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Hong Kong Green Roof Guide
8 August 2008

John Herbert, hong kong energy saving expert, BEAM expert
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 get your copy from the
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.

green roofing solutions lower energy costs

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.

But before we delve deeper it is worth reviewing the standard roof. Typically a composite flat roof structure consists of a structure typically a reinforced concrete slab, covered by ashpalpt and weatherproofing membrane system, finished with a protective layer. Often the final protective layer is provided by concrete paving or tiles. In modern buildings a layer of foamboard is added above or below the slab to improve the insulation (U-Value) to acheive the OTTV criteria.

The Energy Perspective

In the summer months there are primarily three types of heat transfer of concern to the building services engineer in the tropical climate is solar radiation (direct solar gain, and indirect radiation reflected or diffuse solar gain), convective, 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.

Also with its cousin, indirect radiation (also know as diffuse or reflected radiation) that's the solar energy reflected back from any clouds or particles in the atmosphere, green foliage also helps prevent in-direct radiation reaching the roof surface and heat transfer to the room below.

Convection heat transfer is limited to fluids, in this case air and the roof surface. Since a green roof provides a barrier between the roof surface and the air currents it also provides good protection. And hot air rises away from the surface under convection.

Finally the last mode of heat transfer is conduction. The energy transfer by conduction is dependent on the sum of thermal conductivity (r-value) of the materials, and the temperature difference across the structure. If a typical U-value for concrete roof might say 0.5-1.5w/m2/degC. Additional layers used in construction of the green roof, for example, gravel, soil, would improve the U-value and less conductive roof in theory. However, those green roof layers, for example the soil are typically wet (it keep the planting alive) therefore in reality add little resistance to energy flow.

Urban Heat Island Effect

The heat island effect results from our urban lifestyle, the materials used in modern city construction these include concrete, bitumen, and tarmac absorb solar heat energy.

Buildings with green roof technology help mitigate the heat island effect because the living roof doesn't absorb and re-radiant heat energy like concrete.

A green roof is a living structure that breaths water vapour, keeping a cooler environment. Lowering the abmient temperature also benefits the outdoor air intake (if located at roof level) reducing the cooling load.

Storm Water Management

A typical flat roof consists of a waterproof non-absorbent surface designed to direct the rainwater to the drainage system instantly. Whereas a green roof can absorb water and releasing it slowly to the drainage system, therefore in areas prone to flooding the slower release helps prevent flooding events.

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 possible weapon in the arsenal.

Cool White Roof

As an alternative to green, white is a cool roof option., white or light coloured roofing systems can be designed to reflect solar energy lowering the cost for air conditioning upper most floor. Two important criteria 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.

About the Author

John Herbert is a veteran chief engineer with more than 30 years international engineering experience, educated in the United Kingdom, he has worked in the United Kingdom and then across Asia for more than two decades engaged by international and local companies. He is a Hong Kong Registered Energy Assessor (REA), a BEAM Professional, and stationed in Hong Kong.


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