In our recent tech series “The impact of thermal bridging on building elements”, it was proven that the use of NCC2019 thermal bridging provision can result in a significant reduction in building fabric performance. In this tech article, we will explore what would happen if similar thermal bridging criteria were to be implemented for residential developments.
Note – The nominated product types are intended to demonstrate thermal performance differences between the common industry products only. It is important that design advice is sought related to non-combustibility and condensation requirements which are not covered in this article.
The current process to assess residential development compliance in NSW requires the assessor to simulate the proposed building design through approved NatHERS modelling tool. The Nationwide House Energy Rating Scheme (NatHERS) is a star rating system (out of ten) that rates the energy efficiency of a home, based on its passive design. The current protocol does not consider thermal bridging losses within fabric layers, which means the R-value (R) of individual building element are simply combined to determine the total R-value (Rt).
To compensate for the lack of thermal bridging allowance within Class 1, 2 (sole occupancy) and 4 dwellings, the BCA states that where metal frames are used, thermal breaks are to be installed with R0.20 thermal breaks where ceiling or wall lining is directly fixed to a metal frame with external lightweight cladding or roofing. This is a high level ‘band-aid’ fix to a problem that is now only starting to be recognised and addressed within the non-residential sector.
For this article, a typical single storey house was assessed across multiple climate zones and orientations to demonstrate how ‘downrating’ the total R-value due to thermal bridging can affect the NatHERS star rating. This was then compared to an insulated panel which provides continuous insulation and much less thermal bridging.
Wall frame types
Roof frame types:
Model 1 | Model-1 Baseline Model | |
---|---|---|
Layers | Conventional Brick Veneer, metal stud wall | R-Value |
1 | Brick Veneer | 0.09 |
2 | Thermal break layer min. R0.2 / Vapour barrier | 0.20 |
3 | 90mm steel stud (14% F/W ratio, 0.5bmt, 35mm x 90mm) | 2.00 |
4 | 90mm bulk insulation R2.00 | |
5 | Plasterboard | 0.06 |
6 | Ext air | 0.04 |
7 | Indoor air | 0.12 |
Total | 2.51 |
Model types | Thickness (mm) | Total R-Value | Heating (MJ/m2) | Cooling (MJ/m2) | Total | Star |
---|---|---|---|---|---|---|
Model-1 Baseline Wall & Roof | 231 | 2.51 | 61.6 | 29.6 | 91.2 | 6.1 |
Model 2 | Thermal Bridged | Model-1 Baseline Model | |
---|---|---|---|
Layers | Conventional Brick Veneer, metal stud wall | R-Value | R-Value |
1 | Brick Veneer | 0.09 | 0.09 |
2 | Thermal break layer min. R0.2 / Vapour barrier | 1.15 | 0.20 |
3 | 90mm steel stud (14% F/W ratio, 0.5bmt, 35mm x 90mm) | 2.00 | |
4 | 90mm bulk insulation R2.00 | ||
5 | Plasterboard | 0.06 | 0.06 |
6 | Ext air | 0.04 | 0.04 |
7 | Indoor air | 0.12 | 0.12 |
Total | 1.46 | 2.51 |
Model types | Thickness (mm) | Total R-Value | Heating (MJ/m2) | Cooling (MJ/m2) | Total | Star |
---|---|---|---|---|---|---|
Model-1 Baseline Wall & Roof | 231 | 2.51 | 61.6 | 29.6 | 91.2 | 6.1 |
Model-2 Brick Veneer wall + thermal bridging inclusion | 231 | 1.46 | 70.9 | 31.1 | 102.0 | 5.7 |
Model 3 | Thermal Bridged | Individual Component | |
---|---|---|---|
Layers | Conventional Brick Veneer, timber stud wall | R-Value | R-Value |
1 | Brick Veneer | 0.09 | 0.09 |
2 | Vapour barrier | 0.00 | 0.00 |
