A well-planned patch can lower internal building temperatures by as much as four or five degrees
Elizabeth Horscroft – SCMP – Updated on Jul 09, 2008
Hong Kong is getting hotter, and not just during the sweltering summer months. The urban landscape with its closely packed high-rises and limited open space is trapping heat and restricting airflow, with the result that average ambient temperatures are edging up.
That creates more energy consumption, ostensibly to achieve cooling, but instead contributing more to the underlying problem. “Flats have become warmer due to their compact nature,” said Yuguo Li, a professor at the University of Hong Kong’s department of mechanical engineering. “[They are] good for trapping heat, but not for releasing it.”
Furthermore, the canyon effect created by skyscrapers, narrow streets and concrete surfaces, means that solar radiation is absorbed more readily, adding to the rise in temperature indoors and out. Most buildings are not designed to make the most of natural ventilation, so opening a window often does little to cool a room or apartment, even at night.
Recent infrared images show that some buildings in Central, particularly those with dark surfaces and glass facades register temperatures reaching 40 degrees Celsius. This is one of the most obvious manifestations of a phenomenon known as the “urban heat island”, which is now evident in many cities and is becoming a subject of increasing academic study.
In a 2006 research paper entitled Benefits and Potential Applications of Green Roof Systems in Hong Kong, Sam Hui of the University of Hong Kong’s department of mechanical engineering addressed the topic. He pointed out how, in summer, the urban heat island increased the use of energy for cooling and accelerated the formation of urban smog. Dr Hui traced the problem mainly to the lack of vegetation and high absorption of solar radiation.
Several measures are recommended to mitigate these factors. Hong Kong needs more natural vegetation and open areas. And each of us can play a part by lobbying for change on a wider scale or simply starting our own garden on a roof or balcony.
Craig Doubleday, a director of landscape design firm Urbis, explained one of the most tangible benefits of a roof garden. “Research in Singapore shows a net annual saving of 15 per cent for air-conditioning costs. And research in China has shown a reduction of two degrees Celsius for internal room temperatures.” He said that other studies indicated a well-planned roof garden could lower internal building temperatures by as much as four or five degrees.
There was also scope for plants to mask other external surfaces and they could be grown on balconies to contribute significantly to the overall cooling effect. Green roofs work through the evapotranspiration process. This means that plants do what they always do – they absorb the sun’s rays and release oxygen into the atmosphere after photosynthesis.
The good news is that developers are gradually awakening to the wonders of nature and have been spurred by the realisation that a roof garden can add value to a building. “[Roof gardens] are setting a trend in the industry … and are viewed as highly favourable in new developments [because they] increase the attractiveness and value of new properties,” Mr Doubleday said.
Developers like the fact that a green roof reduces energy costs and extends the life of the roof. This quickly translates into savings on maintenance and structural upkeep. Dr Hui said: “An additional benefit is biodiversity. Roof gardens attract insects and birds to urban areas.”
The design of a roof garden depends on taste, practicality and structural loading. Integrated green roofs, which go well beyond the simple positioning and cultivation of potted plants, fall into three main categories. The extensive type requires low maintenance and consists mostly of self-generative plants such as moss.
The semi-intensive type needs a sturdier structure to support the weight of the additional drainage systems, soil and insulation necessary to allow plants such as herbaceous perennials, lavender or grass to grow successfully. The intensive type is the most expensive and difficult to maintain. This type can include trees, bushes and ornamental plants and, depending on the space available, can even be designed to incorporate ponds, benches and winding paths.
Mr Doubleday said that most people should stick to using containerised or potted plants. Bamboo and palms give the maximum greening effect, with minimal need for maintenance or changes of soil, and they provide a noticeable cooling effect. “But remember large pots filled with soil and plants can impose a significant load on the [structure],” he said.
“I have seen instances where three large pots were placed on the balcony of an office building. The loading [was] affected to such an extent that the doors to the balcony in the unit below could not open.”
An integrated roof garden with the need for soil, drainage and irrigation systems requires careful planning and expert engineering advice, particularly if it is installed long after the construction of the building. “Most existing roofs and balconies were designed only to take specific live loads,” Mr Doubleday said.
Individuals or residents’ committees thinking of “going it alone” with a roof garden should, therefore, consider loading issues, drainage, water supply and access for maintenance.
Sufficient thought should also be given to sunlight and the prevailing wind direction, the aspect and possible pollution in selecting the most appropriate plant species.
Costs are variable, but it is especially important to remember that semi-intensive and intensive roof gardens will require retrofitting additional waterproofing and drainage to an existing roof, which can be expensive.
Derek Townshend, a senior landscape architect at Urbis, estimated that the cost of setting up a roof garden with trees, shrubs, a few benches and “decent looking containers” would be between HK$1,200 and HK$1,500 per square metre for a private individual paying retail prices. “The cost of providing potted plants for a roof or balcony is relatively minimal, with a medium-sized ornamental pot, soil and plant starting at around HK$400 each,” he said.
Fixed costs would include water, electricity and equipment, and Mr Townshend put these at roughly HK$7 per square metre per year. Other than that, any expenditure on upkeep would depend largely on the type of plants chosen and how much you enjoy gardening.
Dr Hui said it would be money well spent. “Compared with a conventional bare roof structure, an extensive green roof system would cost two to five times more for the initial investment,” he said.
“However, if planned and designed properly, green roofs could be less expensive than tiled roofs in the long run and provide many other tangible and intangible benefits to building owners,” Dr Hui said.