Thermal Breaks in Hot Climates? 2

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Monday, March 30th, 2015
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With clear climatic zones across Australia, the benefits of ‘thermal break’ technologies in façades are often difficult to value for commercial offices spaces. Is it time we accept that a ‘cold climate’ view of façade optimisation is not a given for all of Australia?

I am fortunate enough to have worked on projects that pose great challenges to the design of high performance façades, from Harbin in north-eastern China, so cold it famously hosts the world’s largest Ice and Snow Festival at roughly minuns-20 degrees Celsius each year to the equatorial challenges of Singapore and Jakarta, where the temperature never drops below 20 degrees Celsius

In these situations, we often see the argument for improving the performance of window frame designs via ‘thermal break’ technologies. Given the climate variation across Australia, however, what are the merits of these technologies and are they always a worthwhile investment?

What is a thermal break?

Before considering such questions, let’s add a definition to the term ‘thermal break’ so we are all on the same page. Conventionally, the term is used to describe a material with low thermal conductivity (≤0.5 W/m.K) that reduces or isolates the flow of thermal energy between conductive elements within a framing system. In layman’s terms, they offer an opportunity to reduce how much we cool and heat our buildings.

The Thermal Break

Thermal break

In practice, this means that fabricators split the frame components into interior and exterior pieces with a material such as polyamide, effectively creating a thermal buffer. Given that frames constitute, on average, 20 to 40 per cent by area of the total vision system (glass and frame), it seems like a no-brainer to want to reduce the heat transfer given that it represents a significant load in typical commercial office spaces where upwards of 50 to 60 per cent window-to-wall ratios are common.

Are thermal breaks a must for commercial office projects across Australia?

Firstly, one thing needs to be affirmed. Thermal breaks, for all intents and purposes, have been designed for cold climates to limit thermal transfer and reduce condensation risk (think cold European or American cities). Such climates can loosely be defined as those where large temperature differences (greater than 10 degrees Celsius) between inside and outside draw heat out of the building to the cold for at least six months of the year.

While moderate or warm climates do see measurable performance enhancements in peak cold and warm seasons, primarily in the form of thermal comfort and to a lesser degree energy efficiency, these are not in the same league as colder climates.

Unlike the classic definition of cold climates, commercial office buildings in Australian climates are most commonly only exposed to high temperature differences during the night for a few months of the year. During the day, when the building is occupied, temperatures are much higher. The image below shows average external minimum temperatures during the day in the major Australia cities.

Assuming an internal temperature of 20 degrees Celsius, it highlights that there are only relatively short durations where the temperature difference is greater than 10 degrees Celsius between inside and outside. In fact, it is only Canberra (and Climate zone 7) that experiences genuine cold climate conditions during the typical occupancy period of a commercial office building. The case for ‘thermal breaks’ is therefore not as clear cut as might be expected.

temps

Where are thermal breaks the greatest of value?

In order to illustrate the variety of performance that a thermal break can provide against a typical framing system, I have modelled both heating dominant and cooling dominant climates in Australia, and thrown in a cold New Zealand city for good measure.

Heating dominant climates

If we scan through the bar graph, we can quickly conclude that during heating season within heating dominant climates, benefits (overall energy consumption of the building) of around four to six per cent can be realised. Given that this is a simple feature added to the frame that has no impact on the aesthetics of the design, this is a fantastic outcome and something of real benefit during winter seasons.

However, if we also look at heating dominant climates but now at the cooling season, we actually see a slight increase in energy consumption. Of course, this is a small and acceptable penalty given the saving during the heating season and the added benefits of condensation risk reduction and improved thermal comfort.

modellingresults

Cooling dominant climates

Less clear, however, are cooling dominant climates during the cooling season. Within this case, if we assume minimal heating requirements for most of the year, the application of thermal breaks for commercial office buildings can be found as neutral.

By reducing overnight heat loss, enhancing the frames can result in a very slight net gain. The likely reasoning for this from a modelling point of view is that the model sees the building as a perfect form (i.e. performance is not impacted by construction quality) and calculates a greater need to remove heat via extraction with a small penalty in terms of energy consumption for heat extraction. If we think about this, it makes sense that the NCC (BCA) is relatively lenient on double glazed requirements in warmer climates, instead focusing on external shading and the solar control (SHGC) properties of the glass to improve design performance.

That being said, while not a direct benefit to the overall energy consumption of the building, thermal breaks still have a role in smart design as they will reduce the heat transfer when compared to a typical framing system. They are also likely to improve thermal comfort, where without the introduction of a thermal break, internal exposed frame details can reach greater than 40 degrees Celsius. This can have a direct impact on occupants, particularly those the west side of an exposed building during summer.

At what price?

Like all things in the construction industry, the longer we wait to introduce something to a project, the more it is going to cost. Thermal breaks are no different; if a design goes through the mill and suddenly is not meeting performance requirements set by an ESD or façade consultant, the costs could be as high as 10 per cent of your façade costs. However, getting in early or looking not at a true thermal break, but a thermal enhancement, this could reduce that number to less than three per cent of the total façade costs, certainly making it worthy of consideration

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2
  1. David Kuhnert

    Good Article Darren , Thermal Break is very important & so is Controlled Thermal mass , example Insulating Concrete Form . Regards

  2. David Kuhnert

    The silence is deafening , such an important subject that is missing in the Building Industry in Australia , Shame .