3 | 90mm Timber stud (14% F/W ratio, 35mm x 90mm) | 1.71 | 2.00 |
4 | 90mm bulk insulation R2.00 | ||
5 | Plasterboard | 0.06 | 0.06 |
6 | Ext air | 0.04 | 0.04 |
7 | Indoor air | 0.12 | 0.12 |
Total | 2.02 | 2.31 |
Model Types | Thickness (mm) | Total R-Value | Heating (MJ/m2) | Cooling (MJ/m2) | Total | Star |
---|---|---|---|---|---|---|
Model-1 Baseline Wall & Roof | 231 | 2.51 | 61.6 | 29.6 | 91.2 | 6.1 |
Model-2 Brick Veneer wall + thermal bridging inclusion | 231 | 1.46 | 70.9 | 31.1 | 102.0 | 5.7 |
Model-3 Brick Veneer wall + Timber Stud frame + thermal bridging inclusion | 231 | 2.02 | 65.1 | 30.1 | 95.2 | 6.0 |
Model 4 | Thickness | Component | |
---|---|---|---|
Layers | Insulated Panel with R3.70 Rated Value | R-Value | R-Value |
1 | 140mm insulated panel R3.70 + 1mm tape to capping top capping - web 0.5mm, width 25mm, depth 35mm bottom capping - web 0.5mm, width 25mm, depth 90mm total capping exposed 2% F/W ratio | 3.07 | 3.70 |
2 | Capping with insulated tape (1mm) | 0.00 | |
3 | Plasterboard | 0.06 | 0.06 |
4 | Ext air | 0.04 | 0.04 |
5 | Indoor air | 0.12 | 0.12 |
Total | 3.29 | 3.92 |
The reference product is Insulwall from Bondor range (SL Grade 140mm). The insulated panel offers continuous insulation along the full height of the wall, effectively isolating the internal and external condition except for minor thermal bridging path on the end capping which can be reduced by taping the contact joints.
Model Types | Thickness (mm) | Total R-Value | Heating (MJ/m2) | Cooling (MJ/m2) | Total | Star |
---|---|---|---|---|---|---|
Model-1 Baseline Wall & Roof | 231 | 2.51 | 61.6 | 29.6 | 91.2 | 6.1 |
Model-2 Brick Veneer wall + thermal bridging inclusion | 231 | 1.46 | 70.9 | 31.1 | 102.0 | 5.7 |
Model-3 Brick Veneer wall + Timber Stud frame + thermal bridging inclusion | 231 | 2.02 | 65.1 | 30.1 | 95.2 | 6.0 |
Model-4 Insulwall panel wall + thermal bridging inclusion | 146 | 3.29 | 60.2 | 31.8 | 92.0 | 6.1 |
Model 5 | Thermal Bridged | Model-1 Baseline Model | |
---|---|---|---|
Layers | Conventional Metal Cladded, Metal Framed room | R-Value | R-Value |
1 | Metal Sheet | 0.00 | 0.00 |
2 | Reflective Sarking | 0.00 | 0.00 |
3 | Metal frame + reflective roof space | 0.92 | 0.92 |
4 | Thermal break layer min, R0.2 | 2.03 | 0.20 |
5 | 90mm steel truss (12% F/W ratio, 0.5bmt, 39mm x 90mm) | 0.00 | |
6 | 165mm bulk insulation R3.00 (k=0.052) | 3.00 | |
7 | Plasterboard | 0.06 | 0.06 |
8 | Ext air | 0.04 | 0.04 |
9 | Indoor air | 0.16 | 0.12 |
Total | 3.21 | 4.34 |
Model Types | Thickness (mm) | Total R-Value | Heating (MJ/m2) | Cooling (MJ/m2) | Total | Star |
---|---|---|---|---|---|---|
Model-1 Baseline Wall & Roof | - | 4.34 | 61.6 | 29.6 | 91.2 | 6.1 |
Model-5 Metal framer roof + thermal bridging inclusion | - | 3.21 | 66.8 | 29.7 | 96.5 | 5.9 |
Model 7 | Thermal Bridged | Component | |
---|---|---|---|
Layers | Insulated Panel with R3.90 Rated Value | R-Value | R-Value |
1 | 150mm insulated panel R3.9 + 1mm tape to end capping web 0.5mm, width 150mm, total capping exposed 2% F/R ratio | 3.67 | 3.90 |
2 | Capping with insulated tape (1mm) | ||
3 | Plasterboard | 0.06 | 0.06 |
4 | Ext air | 0.04 | 0.04 |
5 | Indoor air | 0.16 | 0.12 |
Total | 3.93 | 4.12 |
The reference product is Solarspan from Bondor range (SL Grade 150mm). The insulated panel offers continuous insulation along the full width of the roof, effectively isolating the internal and external condition except for minor thermal bridging path on the end capping which can be reduced by taping the contact joints.
