Taking a performance-based approach to Deemed to Satisfy
There are two pathways for demonstrating a residential building meets the energy efficiency requirements of the National Construction Code: Performance Solutions and Deemed to Satisfy.
Many of you will already be familiar with the Deemed to Satisfy provisions for energy efficiency. It consists of a prescriptive “how to” list of acceptable construction practices that have to be completed in order to demonstrate compliance with the energy efficiency provisions. Currently under the NCC, there are two Deemed to Satisfy options for energy efficiency – energy rating or elemental provisions.
On the other hand, a Performance Solution provides a performance-based framework of what needs to be addressed in order to achieve compliance. While it provides an overview of what needs to be tested or considered when proposing a Performance Solution, it’s not as prescriptive as a Deemed to Satisfy solution. It’s up to you as the designer to determine the best approach that will achieve NCC compliance.
Now, it may sound a bit contradictory to try and apply a performance-based approach to Deemed to Satisfy under the National Construction Code. However, bear with me, as you’ll hopefully discover that it’s entirely possible to achieve the Deemed to Satisfy requirements using a performance-based methodology that isn’t that different to what you may already be doing. It’s an approach that can also produce better outcomes for both compliance and cost.
When it comes to demonstrating energy efficiency compliance, in my experience most builders, architects and building designers will only complete the energy efficiency compliance requirements just prior to submitting for the building permit or licence. The general view seems to be that energy efficiency is just another thing that has to be done in order to obtain a building permit, and so let’s just do the minimum (or whatever it takes) to achieve compliance.
While this may at first appear to be the most time-efficient way to achieve energy efficiency compliance, there are some flaws to this approach. By the time a proposed dwelling is ready to submit for a building permit, the design and layout of the building has been largely finalised, and there are very few changes that can be made at this late stage of the design process. Consequently, if the proposed design doesn’t meet the NCC energy efficiency requirements, expensive construction specification and material upgrades may then become necessary to achieve compliance.
There’s also a possibility that the design still doesn’t comply even with specification and construction material upgrades. This may then mean significant changes to the design or having to apply for amendments to previously issued planning or development approvals; all of which may cause delays and result in additional costs and fees.
For example, one area where architects and building designers often get caught out is with windows. If the design has been finalised with large areas of glazing that have non-optimum orientation (such as facing east or west), it may then be necessary to upgrade the glazing specification with more expensive requirements such as thermally broken frames, double glazing, tinting and/or low-emissivity glass coatings. It’s unlikely these window upgrades have been budgeted for as part of the original project cost, which can then lead to a blowout in the construction costs and a very unhappy client.
Even when windows have the optimum orientation for solar passive gain, another common oversight is to have too much glazing on the north elevation with not enough shading. A glazing to floor area ratio of around 20-25% tends to provide the optimum amount of solar passive gain in winter, while a 600mm eaves overhang will typically provide enough shading to prevent excess sun in summer. However, if these considerations haven’t been allowed for in the finalised design, again expensive window upgrades or unforeseen design changes may be required.
So, what’s an alternative approach we could take? For the clients and architects that I work with, I try to encourage them to look at energy efficiency compliance as a way of informing the design process to achieve improved performance outcomes:
A key part of this approach is to start assessing the performance of the proposed design well before submitting for a building permit, and even before a planning or development application. Once the key elements of the floor plan, layout and elevations have been mostly resolved, there is generally sufficient information for a skilled energy assessor to conduct a preliminary rating on the design’s thermal performance.
Assessing the energy efficiency performance of the proposed design at this earlier stage can then help to inform important design decisions such as:
- How much and what type of glazing is ideal for each elevation or orientation of the building;
- Levels of floor, wall and ceiling insulation that are likely to be required, so that appropriate spaces and dimensions can be factored into the design;
- Where thermal mass such as exposed concrete floors or brickwork are best located to help improve indirect solar gain and regulate internal temperatures;
- What types of window openings are ideal for improving cross ventilation; and
- Where shading may be required to prevent excess solar gain during summer, without compromising solar access during winter.
These preliminary results can then be used to help shape and finalise the design and selection of materials and finishes, before submitting for development or planning approval. Because the key performance issues have already been considered before approvals and/or permits have been obtained, major design or specification changes are then unlikely to be necessary later on. You could then instead look at upgrades that will allow you exceed the minimum compliance requirements, if a higher level of performance is desired.
The other advantage to taking a performance-based approach is the ability to budget and plan for specification upgrades earlier in the design process. Take the previous example of glazing with non-optimum orientations – we may have a house design for a site with spectacular ocean views to the east or west. Understandably, the client may want more windows to take advantage of these views, even though these aren’t the optimum elevations for large areas of glazing.
By commencing the energy rating during the earlier stages of the design process, we can then start to fine-tune the windows on these elevations. We can work out the optimum glazing surface areas and where to position windows for views without compromising the thermal performance. Or if we were to decide to increase the glazing area by more than the optimum amount, we can then make an appropriate allowance in the construction budget for a higher window specification, so that it doesn’t come as an unpleasant surprise later. We may also find the addition of shading devices such as adjustable external louvres may also help to improve the thermal performance, which can then be integrated into the design rather than added as an afterthought.
A performance-based mindset to Deemed to Satisfy can even help to achieve performance outcomes better than the minimum requirement. Almost all of my clients are well informed about building sustainability & energy efficiency and so expect better than minimum compliance. By taking a performance-based approach to the design, we can often achieve energy ratings of 7.0 stars and above for modest increases in the construction cost, and sometimes at no additional cost at all.
Now it’s important to acknowledge that a performance-based approach may cost more in terms of consulting fees, as iterative testing of different design changes takes more time. However, I have generally found a few hundred dollars extra in consulting fees can result in cost savings of thousands of dollars in the construction budget.
As you can see, there are a number of advantages to taking a performance-based approach to complying with the NCC, even when using a Deemed to Satisfy solution. Instead of asking “What is the minimum that needs to be done to comply?”, the better question to ask is:
What’s the optimum way to achieve compliance by testing the performance of the design?
Rather than seeing energy efficiency under the NCC as just another compliance chore, I encourage you to see it as a valuable opportunity to improve and inform the design process. This approach will help you and your clients to achieve a better, more cost-effective design that also has great performance!