February 2010 – Work is underway at 1 Bligh Street, Sydney, to construct Australia’s first major high-rise building with a full ‘double-skin’ facade. JOHN POWER investigates whether two sets of glazing skins are really twice as good as single-skin options.
When 1 Bligh Street, a 29-storey office building overlooking Circular Quay in the heart of Sydney, is completed in April 2011, this $270 million structure will be a visually and functionally unique landmark.
The 42,000-square-metre development will deliver 6 Star Green Star (5 Star NABERS Energy) performance through a range of tightly integrated ESD (ecologically sustainable development) solutions, the most notable of which is a ‘double-skin’ facade.
As the phrase suggests, a double-skin facade consists of two separate glazing systems – in layman’s terms: two layers of windows. The design being incorporated into 1 Bligh Street has an INNER skin of high-quality, double-glazed windows, and an OUTER skin of single-sheet laminated glass. There is a 600mm cavity between the two skins – providing sufficient space to accommodate a sophisticated automated venetian blind system, as well as walkway gantries at each level of the building for access by cleaners and maintenance personnel.
While double-skin facades are popular in the Northern Hemisphere, where such energy-efficient designs are highly prized for their superior insulation and anti-glare properties, there are fewer examples of double-skin systems in Australia. The 5,500-square-metre Bendigo Police Station in Victoria is a good example.
The outermost glass skin at 1 Bligh Street has two primary functions: first, fixed horizontal ventilation slots at all levels of the building encourage upward airflow in the cavity between skins, helping to expel unwanted hot air; second, this outer skin serves as a weather shield to protect the motorised venetian blinds from severe winds.
The inner skin, utilising double-glazing for world-class thermal efficiency, provides an effective barrier against heat gain in summer and heat loss in winter, and maximises the benefits derived from the external venetian blinds.
According to Ray Brown, director of the Australian architectural firm Architectus – which designed the building in collaboration with German colleague Christoph Ingenhoven – the geographic and climatic conditions at 1 Bligh Street were major influences on the specification of the double-skin façade.
“The siting was really a fundamental issue; it all comes down to the siting, the use of the building, and the natural attributes of the site,” Brown says. “The building is at the heart of the commercial core of the city and looks out over the harbour. Even though it is set back several blocks, there are panoramic harbour views from the fourth level.”
Brown says the venetian blinds between the glazing skins are intrinsic to overall building performance. “Normally, external blinds of this kind can’t survive the conditions of a high-rise building more than 40 to 50 metres off the ground, so the second (outer) skin of glass is a wind shield for the blinds, which are the main solar shading system.”
Co-architect Christoph Ingenhoven, speaking recently1 with Professor Steffen Lehmann, UNESCO Chair in Sustainable Urban Development for Asia and the Pacific, and chair of the School of Architecture and Built Environment at The University of Newcastle, adds that double-skin sun protection is vital for both energy efficiency and user comfort.
“Our first eco-high-rise was the RWE Tower in Essen 25 years ago, which was all about the building’s envelope,” Ingenhoven says. “Since then, we have done over 40 buildings with dual-glass skin facades, and the technology has greatly evolved over this time. The Bligh Street tower will be the first high-rise to receive a 6 Star certificate on the Green Star rating system. This tower will be equipped with a real double-skin façade and will be ventilated by an atrium stretching the whole height of the tower. Fifty percent of the ventilation will be provided by the double-skin façade.
“The building will capture great gap views to Circular Quay, and there is a whole range of things we have introduced that will make the project work well. For instance, the façade will allow us to have a 100 percent shading solution and glare protection, with perforated internally adjustable blinds within the 600mm double-skin cavity. The sun protection is very efficient, while maintaining the views, so we can use non-tinted glass on the outer skin. This makes the building extremely transparent and will offer the user a different experience. The ventilated outer skin is made of clear glass, which will ensure a highly transparent building.”
The façade of 1 Bligh Street in Sydney will feature two distinct sets of glass skins. Automated venetian blinds will function between the skins, shielded from the elements.
BUILDING WITH VISION
Indeed, the clarity of the glass used in both glass skins of the façade will be one of the most eye-catching elements of the design.
