Thermal Breaks

Preface: On the Nature of Building Codes and This Analysis

As we delve into the complexities and implications of thermal breaks and thermal bridging mitigation within the framework of NCC 2022, it is essential to acknowledge the foundational basis of our analysis. This article draws upon the current standards and regulations as stipulated in the 2022 edition of the National Construction Code (NCC). It aims to provide a comprehensive understanding of the research conducted by LittleShrub Pty Ltd for the Australian Building Codes Board (ABCB), focusing on the significant strides made in building insulation practices, energy efficiency, and thermal performance enhancement.

However, it is crucial for readers to recognise that building codes are living documents, subject to revisions and updates that reflect new knowledge, technologies, and societal needs. As such, future versions of the NCC may present different requirements or interpretations of thermal breaks and thermal bridging mitigation. Our discussion is rooted in the present, with an eye towards sparking a broader conversation about the evolution of construction practices and the pursuit of sustainability and energy efficiency in building design.

This preface serves not only as an introduction to the subject matter but also as a disclaimer: the insights and recommendations provided herein are based on the NCC 2022 and may not fully align with future iterations of the code. The intent is not to prescribe definitive solutions but to contribute to an ongoing dialogue among professionals in the construction industry, encouraging the exchange of ideas and the exploration of innovative approaches to thermal management in buildings.

We invite readers to engage with the content critically, considering both the current state of the art and the potential for future advancements. By fostering a culture of continuous learning and adaptation, we can collectively advance our goals for more sustainable, efficient, and comfortable living environments.

Walls

Roofs

What is a Thermal Bridge

To understand this better, we need to identify how heat moves through certain parts of a building. In some cases, it's easy to see, like in a wall where both the outer layer and the inner layer are fixed to the same supports. This is typical of a framed wall (like metal studs) with both external cladding and internal lining fixed directly to the framing material. In this example, there are two main ways heat transfers: through the support frame and through the insulation.

As buildings get more complex, there are more ways heat can flow, which we call thermal pathways. For example, imagine awall with horizontal metal battens across the stud frame as show in the 3d sketch. In this case, there are four distinct pathways through which heat can travel.

Through the air-space and insulation.
Through metal battens and insulation.
Through the air-space and metal studs
Through the metal battens and metal studs

Based on what we've described, we can identify the parts of the wall where heat is more likely to transfer, which are pathway 3 and pathway 4, as these areas lack proper insulation.

When dealing with more complicated structures, it becomes challenging to figure out these thermal pathways. Here, we use a method described in AS/NZS 4859.2: 2018 to calculate the thermal resistance of these areas. This method helps us determine how much heat is lost in these bridged areas. By doing this, we can find out the total R-Value of the system, which indicates how effective the insulation is.

The process might sound technical, but it helps us figure out where the thermal bridges are in a building. In our example case, whenever there are metal support frames in the structure, there's a chance for thermal bridging. So, it's essential to understand and address these areas to improve energy efficiency and keep our buildings comfortable. The diagram indicated shows the four potential thermal pathways in this wall as ordered above.

Importance of Thermal Breaks

Enhancing Metal-Framed Construction

Thermal breaks emerged as a critical component in building insulation, especially for metal-framed construction elements like walls, roofs, and floors. Metal frames possess high thermal conductivity, leading to localised excessive thermal conductance, which adversely impacts energy efficiency. The incorporation of thermal breaks helps create insulating barriers that prevent heat transfer and significantly improve the building's overall thermal performance. Understanding the intent of thermal breaks clauses of the NCC was essential in addressing the challenges posed by metal-framed structures.

What is a Thermal Break

The NCC defines a thermal break as a material with an R-Value ≥0.2. While this might seem straightforward, there's more to consider when understanding their usage. Thermal breaks are typically employed in locations where structural materials will compress them. In the calculations undertaken by the ABCB for the new NCC 2022 and reviewed by LittleShrub, thermal breaks were considered to have an R-value of 0.2 in-situ. One common oversight is assuming that a thermal break with a declared R-value of R0.2 will perform at R0.2 in its compressed state. This is not necessarily true, if the material is compressible, it will lose some of its thermal performance when compressed. If that means its in-situ performance is less than R0.2, then it is no longer a thermal break. According to NZS 4214:2006, a bulk insulation material used as a thermal break experiences a deration factor resulting in a 1% decrease in thermal resistance for every 2% compression.

The Impact of Ventilation on Thermal Breaks

The analysis revealed a significant impact of ventilation on thermal performance effectiveness. Walls are commonly considered to have a ventilated air-space when battened out, resulting in a considerable deration of thermal performance compared to an unventilated air-space, regardless of the presence of thermal breaks. LittleShrub's analysis for the ABCB noted that walls were consistently assumed to have unventilated air-spaces, disregarding the more conservative and typical ventilated scenarios. This oversight in determining the cavity's ventilation status had cascading effects on the optimal thermal bridging mitigation strategy, which was previously unaccounted for by the ABCB.

