Climate scientists agree global weather patterns are changing: some regions becoming hotter and drier, others wetter. The built environment is a high contributor to human induced climate change, and the residential sector is the land use type contributing the most to emissions. Improved sustainability in new buildings is not enough. 90% of the residential buildings Australia will have in 2050 are already built; clearly we need to adapt them. On this basis, climate change mitigation should focus on adaptation and sustainable retrofit for existing buildings. The question is, what are viable solutions in terms of our existing buildings? This article reports on research into retrofitting lightweight green roofs for residential property to reduce heat gain and cooling loads primarily, although it will lead to increased biodiversity and improve air quality. Furthermore, the technology could be adapted for other building types.
Why retrofit with green roofs?
Green roofs are a living vegetated roofing alternative to traditional impervious roofing materials. They have environmental, economic and social benefits. If widely adopted, green roofs could dampen the urban heat island effect by reflecting the sun’s radiation, by providing shade, or by evaporative cooling. It is estimated that ambient air temperature in the USA would be 2ºClower in urban areas if 8% of buildings had green roofs. As plants photosynthesise they absorb carbon dioxide and produce oxygen improving air quality around the site of a green roof. Green roofs attract bugs and insects that pollinate the plants.
A further benefit is their ability to absorb rainwater, thereby reducing water flow into sewers and drainage systems. The rate of storm water flow reduction varies according to the depth and the type of substrate specified, and also the temperature, amount and timing of the rainfall. For example, if the roof is already sodden through extended rainfall, the attenuation rate is diminished, however if the roof is dry attenuation rates as high as 90% are recorded. Some cities have mandated green roofs on this basis as it is deemed a better option than digging up roads to enlarge sewer pipes, avoids inconvenience to residents and costs less.
Green roofs can provide much needed social spaces in CBDs, where problems of social isolation are highest. There is a considerable body of research that affirms the feelings of wellbeing humans experience when they come into contact with the natural world (known as the biophilia effect). Finally there is the benefit of improved thermal performance from retrofitting a green roof that occurs in both hot and cold climates. In cold climates green roofs provide an insulating layer that reduces heating energy consumption. In hot climates, green roofs prevent heat gain and reduce cooling energy consumption.
Barriers to take up
Key obstacles include perceptions of high installation and maintenance costs, structural adequacy, and risk of water damage.
Few green roofs are found in Australian housing. It is a little known technology, and a design and specification unfamiliar to designers, contractors and homeowners. This makes people nervous and sceptical. Although the technology to design and retrofit green roofs exists, the take up and the demand have not been high. The existing numbers of residential green roofs confirm this observation. Only by specifying and building more green roofs will we overcome this fear and risk-averse position.
Adding a green roof to an existing metal roof can double its lifespan, and choosing drought tolerant plants which do not require irrigation can help reduce maintenance costs. Using plants which do not require a deep growing medium (typically 50mm) reduces additional loads, but a structural engineer should always confirm the proposed retrofit green roof would not overload the existing structure.
Drainage must be considered to ensure water does not get trapped between the metal covering and the green roofing system, as shown in figure 1. Some systems are available, although there is scope for entrepreneurial design of lightweight, modular systems to simplify installation and removal.
Typical existing residential roofs
Many Australian residential buildings have profiled metal sheet roofing – a lightweight material with poor thermal performance and high rates of heat transfer. Often buildings have little or no insulation. Sydney and Melbourne summer temperatures can reach 45ºC and rainfall patterns are variable and changing. Much stock has pitched roofing comprising a structural timber or metal frame with a profile metal sheet covering. These structures have limited capacity to support additional loads unless strengthening work is undertaken. Green roofs may be retrofitted to roofs with a pitch of 22 degrees and less. Much existing stock meets these requirements.
Case study – thermal performance at University of Technology, Sydney (UTS) test site
An experiment has been undertaken on two small scale profiled metal sheet roofs in Sydney to assess thermal performance. One roof was left as a control, whilst the second roof was planted with succulent plants in trays. Data was collected using thermal data loggers over a summer and autumn season. Urban densification exacerbates heat islands, leading to uncomfortably high internal housing temperatures. This experiment is predicated on the idea that simple technologies can combat this. The researchers aimed to use adaptive techniques that minimised initial and ongoing costs. Hence this project used lightweight removable modules of vegetation of low thickness. This modular system enables offsite planting, cultivation and maintenance.
Succulent plants were chosen for their higher drought resistance and lower fire risk. Furthermore these species can develop easily in shallow soils, and thus, structural reinforcement of existing roofs is unnecessary. Additionally, due to the modular characteristics of the planting containers, the modules can be applied directly onto the roof covering be it profiled metal sheeting or tiles. 190 x 330 mm rectangular plastic containers, produced on a 3D printer, were used.
Despite the short experiment period, a significant green roof cooling occurred. According to the characteristics of the site where the experiments were undertaken, a particular pattern in temperature registers. As shown in Figure 2, a sudden reduction in temperature for non-green and green roofs occurs around 3pm due to the shadows caused by adjacent buildings. This same figure shows green roofs have a slower response to temperature variation, consistent with their insulating properties.
Maximum, minimum and average temperatures for Sydney green and non-green roofs are shown in Table 1. The greatest differences occurred around noon on the warmest days.
Figure 3 compares the non-green and green roof inner temperatures over 97 days of summer in 2014. Temperature differences between green and non-green roofs varied from -1.6ºC to +14.8ºC. Negative differences were evident during all night time periods, indicating the green roof’s efficiency in attenuating high and relatively low temperatures.The green roof cooling potential in Sydney is very good and may be attributed partly to positioning of the temperature data loggers. They are located about 50 mm below the roof, whereas if placed 250 mm below the roof, results may vary. To evaluate this influence, new loggers were placed in both Sydney sheds to provide a future temperate comparison in a lower position.
Another aspect to consider is the effect of water. Water’s high specific heat should enhance insulating properties of the system. Water levels were not monitored in the studies so far, and its influence remains unknown in the present work. This is the object of further research in Rio de Janeiro. Furthermore only temperature data, not solar radiation levels, was collected. Therefore the temperature attenuation provided by the green roofs must be directly related to high solar radiation levels. During cloudy days this effect tends to be less pronounced.
The Sydney experiment presented a very high green roof performance that could be adopted in retrofitting housing. The research demonstrated that roofs planted with succulents are viable and could provide a low cost, drought tolerant option to reduce heat gain and heat loss through roof structures in some regions of Australia. In urban areas, further research could be conducted in order to assess not only the thermal insulation provided by green roofs but green walls. In tall buildings, green roofs affect only the top floors, highlighting a need to consider both green roofs and green walls.
This research will be documented further in a forthcoming book titled Building Resilience in Urban Settlements: Green Roof Retrofit.
Two sets of experiments were performed in Australia (Sydney) and in Brazil (Rio de Janeiro). The Australian site is located at the University of Technology, Sydney in Ultimo; the Brazilian location is an existing building at the Oswaldo Cruz Foundation (Fiocruz). This article covers the Australian experiment only.
Wilkinson has also published a RICS Research Report Sustainable Urban Drainage – Retrofitting for Improved Flood Mitigation in City Centres.See www.rics.org.This research models the effect of wide scale green roof retrofit in the Melbourne CBD and also features a checklist for assessing the potential of a roof for green roof retrofit.