Today, most Australian design teams will have experienced the post-2010 industry shift from using the BCA Section J calculators to assessing the façade design ‘allowance for glazing’ by adopting the JV3 alternative energy modelling pathway.
Through this alternative pathway, we have become accustomed to a more flexible approach of ‘trading off’ performance elements of the envelope to ‘outperform’ a theoretical reference design. While this has brought some freedom to designs, in particular more glass (by area), it carries a very large caveat; the reference design that we are trying to outperform is, to be frank, absolute (theoretical) rubbish.
The reference design is made up of NCC (BCA) default values for all building services and building fabric components (all good) except for the glazing systems (not so good). In the case of glazing, a consultant or engineer will often generate the absolute worst performing glazing based on the designs required window to wall ratio. This makes 100 per cent sense, as their duty is to the client in order to make the design ‘work.’
For example, if 40 per cent of a façade is designed as vision area and the rest made up of various opaque elements (spandrels, composite finishes, standard brick walls, and so on), the industry’s informal protocol is to tweak glazing performance values (Total SHGC [solar control] and U-values [conductance]) to generate a relic of glass thermal performance more common in the 1970s than in 2015.
For example, in the case below for Melbourne, a 50 per cent window-to-wall ratio means that our reference building glazing has Total SHGC and U-values of 0.28 to 0.33 (depending on orientation) and 5.5 watts per metre squared Kelvin (W/m2K) respectively.
To the uninitiated, this typical tale means dark glass on north, east and west façades and highly transparent glass (SHGC 0.80) to the south, while we can freely throw in some single glazing options to round off this disconnection with modern design.
Are we really suggesting that our reference of good design is so unambitious?
Could following this trend not directly facilitate an ever increasing gap between the global building stock and the structures we are building today?
Today’s ‘generation’ of performance requirements for the reference design are completely at odds with other advanced building markets and indicative of a need for greater discussion around how we set the minimum performance of our future building stock.
Without any real responsibility or system to ensure we are using a common method (such as the accreditation body ABSA on the residential side of compliance modelling), commercial modelling is arguably unaudited and has no governing body to bring those undertaking the modelling into line.
In Shanghai, if we use the local energy design standard (DGJ08) as we would in Australia for our reference design, the performance values would be set as a function of our window to wall ratio. Meaning, if we have 50 per cent WWR, then SHGC and U-values are non-negotiable. This makes sense if we want an easy to identify reference that is indicative of modern building performance.
Moving further south, in Hong Kong, Jakarta or Singapore, the amount of heat transfer is directly limited via thermal transfer calculations (OTTV, ETTV, RTTV), creating an onus on the design process to ensure performance glazing is specified and purposeful shading devices are designed for.
The same would apply if you opted for the American/International standard, ASHRAE 90.1 (the most commonly adopted reference design standard for performance). There are no magic tricks to set a reference design; a standard sets the values you must use. The UK is not exactly the same, but it is analogous.
We are already struggling to surpass this watered down reference design in many of today’s design teams. If we were to adopt international standards, most of our buildings would be forever stuck in the planning cycle, never to be realised.
What we need is a more unified (a single calculation engine) and challenging (mandatory air-tightness) approach to setting higher minimum façade performance within our buildings. Otherwise those that are going up around us will simply stand as testaments to our design and construct shortcomings and lack of foresight.