditions and local area climate. Site wind loads and solar exposure to the roof area must be considered when selecting the plants.

    A Green Roof is generally classified as either Intensive or Extensive.  The Extensive version contains a lighter layer of vegetation, consisting of succulents, grasses, perennials and sub-shrubs, generally growing between 4-20cm. They are low in maintenance and don't require irrigation and therefore are not a garden that is frequently occupied by occupants.

    On the other hand an Intensive Green Roof contains a thicker and denser plant layer and therefore supports a wider variety of plants.  It supports 20-100cm of growth, consisting of herbaceous materials (perennials), shrubs and small trees. It is heavier, having a weight capacity around 300-1000 kg per square meter, depending on the depth and type of substrate selected and the plant selection.  An Intensive Green Roof will require regular maintenance and irrigation, but provides a park-like environment for occupants.

    The benefits a Green Roof offers to a building include provision of additional insulation and shading to the roof of the building, insulation of the building from sound and increasing the life of the roof, by protecting the waterproofing of the building from harmful UV rays. However, the benefits of a Green Roof do extend well beyond the building envelop.  A Green Roof will reduce the heat island effect, reducing the urban air temperature of the surrounds. 

    Figure courtesy of Norman Disney & Young

    A standard building roof (one with a hard surface such as concrete or asphalt) will reflect, absorb and re-radiate solar radiation into the surrounding atmosphere, which increases the local air temperature. Installation of a Green Roof reduces this affect as it allows the solar radiation to be absorbed by the plants and not reflected back into the atmosphere. 

    The surrounding micro-climate is further improved by the plants acting as a filter to pollutants and CO2 from the surrounding atmosphere and, by creating a habitat for wildlife, the Green Roof can increase the biodiversity of the area.  The local stormwater system also benefits from the installation of a Green Roof as it filters pollutants and heavy metals from the rainwater and the substrate retains rainwater, reducing the peak and average stormwater levels leaving the site.

    To quantify the stormwater benefits, the University of Auckland and Landcare Research undertook a study to investigate the rainwater retention of an extensive green roof.  The green roof was setup on top of a 13 story high-rise at the University of Auckland in Auckland’s CBD.  The green roof encompassed 94% of the total roof area (235m2) and consisted of 6 hydraulically isolated plots. 

    The study found that for over 17 rainfall events, the peak reduction in stormwater was no less than 66%, with an average peak reduction of 92% per storm. For the results that were able to be measured between 60% - 65% of the total rainfall was able to be retained.  The study was conducted over a 6 month period over summer and spring and for the analysis that was conducted, concluded that a significant difference between the 50mm and 70mm substrate areas was not observed. 

    Another recent study, conducted in Melbourne Australia, by the University of Melbourne in collaboration with CSIRO, attempted to quantify he thermal benefits of a Green Roof.  It investigated the difference in internal temperature and HVAC energy consumption for a room with a green roof, compared to one without.

    The study involved setting up a live test and control site and comparing the internal temperatures of the two rooms to those achieved when the same rooms were simulated through computer modelling, using AccuRate. Once they were able to confirm a correlation between the computer simulated internal temperatures and the internal temperatures from the live site, they used the AccuRate model to predict the HVAC energy consumption for each room. 

    The computer simulation was based on the rooms being conditioned between 8am and 6pm Monday to Friday, with a heating set point of 18 degrees and a cooling set point of 24 degrees and allowance for natural ventilation when external conditions are favourable.

    The study found that during the summer period, the room with the green roof was on average 1 degree cooler than the room that had the conventional concrete roof and during the winter months on average 0.2 degrees warmer.  This difference is thought to be due to the thermal mass of the green roof, the shading effect of the vegetation and evaporative cooling.  It was also found that the peak average indoor air temperature during the summer months was around 1.5 degrees lower and in winter 0.6 degrees higher with the green roof.

    The study found that the green roof reduced the HVAC energy by 48% in summer and 13% in winter.  While this result does seem impressive, it is important to note that the construction of the room was double brick external walls, concrete roof and concrete slab floor.  Adding insulation to the rooms was not assessed, but it would be expected to reduce the temperature fluctuations noted and thus possibly narrow the HVAC energy consumption differences reported.

    Figure courtesy of Norman Disney & Young

    When trying to calculate the thermal benefit of a Green roof it is not as simple as calculating the different R-value of each layer, as you would a standard roof structure, as this does not consider the complexity of a plant and its soil's ability to transfer heat.  Like humans, plants have a unique ability to use a number of heat transfer means to maintain their temperature.

    While a traditional roof (which does not have a green roof) has no means of controlling its temperature, so it continues to absorb the suns radiant heat and can heat up to as much as 80’C, the foliage and soil layers of a green roof are able to maintain their temperature to that of the ambient temperature.

    Plants have this ability by using a number of heat transfer methods, convection, conduction, reflection, thermal mass and evaporation and evaportranspiration. Heat from the sun, absorbed by the leaves, is transferred to the air through convection, with small pointy leaves being the optimal shape and size for maximum convection. Heat absorbed by the plant is transferred by conduction to the soil/substrate and foliage will also reflect solar radiation.  When using succulents, in many cases the surface of the plant will get more reflective as a drought period extends.

    Where plants are able to store water, such as succulents, they will have a large thermal mass, so have a greater ability to absorb more energy, thus not transferring excess heat to the medium below.  Evaportranspiration is another means by which plants are able to maintain their temperature. This is essentially a plants version of sweating, it is the process by which water is forced up from the soil, through the roots, up the stem and out through the leaves by forced evaporation.

    The ability of the green roof to maintain its temperature to near ambient conditions means that the concrete slab has a lot less thermal energy to transfer to the adjacent space.  A study conducted in Canada compared the roof membrane temperatures of a plane bitumen roof to that of a green roof and found that where the outside temperature peaked at 95F (35’C), the membrane of the green roof remained around 77F (25’C) and the reference roof reached 158F (70’C).

    This temperature difference for a concrete slab would mean there was an additional 14256J of thermal energy per m3 of the roof slab to be transferred into the conditioned space below.

    The other thing to keep in mind, when discussing the thermal benefits for a building, is that the thermal benefit is only going to benefit one, maybe two levels of a building. In a high rise building, the glazing of the building will have the greatest thermal impact on the building.    

    Peta Earley is a Senior Sustainability Consultant in the Canberra office of leading engineering consultancy Norman Disney & Young. She has 10 years experience in the industry collaborating with clients to establish a set of environmental outcomes, which improve energy efficiency resulting in reduced overheads. Peta also has extensive experience using the sustainable benchmarking tools such as Green Star and NABERS.

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