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Green Roof in Asia
8 August 2008 | Link | John Herbert
The Hong Kong Architectural Services Department (archSD) commissioned an invaluable report titled "Study On Green Roof Application In Hong Kong" published in February 2007, if you have not read it yet here is the link to download your own copy:
Study On Green Roof Application In Hong Kong (PDF, 10MB)
The details study primarily focuses on the architectural, planning, maintenance, and other building related aspects of green roofing but does not cover possibly the most important point, namely the benefits of lower energy consumption for the building owner.
Work in Canadian universities and elsewhere has started to uncover the other advantages of green roofing technology, with initial studies indicating upto 25% lower cooling operating costs for buildings with a green roof.
Before we delve deeper it is worth reviewing the standard roof. Typically a composite flat roof structure consists of a structure concrete slab, covered by a weatherproofing membrane system, and finished with a protective layer. Often the final protective layer is provided by concrete paving or tiles. In more modern buildings a layer of foam board is added above or below the slab to improve the insulation (U-Value).
The Energy Perspective
In the summer months there are primarily three types of heat transfer of concern to the engineer for building in the tropical climate, direct radiation (direct solar gain), indirect radiation (reflected or diffuse solar gain) and conduction.
Direct radiation is the most obvious, it is the direct energy from the sun, the most effective energy reduction method being provision of shade to prevent the solar energy actually reaching the roof surface. Here a green roof scores well, planting trees, shrubs and even grasses prevents direct solar radiation reaching the roof. Lowering the solar gain directly hitting the roof lowers the cooling demand requirement.
Indirect radiation (also know as diffuse or reflected radiation) is the solar energy reflected back from any clouds or particles in the atmosphere. And again green foliage also helps prevent in-direct radiation reaching the roof.
Finally the last mode of heat transfer is conduction. The
energy
transfer by conduction is dependent on the sum of thermal conductivity
(u-value) of the materials, and the temperature difference across the
structure. If a typical U-value for a concrete roof is 1.5w/m2/degC,
the additional roof material used in construction of the green roof,
for example, soil, etc. will
create a deeper, and less conductive roof, having a higher
U-value.
Urban Heat Island Effect
The heat island effect results from our urban city lifestyle and the materials used in modern city construction these include concrete, bitumen, and tarmac that absorb heat energy. Building with green roof technology helps mitigate the heat island effect because the live roof doesn't absorb and re-radiant heat energy. A green roof is a living structure that breaths water vapour, keeping a cooler environment.
Lowering the roof temperature also benefits the outdoor intake (normally located at roof level) lower temperatures lower the cooling load.
Storm Water Management
A typical roof consists of a water
proof hard non absorbent surface and is designed to direct all the
rainwater to the drainage system. ponding is a concern for flat roofs.
A green roof absorbs water releasing it slowly to the drainage system,
therefore areas with drainage problems the slower release
helps prevent flooding.
One example, a green roof offers the
opportunity to provide shade, therefore lowering the direct solar gain
impinging upon the roof. However, calculating the shading effect
created by one million blades of grass, plus planters, shrubs trees
and bushes, is beyond reasonable calculation, and arguably has delayed
the introduction of green roofing where the quantum benefit is
ambiguous.
It is clear that green roof will reduce the
heat transfer on the roof, and therefore the
operating cost for building owners, the outstanding question - what is
the extent of the energy saving? The Canadian studies indicate that
savings
of 25% are achieved, however more research is needed to validate the
results in a tropical climate.
In vertical
cities like Hong Kong where the ratio of the roof, and podium
roof compared with the entire building envelope is a relatively small
area, we need to remember there is no silver bullet solution, no single
solution or technology will solve all the problems, green roof is one
weapon in the arsenal.
Cool Roof
As an alternative to green, white is a cool option. White or light coloured roofing systems help reflect solar energy lowering the cost for air conditioning upper most floor. For a cool roof two criteria are important:
Reflectivity is term used to describe the ability to reflect solar energy values exceeding 0.7-0.85 are desirable. And Emittance no matter what material is used, some portion of the solar energy will be absorbed, and the term emittance describes how effectively the roof surface will re-release that absorbed heat energy. 1.0 being the perfect emitter, values exceed 0.75-0.85 are desirable.
As with every system maintenance
is key to long term success.
Further Information
If you need further information regarding Kelcroft and the services we provide to help create sustainable business in Asia call Mr John Herbert at our Hong Kong office telephone +(852) 2335 9830 or by fax +(852) 2335 9862.