When to Use Building Simulation for Assessing Energy Compliance in Australia
The use of building simulation to determine the thermal performance of both residential and commercial buildings is on the increase. This is being driven by several factors including the availability of better software. However, building simulations involve more work and therefore costs more. This article examines this trade off.
The Building Code of Australia (BCA) describes the requirements for energy efficiency in Section J of Volume One and Part 3.12 of Volume Two. Both provide the choice between a rules-based approach called deemed-to-satisfy or DTS and computer-based modelling and simulation. The Australian Building Codes Board (ABCB) makes it clear that the DTS option is provided to provide a simpler and therefore lower cost approach.
DTS is rules based and therefore very prescriptive and inflexible. Generally, each facade or orientation is examined independently and the over performance of one facade or orientation cannot be used to compensate for the poorer performance of another part of the building fabric or glazing. DTS also includes many simplifications. For example, overshadowing by adjacent structures is often not recognised even though this may have a significant impact on the heating and cooling loads.
The aspect of DTS that causes the most grief is the glazing calculator. This tool attempts to assess the energy consumption resulting from windows. The ABCB provides a spreadsheet for these calculations where orientation, window size and specifications are entered.
Results with the glazing calculator can be mixed. For simple and non-challenging glazing, the outcome is usually fine. However, more complex designs typified by large ratios of glass area to wall area can result in very expensive glazing specifications such as high-performance double glazing plus heavy toning. Occasionally this might be the right answer but more often than not it is more a limitation of the DTS approach.
The prescriptive DTS approach is an option for commercial buildings (Section J) and residential buildings (Part 3.12). In New South Wales where BASIX applies for residential construction, it is called DIY. Again for all but the most straightforward designs, DIY is likely to lead to poorer design outcomes because of its limitations.
So why do so many architects and designers persist with DTS or DIY? One reason is that it does not require the involvement of a qualified energy assessor. However, a good assessor will work closely with the designer to ensure the best outcome so this issue is really one of education and experience.
The second reason is cost. A building simulation is going to cost more as a computer model of the building has to be created with all materials defined. However, it is important to keep in mind that once the model is built, the assessor can quickly evaluate different approaches to insulation, windows and shading. An improvement here can result in reduced construction costs and reductions in the ongoing operating costs of the building. These can be many times the investment in the computer modelling.
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