Model Types | Thickness (mm) | Total R-Value | Heating (MJ/m2) | Cooling (MJ/m2 | Total | Star |
---|---|---|---|---|---|---|
Model-1 Baseline Wall & Roof | - | 4.34 | 61.6 | 29.6 | 91.2 | 6.1 |
Model-5 Metal frame roof + thermal bridging inclusion | - | 3.21 | 66.8 | 29.7 | 96.5 | 5.9 |
Model-6 Solarspan panel roof + thermal bridging inclusion | - | 3.93 | 63.6 | 30.9 | 94.5 | 6.0 |
The NatHERS summary below demonstrates the effect of thermal bridging within a residentials dwelling.
The results indicate the following:
Building Element | Model Types | Thickness (mm) | Total R-Value | Heating (MJ/m2) | Cooling (MJ/m2) | Total | Star |
---|---|---|---|---|---|---|---|
Baseline Reference | Model-1 Baseline Wall & Roof | 231 | Wall 2.51 Roof 4.34 | 61.6 | 29.6 | 91.2 | 6.1 |
Wall Frame Type | Model-2 Brick Veneer wall + thermal bridging inclusion | 231 | 1.46 | 70.9 | 31.1 | 102.0 | 5.7 |
Model-3 Brick Veneer wall + Timber Stud frame + thermal bridging inclusion | 231 | 2.02 | 65.1 | 30.1 | 95.2 | 6.0 | |
Model-4 Insulwall panel wall + thermal bridging inclusion | 146 | 3.29 | 60.2 | 31.8 | 92.0 | 6.1 | |
Roof Frame Type | Model-5 Metal frame roof + thermal bridging inclusion | - | 3.21 | 66.8 | 29.7 | 96.5 | 5.9 |
Model-6 Solars pan panel roof + thermal bridging inclusion | - | 3.93 | 63.6 | 30.9 | 94.5 | 6.0 |
I’ve known Stuart and BCA Logic for approximately 15 years, and I can comfortably say that due to his experience and his team’s extensive knowledge of the BCA, we’ve had numerous successful projects over the years. As a client of BCA Logic we are pleased with their flexible and innovative approach that results in high quality services. Their ability to resolve issues raised during construction onsite with various options makes them a critical consultant in our team.
I have been working with Allan Harriman over the past 10 years in a wide variety of complex advice and litigation work involving defects, upgrades of fire measures and claims regarding negligence practices of certifiers. Allan and his team are at the forefront of fire engineering and certification practices and deliver material at an A Grade Level and I have every intention of continuing to work with him and his team in the future. If you need a fire engineering or building certification solution or report, look no further.
Caverstock Group provides Project Management and Superintendent Services to private developers, asset owners, and financiers. We have used the services of BCA Logic continually since 1997 because of their proactive, innovative approach to the excellent advice they give us, our design teams, and builders. They are a preferred BCA Consultant. We have no hesitation in recommending their services to our clients due to BCA Logic’s culture of integrity and client focus.
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