The diagram below shows the impact of ventilation on thermal performance. The graph indicates the thermal performance of a wall with and without a thermal break and an R-Value of R0.2, also with a ventilated air-space and with an unventilated air-space. It's evident from the graph that the thermal performance is primarily governed by the wall either having a thermal break or not. The ventilation status of the cavity has a secondary effect that only has a marginal effect if the wall has a thermal break. However, if the wall does not have a thermal breaks and is also ventilated, then this has compounding effect on the thermal performance. This is why it is important to consider the ventilation status of the cavity when determining the thermal performance of a wall.

Note: the graph is based on the analysis undertaken by LittleShrub for the ABCB. The graph is not intended to be used for design purposes.it considers the assumption that are outlined in the full report

When to use a Thermal Break

One of the more practical outcomes of the research was to answer the question: When do you need to use a thermal break to satisfy 13.2.5(5)? The time this question became most relevant was when a metal stud frame wall had metal battens fixed to the studs. More specifically, what is the definition of a metal frame? Does this include the metal battens? The research used both grammatical and technical analyses to determine the intent of the clause. The outcome of this analysis demonstrated that the thermal breaks are required both when a wall has a metal frame without battens and when the wall has metal battens. This is then illustrated by the flowchart below

What about the explanatory information of 13.2.5?
There is an explanatory information section in 13.2.5 that states: 'Because of the high thermal conductance of metal, a thermal break is needed when a metal framing member directly connects the external cladding to the internal lining or the internal environment...'. An argument is made by some, that because this explanatory information states 'directly connects', then this means that the metal battens are not included in the definition of a metal frame. However, this a logical fallacy known as denying the antecedent. It is equivalent to stating:

If you are an engineer, then you have a job.
You are not an engineer.
Therefore, you do not have a job.

It's clear to see that this is not a valid argument. The same logic applies to the explanatory information. It demonstrates a specific case where a thermal break is required. This does not mean that this is the only circumstance where a thermal break is required. The full research then uses the demonstrated methodology of AS/NZS 4859.2 to validate this statement. Indeed, the research supports the explanatory information statement, but it also supports the fact that a thermal break is required when a wall with a metal stud has metal battens.

Enhancing Thermal Performance through Mitigation Options: An Analysis of Thermal Bridging Provisions

Introduction

Thermal bridging in metal-framed constructions poses a significant challenge to achieving optimal energy efficiency in buildings. To address this issue, NCC 2022 introduced thermal bridging mitigation provisions. This section analyses the findings of these provisions, discusses their best use, and highlights the most effective strategies for mitigating thermal bridging.

Findings and Analysis

The thermal bridging mitigation provisions, outlined in Table 13.2.5t, present three key options to combat thermal bridging: using reflective insulation, increasing insulation between frames, or adding continuous insulation with a minimum R-Value of R0.30 on the inside or outside of the frame. The analysis for the ABCB revealed that these provisions must be used in conjunction with thermal breaks to achieve compliance with Table 13.2.5q requirements.

Best Use

The thermal bridging mitigation provisions must be applied in combination with a thermal break. The continuous insulation method with a plus R0.3 rating demonstrated the highest robustness and overall performance, regardless of whether an unventilated air-space was present or not. The reflective insulation method also showed promise but required an unventilated air-space for optimal results, making it suitable for limited scenarios. On the other hand, the plus R0.5 method placed between frames proved to be the least effective and is not recommended unless used in conjunction with an unventilated air-space, provided the aggregation opening is less than 500mm²/m

Effectiveness

Based on the graph, the effectiveness of the three methods can be ranked as follows:

Continuous R0.3 insulation with an unventilated cavity
Reflective insulation with an unventilated cavity
Continuous R0.3 insulation with a ventilated cavity

Table 13.2.5q

Plus R0.5 insulation between stud frames with an unventilated cavity
Plus R0.5 insulation between stud frames with a ventilated cavity
Plus R0.5 insulation without battens

Please note that currently, the R0.3 continuous method is not permitted by Table 13.2.5t for 0 to ≤1.5. However, it is the most effective method and was recommended to the ABCB to be considered as an option across the full range of added R-values.

Conclusion: Advancing Building Insulation Practices

In conclusion, the text underscored the pivotal role of thermal breaks and thermal bridging mitigation provisions in modern building insulation practices. As societies increasingly prioritise energy efficiency and sustainability, the construction industry must continue to embrace these provisions to create environmentally responsible and energy-efficient buildings.

By understanding the research objectives and the intent behind thermal breaks and thermal bridging mitigation provisions, professionals can make informed decisions in their building design and construction processes. The analytical approach to identifying thermal bridges offers targeted solutions to improve thermal performance. Furthermore, the impact of ventilation highlights the need for careful consideration in designing thermal break provisions.

The effectiveness of thermal bridging mitigation strategies demonstrates their potential to significantly enhance overall building insulation and energy efficiency. As construction practices evolve, implementing the recommended strategies will contribute to a more sustainable future, with buildings that offer improved thermal comfort and reduced environmental impact. The key takeaways from this text pave the way for a more energy-efficient and environmentally conscious construction industry, advancing building insulation practices for generations to come.

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