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NCC 2019: Thermal Breaks, To Be Or Not To Be?

After finally getting a post-up last week after a year hiatus, the proposed NCC Section J – Energy Efficiency changes have kicked me back into the blogging space with what I personally see as a very confusing vision for the Australian design and construction Industry, even to those experienced in this space.

Ringing around some of my Industry peers, many shared similar concerns. While mostly supportive, I have found generally agreement that a greater level of clarity and readership is required for these academically developed changes to be ultimately useful to a practical, cost sensitive industry reluctant to even the smallest of changes.

Supportive, I remain, but the more I dwell on the proposed changes, the more concerned I am that the powers that be are not on the right track chasing Performance Solutions and killing off the Deemed-to-Satisfy pathway with onerous or unachievable requirements!! However, if there was a time for pushing our industry, it is now. With Australia’s emission trajectory set against the Paris Agreement (2015) continuing to go in the wrong direction, the projected reduction in GHG emissions to 26-28 % on 2005 levels by 2030 are a must in the overall objective to ensure global warming remains under 2 degrees.

Of the many ‘deemed-to-dissatisfy’ conversations I had this week and last, the importance of higher performance façades and specifically thermally broken framing systems came up many times as they stand to become far more common across climates and building types as a result of the proposed changes. How do you define a thermally broken system? Are thermally broken framing systems a good investment? In heating dominated climates, sure, but what about cooling dominated climates? Thermal breaks, to be or not to be?

These questions, in part, can be answered in my first ever self-published post – ‘Thermal Breaks: A worthwhile investment for the Australian Office?’, a quick study to identify (mainly to myself) if improvements in window framing can lead to better office buildings. The conclusions supported heating dominated climates but left the advantage in cooling dominated climates a bit in the air. Luckily, I had the chance last year to work with an awesome graduate-to-be (props Miss Sophie Weiner) on a dissertation submission named ‘Performance of Thermally Enhanced Façade Systems Across Australian Climate Zones’.

While I cannot show the full details of the study (it is going through a peer review process to formally publish Sophie’s findings), the aim was to compare the performance of different quality frames in eight Australian climate zones. Unlike most ‘cold climate’ thermal break studies, this is the first I know of that aims to contextualise the Australian construction industry and various climates with real façade details to ascertain a performance benefit.

Through the quantification of internal surface temperatures on the back of a frame, modelled in Therm while utilising Honeybee and Ladybug, the following three frame details were studied:

1. Thermally Enhanced (Typical in colder or premium buildings: 3 – 4 % cost premium),

2. Thermally Broken (Non-typical European: 10 – 15 % + cost premium),

3. Standard Aluminium (Very common and today’s norm: no cost premium).

Take the cost premiums with a pinch of salt as you never know how keen someone is to win the work or the power of negotiation! So let’s call these indicative for now.

The point of difference in this study was the aim to highlight the role of solar loads onto the external surface of the framing system and the impact of heat transfer as a result. Typically, we exclude these when undertaking U-value assessments but in the real world, solar loads are a very high source of heat transfer on vertical and horizontal surfaces and thus important to understand if your designing high performance buildings. That is why we have external shading after all!

The simplified graphs (we have tonnes of data) below aim to show the difference between no solar load and a high solar load (750 W/m2) on the framing options mentioned above. Along the x axis, we can see exterior temperatures and the y-axis shows the average surface temperature from the full back length of the mullion. The graphs are presented so you can interpret the likely internal surface temperature for your climate based on the temperature you are interested in.

If we focus on the standard aluminium Frame line on both charts, we can see how this has the greatest range in surface temperature as you run from low (left) to higher (right) external temperatures. Without a solar load, the internal surface temperature could fluctuate between 11 – 28° C, indicting a condensation risk in colder climates and overheating. Including peak solar loads at peak temperatures of 40°C, we can see that the interior surface temperature shoots up to 39.8 °C. I can personally confirm much higher values through direct measurements on standard frames, so nothing too surprising here!

But what is interesting and not previously presented as an advantage in warmer climates is the significant reduction in interior surface temperature when we compare the standard frame to a thermally enhanced or thermally broken system. Simply put, our thermally enhanced or broken surface temperatures follow the internal (air) temperature, reducing the impact of heat loss or gain and the result need for additional heating or cooling. It is also interesting to note that the thermally enhanced frames provide a similar interior surface temperature than the thermally broken, indicating that European thermal breaks are not always required In Australian climates.

So, while these preliminary results should be deemed as conservative, I am personally of the view that actual results of the internal surface temperatures are likely to be more extreme (lower and higher) and thus the role of thermal improvements is still valid in all Australian climate zones with high levels of solar exposure. For some climates, solar exposure is a far more influential parameter than simply looking at external temperatures as a justification for better framing. While the proposed changes don’t directly show this, it should be appreciated that we have still much more to learn about this area and thus support frame thermal improvements in all Australian climate zones.

Again, the 20th April, 2018 deadline is getting closer. Drop me a line to discuss or meet up for a coffee. The clock is ticking!!