External vertical shading devices can be awesome things.

Done well, they provide more visual interest to a façade design, improving local comfort while reducing solar loads and cooling requirements. Done poorly, they may actually be achieving the exact opposite.

As our buildings continue to climb to greater heights, with a strong architectural preference for the clean, glass box aesthetic, ESD and mechanical services teams are more challenged than ever to make buildings function comfortably and efficiently within a clearly defined budget.

Vertical shading devices serve as a typical design response that can tick boxes in both engineering and architectural camps. Through thoughtful application, vertical shading can bring life back to a glass box and offer a unified design and engineering solution. This is particularly true on exposed western orientations where frames, spandrels, closed cavity façades and even in-between the vision glass and internal shade can heat up to 60 degrees Celsius during peak summer. It’s worth noting that this fact is ignored in most energy modelling processes, a significant omission in the equation of performance design.

As we climb higher and higher above the Australian state capitals, the application of vertical shading devices offer combined structural challenges that can begin to dictate the expected thermal performance criteria in the built form. With greater wind loads and external/internal pressure differences as our buildings get taller, our need to ensure vertical shading devices are fixed more securely and frequently within framing systems also increases to avoid rattling, whistling or frequent maintenance inspections.

Today, we are seeing various frame design responses to this engineering challenge that are resulting in significant thermal impacts within Section J compliance and or performance design. While these are not as thermally significant in the mild to sub-tropical climates of Sydney, Perth and Brisbane, they can be risky in Melbourne, Adelaide and Hobart if not designed correctly.

The primary challenge with vertical shading devices in colder climates is that we often need to structurally support them along the height of the frame at key locations and balance the thermal bridging effect on the System U-value or overall performance of the façade design.

The support or bracket required to maintain the vertical shading device acts as a major thermal bridge, significantly impacting frame thermal performance.

For those exposed to the world of U-values, a standard double glazed unit might have a U-value of 1.9 W/m²K (which is ok) while a poor performing three-millimetre piece of glass might have a U-value of 6 W/m²K (which is bad). In the case of the common bracket fixing above, it has a U-value of 25 W/m²K (yep, crazy), 15 times more heat transfer than the double glazed unit or 70 times more heat loss than your standard wall (R 2.8)!

While this story is not one commonly told as it is typically not picked up if you choose an off the shelf solution in the WERS glazing database or select a glazing product from a drop-down menu within an energy modelling tool, it is a significant challenge for façade designers and engineers to deal with as our desire for taller buildings continue to dominate planning applications.

Depending on the frequency of brackets, their structural requirements and architectural intent, it is my view that the vertical shading on a western elevation in colder climates is not a foregone conclusion that it is a good thing. As we balance the benefits across the entire year and not just peak periods, it can easily be argued that we are increasing our heating period, heat transfer and comfort at the expense of alleviating a short-term cooling period.

Undoubtedly, with a little bit more love in the design process, we can assist in reducing this effect by using carbon fibre brackets or minimising bracket frequency through good design. However, this comes with a major cost caveat that many projects would not be in a position to absorb. So the next time you see vertical shading devices on a western façade system in a colder climate, it may be worth asking, what are they actually achieving and would my money be better positioned elsewhere?

  • Thermal benefits aside, a far more important design parameter for these external sunshade devices is the attachment to the building in question. Most designers have no idea what the appropriate wind load is, since AS1170 gives no useful advice. Additionally, almost all of these large sunshade elements are not designed for dynamic loads, possible resonance; let alone the resulting fatigue failure. Several sunshades have failed, in recent years, at relatively new buildings in major Australian cities (two here in Brisbane just last year) and, fortunately, no one has died. But I predict that such a public tragedy will happen soon and the designers of these external architectural sunshade appurtenances will then be in the spotlight. Low-windspeed, dynamic, loading on external sunshades can result in a large-cycle-number, modest-stress, cumulative fatigue loading that many designers ignore. On the rare occasions when a full-scale, wind-tunnel, testing is performed the bolt-attachment methodology is almost always shown to be inadequate.

    One cause of this problem is a common misinterpretation of AS1170.2. Many designers calculate the sunshade's fundamental natural frequency and find it is, say, 10 or 20 Hz. The AS1170.2 Wind Load Standard speaks of dynamics being an issue at frequencies less than 1 Hz. So, all is OK, right? Wrong! Sadly, the Standard is referring to a whole building, not a sunshade appurtenance. The accumulation of fatigue stress from alternate-direction wind loads will still occur on small components, like sunshades. For all their thermal benefits, it will be the structural security of these shading devices that will ultimately define their success.

Dulux Exsulite Architecture – 300 X 250 (expire Dec 31 2017)