Conventional office buildings usually incorporate some form of tinted or reflective glazing in order to minimise the amount of direct sunlight and heat entering the structure. The trade-off is a darkened or pearlescent finish that can visually isolate the occupants of the building from the natural environment and create unwanted reflectivity, particularly at night, when the inner glass can resemble a mirror.
The Bligh Street glass, supplied by G. James, has a 60 percent VLT (visual light transmission), compared to normal office glazing specifications of approximately 25–40 percent. In other words, onlookers will be struck by the crystal clear views into the building; meantime, the occupants will experience ‘true-to-life’ panoramic views of the harbour and the city’s genuine colours.
The double-skin façade will “definitely stand apart”, says Kerryn Coker, from engineering consultancy ARUP, who has worked closely on the project.
“In terms of the overall look, you have to realise that most commercial buildings have a VLT of no more than 35–40 percent, used with internal blinds that mean ‘no views’ when drawn,” she explains.
“So the immediate benefit of a double-skin façade is that you introduce operable external blinds, which typically you can’t have on a high-rise, to produce a shading coefficient of 0.15 with the blinds down and uninterrupted views when they’re up.”
Coker says that the external skin’s fixed (open) ventilation slots, measuring approximately 100mm wide, will allow wind to circulate fresh air through the cavity between the skins and stop excessive heat build-up. This means the internal skin will never be exposed to air temperatures that are vastly higher than the outside ambient air temperature.
“Wind rather than convection will typically drive the air movement,” Coker explains. “In Sydney you practically never experience a completely still day.”
The outer glazing skin, i.e. the external glass panel exposed to the elements, has been designed to promote airflow through fixed horizontal vents at the top and bottom of each level of the building, thereby preventing excessive heat build-up during summer.
Strong winds, of course, are not compatible with sophisticated venetian blind systems. As already mentioned, the outer skin of the building is a protective barrier against the elements for these units.
Jason Turner, whose firm Turner Bros is responsible for the installation of the motorised venetian blinds and accompanying façade control system – all supplied by Horiso – says there will be a total 1774 separate blinds throughout the building, i.e. an average 64 units per typical level.
Each unit, measuring 3300mm high and 1702mm wide, will be positioned in the cavity between the two skins, “on the inside of the outer glazing,” Turner notes, “where the pelmet attaches to the bottom of the vent at the top of each section.”
Overall, these units will form part of a powerful management platform to conserve energy and optimise user comfort. The aluminium blades [colour RAL 9007] will have a width of 80mm each.
Turner says a Horiso Dynamic Façade Controller has been programmed to track the path of the sun, which changes slightly each day of the year. The system will automatically adjust the angle of the blades in each blind depending on the orientation of the façade and the momentary position of the sun.
One of the strengths of the system, Turner says, is its ease of operation. The facility manager will be able to make use of a GUI (graphical user interface) for on-screen views of the positional settings of each blind on each floor. This system provides complete centralised control, which is vital, for instance, when overriding automatic functions for scheduled cleaning.
Similarly, individual building users can manually override pre-set functions for personal privacy, or to darken a room for video conferencing presentations, etc. Automatic functions will resume after a specified period.
It goes without saying that a double-skin façade requires more cleaning than a single-skin glass façade: “about twice as much,” Ray Brown jokes.
However, as Kerryn Coker observes, while the exterior of the outer skin – the surface exposed to the elements – will probably require six-monthly cleaning, the other surfaces will require less regular attention.
Coker says a customised, permanent BMU (building maintenance unit), comprising a cantilever-lowered cradle, will be used for the outer face of the building.
The cavity between the skins should be less susceptible to weather-related grime, and the gantry set-up will greatly enhance the speed and safety of this cleaning task.
1. Professor Christoph Lehmann spoke to Carl Ingenhoven recently as part of an interview for the article ‘Beauty in Necessity: Christoph Ingenhoven’, which appeared in the August-September 2009 issue of Architectural Review, a sister publication of Facility Management.
1 BLIGH STREET AT A GLANCE
PROJECT NAME 1 Bligh Street, Sydney
ARCHITECTS Architectus + Ingenhoven Architects
BUILDING CO-OWNERS DEXUS Property Group and Cbus Property