Tech Article: Thermal bridging within residential building design
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.
Summary
- This article will indicate the following:
- Brick veneer timber-framed dwelling outperforms metal-framed due to less thermal bridging – 6.0 vs 5.7 star
- For wall and roof, the continuously insulated panels outperform the brick veneer timber and metal frame construction – 6.1 star
- Even with the decrease in thermal mass (due to the loss of the brick skin) the higher R-value and less thermal bridging leads to a higher star rating
- The insulated Panel allows for a much thinner wall construction 146mm vs 231mm for the standard brick veneer wall
Current Residential Compliance Overview
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.
Model Parameters
- Location – climate zones 10 (Hot), 28 (Warm), 56 (Mild) and 24 (Cold)
- Orientation – North, East, South and West
- Roof – Metal sheet hip roof with R0.2 thermal break, metal truss and plasterboard ceiling
- Wall – Brick Veneer with R0.2 thermal break, 90mm stud and plasterboard finish
- Glazing – standard single glazed, clear in aluminium frames (U 6.7 and SHGC 0.70)
- Ground – Concrete slab-on-ground
Wall frame types
- Model 1 – Brick veneer wall with metal stud + thermal break (No thermal bridging allowance)
- Model 2 – Brick veneer wall with metal stud + thermal break
- Model 3 – Brick veneer wall with timber stud
- Model 4 – EPS insulated panel with no internal frame and continuous insulation
Roof frame types:
- Model 5 – Metal clad roof with metal frame + thermal break
- Model 6 – EPS insulated panel with no internal frame and continuous insulation
Baseline Model 1 – No Thermal Bridging Allowance
- Theoretical best with a star rating of 6.1/10
| 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 |
Thermal Bridged Model 2 – De-Rated Metal Frame
- Construction detail remains identical to baseline Model 1
- Thermal bridging has been calculated within the wall make up only, yielding Rt1.46 compared to Rt2.51 for the baseline
- Star rating decreases to 5.7/10 – in line with true value
| 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 |
Thermal Bridged Model 3 – De-Rated Timber Frame
- Construction detail similar to Model 1 and 2
- Timber stud frame used instead of metal
- No thermal break required
- Thermal bridging has been calculated within the wall make up yielding Rt2.02
- Star rating of 6.0/10 compared to 5.7/10 for metal frame
| 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 |
Thermal Bridged Model 4 – De-Rated EPS Insulated Panel Wall
- 140mm EPS Insulated Panel wall used instead of brick veneer and stud wall
- No thermal break required apart from capping with insulated tape
- Thermal bridging has been calculated within the wall make up yielding Rt3.29
- Star rating of 6.1/10, equalling the theoretical best with a much thinner wall thickness (146mm vs 231mm)
| 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 |
Thermal Bridged Model 5 – De-Rated Metal Roof
- Construction detail remains identical to baseline Model 1
- Thermal bridging has been calculated within the roof make up only, yielding R3.21 compared to Rt4.34 for the baseline
- Star rating decreases to 5.9/10 – in line with true value
| 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 |
Thermal Bridged Model 6 – De-Rated EPS Insulated Panel Roof
- 150mm EPS Insulated Panel roof used instead of metal sheet and metal framed roof
- No thermal break required apart from capping with insulated tape
- Thermal bridging has been calculated within the roof make up, yielding Rt3.93
- Star rating of 6.0/10 which is higher than the metal framed roof with less thickness
| 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 |
NatHERS Output Summary
The NatHERS summary below demonstrates the effect of thermal bridging within a residentials dwelling.
The results indicate the following:
- Brick veneer timber framed dwelling outperforms metal framed due to less thermal bridging – 6.0 vs 5.7 star
- For wall and roof, the continuously insulated panels outperform the brick veneer timber and metal frame construction – 6.1 star
- Even with the decrease in thermal mass (due to the loss of the brick skin) the higher R-value and less thermal bridging allows for a higher star rating
- The insulated Panel allows for a much thinner wall construction 146mm vs 231mm for the standard brick veneer wall
| 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 |
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