Building Code: An Important (Thermal) Bridge to Span

Wednesday, October 12th, 2016
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The Australian Building Codes Board (ABCB) recently confirmed that they’re undertaking a process for developing Section J 2019.

This is a sure sign that industry is being engaged and workshops are taking place. Although a three-year timeline is of concern if the code is falling short, it finally allows time to really explore and develop the code response and properly engage with stakeholders.

One of the more shocking elements to Australia’s building industry is that it seemingly ignores the impact of thermal bridges when assessing the energy consumption of buildings. This is potentially going to change with the revised code release in 2019, as thermal bridging has been confirmed as a topic that’s up for discussion in the working groups.

So what is a thermal bridge? In essence, bridges are the thermal weak points in the fabric of a building. They are typically caused when there is a break or change in the insulation layer, due to either the geometry of the construction or its make-up.

Thermal bridges can affect not only the energy consumption of a development, but also the indoor environment quality, both from a thermal comfort and health perspective. Internal surface temperatures play a vital role in ensuring a space is comfortable. Typically, a temperature difference of five degrees between the internal surface and the internal air is regarded as the higher end of the range for preventing draughts and uncomfortable temperature gradients. And of course, cold surfaces can lead to condensation and therefore mould growth, particularly in residential developments which rely on natural ventilation.

There are a multitude of different types of thermal bridges, spandrel panels being particularly troublesome for many.

A spandrel panel is a solid wall element in a curtain wall. They can have various different claddings as their exterior skin – clear glass, opaque glass, metal, fibre cement sheet and so on – but all will (or at least should) feature some insulation stuffed in the cavity between the outer skin and the internal lining. They are most commonly located at the concrete slab level, but they are likely to also be present as full height panels. The image below shows a cut-through of a typical curtain wall system with the vision system (transparent glass) above and the spandrel panel (opaque elements with insulation) below.

section j - building code of australia

When the thermal performance of transparent windows is determined in accordance with Section J, the frames that hold the glass in place must be accounted for and the overall performance becomes an area-weighted calculation incorporating both the glass and the frame. This ensures that the highly conductive metal portion is represented in the thermal performance value used in the modelling. As metal is a fantastic conductor of heat, the higher the proportion of the metal frame, the higher the resulting U-value and the greater the heat flow through the system.

However, for spandrel panels, this loss through the frames is simply written off by the code and ignored. There is a part within Section J that talks about incorporating thermal breaks into metal framed walls. However, upon seeking clarification from the ABCB it has been confirmed that does not apply to spandrel panels.

So how does a spandrel panel really perform from a thermal perspective? Based on a typical structurally glazed curtain wall system and a centre-pane R-value of 2.8m2K/W, a spandrel panel would achieve overall a system R-value of between 0.5 and 0.8m2K/W, with the final value depending on the geometry. This shows that it’s rather likely that spandrel panels often have less than one-fifth the required thermal resistance.

section j

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The edge and frame thermal losses represent such a large proportion of the overall heat transfer that it’s impossible to achieve a total system R-value of 2.8m2K/W by insulating the cavity alone. To achieve the stated Section J deemed-to-satisfy figure, either thermal breaks in the frames or insulated internal linings are required.

So what does this mean for the potential revision to the code? Will Section J require all curtain walls to be thermally broken? It’s probably unlikely as undoubtedly there would be uproar from certain parts of industry, and to get past that there would need to be some political will. Maybe the deemed-to-satisfy figure will be updated for curtain wall systems so that the realistic performance of the spandrels is more truly represented?

Either way, the framing of spandrel panels is just one example of thermal bridging in our construction techniques. To reduce the impact, there are typically two design solutions – removing/breaking any high-conductivity material that connects the interior to the exterior, and maintaining a constant line of insulation. It’s pretty simple, really. Energy consumption is reduced and thermal comfort is improved. Surely we can’t keep our heads in the sand and carry on ignoring these flaws past 2019?

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