Using glass to reduce heat gain proves a clear winner

With temperatures of up to 50oC just a couple of months away, heat gain in the Middle East is now back on everyone’s agenda. Construction Week takes a look at how the type of glass used in a building has a direct impact on how much solar radiation is allowed to enter the structure.

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By  Sean Cronin Published  April 30, 2005

|~||~||~|High-rise buildings have developed considerably since they were first pioneered in the USA. The early skyscrapers such as the Empire State Building have predominantly stone façades perforated with windows. As tall buildings developed, so did their façades, and by the 1980s large monolithic glass elevations became the order of the day. This trend has continued, and today the majority of high rises feature a curtain wall that is predominantly made of glass. This has been rationalised to some extent as solid cladding is now often used for the façade to conceal services and the building’s structural frame, so that the glazing appears uniform. Buildings such as Emirate’s Towers have a mixed façade. Modern glazing technology allows architects to design buildings with vast expanses of glass without losing building performance. Products such as low-emissivity and reflective glazing have now been developed to give a building the desired performance. As any car driver will testify, glazing performance in the UAE is all about blocking out the fierce sunlight. By shielding the building’s interior from the sun, a comfortable environment can be created. This is of obvious benefit to the occupants, and at the same time of benefit to the environment, as the level of artificial temperature controls such as air conditioning declines and reduces power consumption. With a rapidly expanding population, controlling power consumption is high on the agenda for the city of Dubai. One way in which this dilemma is being tackled is through legislation that stipulates glazing must meet certain performance criteria. In essence, glazing must cut out the sunlight to lighten the load placed on air conditioning and electricity consumption. The legislation, which came into effect a year ago, states that if the vision area (glazed area) of a building is 10-40%, then the U-value of the glass should be a maximum of 3.27 and the shading coefficient should be no less than 0.4. If more than 40% of the façade is glass then the U-value should be a maximum of 2.1 with a shading coefficient of 0.35. In case of showrooms, the U- value should be 2.5 and the shading coefficient should be 0.76. Ground floor areas are excluded from these calculations. This legislation reflects the fact that glass is the weakest link in a building’s façade, as it allows light through as well as heat. By promoting these green building principals the authorities have told architects that they must consider this issue, because there is now a set of minimum target levels that must be met. The legislation represents a big step forward, as no other state in the Middle East has adopted such a policy for energy control. The policy is also enforced, as a building permit will not be granted unless it is demonstrated that the installed glass meets the requirements. The same is true in some other Gulf States: Bahrain, for example, now has a set of performance requirements for glass that are much more severe than those currently applied in Dubai. Unlike most other Gulf States, Bahrain does not have much oil — which makes energy production more expensive — therefore it makes sense to reduce the level of heat entering the building. Although constraints have now been placed on the use of glass, the options available to architects are still quite considerable. Modern technology means high performance glass can be completely transparent. These are mainly multifunctional low ‘e’ glass types that have good light transmission and a low internal reflection. These glasses offer solar resistance with very good thermal insulation. That means it offers protection from two kinds of heat; the direct heat from the sun, and the ambient heat that is conducted through the glass from the air outside. The product comes in a range of reflectivenesses, but all have one very important desirable feature — the internal reflection is minimised so that it almost gives the appearance of tinted glass when viewed from the inside. This means that the view from a building can be seen clearly at night, which is particularly important in marina-type properties with coastal views. With so many beach developments in the region, it is not surprisingly that low indoor reflection is currently a hot topic of conversation in the market.||**||

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