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    1 Bligh Street versus 100 Mount Street: the tale of two double skin facades

    Nathan Johnson

    There is a deep interest in the Australian commercial market from developers, among other parties, for full height glazing for commercial office projects.

    On the other hand, there is a growing concern from sustainability experts (and studies to show) that full height glazed buildings use more energy to run than one with punched windows and are detrimental to the environment.

    One study from Cundall sustainability consultancy that tested the thermal and daylight performance of nine different glazing and shading configurations concluded that from an energy efficiency, daylight and comfort perspective, punched windows perform better than unshaded full height glazing.

    But there is one floor-to-ceiling glazing system that challenges this assertion, and although costly, is becoming increasingly popular in Australia. The system is a double-skin façade (DSF) with cavity venetian blinds and has been used by Australian architecture firm Architectus for two significant projects.

    DOUBLE SKIN FACADES WITH MARK CURZON, ARCHITECTUS. Left: 1 Bligh Street by Architectus and Ingenhoven. Right: 100 Mount Street.

    DSFs have been used on European buildings for over 20 years. However their uptake in Australia has been slow primarily because Australia’s glazing market is smaller and more expensive.

    A DSF usually consists of two layers: an ultra-clear single pane of glass (usually low iron) on the exterior and an interior double glazed unit (DGU), separated by a cavity containing a blind.

    There are many variations to the type of glazing and frames that can be used for DSFs but the main difference seen in Australia has thus far come from variations in the cavity.

    1 BLIGH STREET SYDNEY VERSUS 100 MOUNT STREET NORTH SYDNEY

    1 Bligh Street, completed in 2011 by Architectus, Ingenhoven and SOM architects, is an award winning exemplar of a naturally ventilated double skin façade (DSF). The system comprises a low-iron exterior pane of glass and an interior DGU skin with low-e glazing and thermally broken frames. The two skins are separated by a 600mm open air cavity with venetian blinds, naturally ventilated by air passing through a void at the bottom and top of each floor plate. Air passes through the cavity, cooling the temperature before being expelled through another void at the top of the ceiling slab.

     

    100 Mount Street on the other hand is a Closed Cavity Façade (CCF) that has a sealed and pressurised cavity rather than a naturally ventilated one.  It is designed to have a slight leakage so a small amount of dehumidified pressure can be pumped into the cavity to stop the ingress of dust. (click to enlarge image)


    Image by Aecom.

    Mark Curzon, Principal of Architectus was project architect for both buildings, and maintains that 100 Mount Street is not an upgrade from 1 Bligh Street, but a sub species, chosen because of different council envelope restrictions and budget.

    “The CCF is a product that had been tested over the last five years by Permasteelisa and is used more widely in Europe,” says Curzon. 

    “We selected it because it reduced the thickness of the façade zone, used less material and is effectively cheaper, but also delivered the performance of the naturally ventilated double skin system.”

    “It takes up less space, instead of having a 600mm façade zone running around your floor plate you have 150mm which effectively sits within the depth of a traditional commercial DGU façade system.”

    “In North Sydney where 100 Mount Street is, we are controlled only by envelope restrictions for which the building sits in so it made sense to use a Closed Cavity Façade (CCF) in order to use space effectively. But at 1 Bligh we were controlled more so by restrictions on interior floor space (GFA) for that site so we didn’t have the pressure on the envelope that 100 Mount Street has.”

    WHY A CLOSED CAVITY FAÇADE (CCF)

    The venetian blinds are never closed and only ever reach a 45 degree angle to keep the direct solar out. This means you can always look outwards and downwards which is the predominant viewing direction from a tower. Image: Stephan Liebl, Dillingen

    The venetian blinds are the key to the CCF at 100 Mount Street and the Double Skin Façade (DSF) system at 1 Bligh. They mitigate direct solar, reduce heat gain and control glare and are completely controlled by a Building Management System (BMS) to adjust at different times of the day in different areas of the building. Because the blinds are protected from the elements by the exterior pane of glass the blinds are not wind effected and expected to have a long life span.

    “Double skin facades perform much better that other commercial facades on several fronts,” says Curzon.

    “Firstly, environmentally because the solar loads on the building are massively reduced less energy is required to control the inside environment. Secondly, the external views are optimised as the venation blind rarely needs to be closed as the blind mitigates direct sun at 45 degrees and hence allows excellent views outward. Thirdly, they give you the ability to future proof buildings, for example they can provide natural ventilation to the floor plate and night purging opportunities.”

    “One of the things that double skin facades do is that if you have a massive solar load at 6:00 am in the morning because the sun is coming at a direct angle you can program your blinds to be closed at that time to mitigate that super loading,” notes Curzon. 

    <A CCF’s compatibility with a BMS is also central to the overall performance of the building as the entire HVAC system and amenity can be tailored to how the façade is working at different times of the day. Image: Stephan Liebel, Dillingen. 

    “A lot of buildings have trouble with super solar loading in the morning and afternoons but the automated blinds can completely close at these out of office hours times.”  

    Venetian blinds are also a great performer against discomfort from glare, says Curzon.

    “Glare is usually at its peak on a bright cloudy day.”

    “So with the venetian blinds you can simply drop them down and have them at horizontal or on a slight tilt, meaning you can take out the sky glare but maintain the view.” 

    COMPARISONS:

    The Westpac Building Sydney by Johnson Pilton Walker recruits full height glazing with a low Visible Light Transmittance glass. Image: Dexion

    Curzon explains that CCFs not only meet the demand from developers for highly transparent floor-to-ceiling glazing but also provide the energy performance actively sought by current day developers as well.

    “In other traditional systems you have to add layers such as internal blinds or use a low visual light transmitted (VLT) glass (darker) which would affect views in and out,” he explains.

    “If you also wanted to match the energy performance of a CCF with a traditional DGU you’d also probably have to reduce the glazed area. Whereas a CCF’s area might be 3.2 metres for example, a DGU’s might have to be reduced to a 2.6m unshaded area to have the same result.”  

    “Basically, with CCFs you’ve got a blind that’s part of base building, energy costs are low, you’re getting very clear glass and great views out, transparency day and night, an excellent indoor environment, and the venetian blinds are doing the glare job as well.”

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