Green Roof design

Green Roof design

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:

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).

In the summer months there are primarily three types of heat transfer of concern to the engineer for building in the tropical climate, 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.

Convection heat transfer occurs between a fluid, 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.

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/m 2 /degC, the additional roof material used in construction of the green roof, for example, soil, etc. will creates a higher U-value, and less conductive roof.

The heat island effect results from our urban city lifestyle, 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.

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.

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.

We want to help spread the word about the benefits of green roofing — you can help by send us your green roof snapshots. Of course, they must be your photos that you have taken since we’re going to post them here. Don’t send them if you don’t have the rights and permit us to publish them.

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