Tech Article: Thermal bridging within residential building design

1 September 2020

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 1Model-1 Baseline Model
LayersConventional Brick Veneer, metal stud wallR-Value
1Brick Veneer0.09
2Thermal break layer min. R0.2 / Vapour barrier0.20
390mm steel stud (14% F/W ratio, 0.5bmt, 35mm x 90mm)2.00
490mm bulk insulation R2.00
5Plasterboard0.06
6Ext air0.04
7Indoor air0.12
Total2.51
Model typesThickness
(mm)
Total
R-Value
Heating
(MJ/m2)
Cooling
(MJ/m2)
TotalStar
Model-1 Baseline Wall & Roof2312.5161.629.691.26.1

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 2Thermal BridgedModel-1 Baseline Model
LayersConventional Brick Veneer, metal stud wallR-ValueR-Value
1Brick Veneer0.090.09
2Thermal break layer min. R0.2 / Vapour barrier1.150.20
390mm steel stud (14% F/W ratio, 0.5bmt, 35mm x 90mm)2.00
490mm bulk insulation R2.00
5Plasterboard0.060.06
6Ext air0.040.04
7Indoor air0.120.12
Total1.462.51
Model typesThickness
(mm)
Total
R-Value
Heating
(MJ/m2)
Cooling
(MJ/m2)
TotalStar
Model-1 Baseline Wall & Roof2312.5161.629.691.26.1
Model-2 Brick Veneer wall + thermal bridging inclusion2311.4670.931.1102.05.7

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 3Thermal BridgedIndividual Component
LayersConventional Brick Veneer, timber stud wallR-ValueR-Value
1Brick Veneer0.090.09
2Vapour barrier0.000.00
390mm Timber stud (14% F/W ratio, 35mm x 90mm)1.712.00
490mm bulk insulation R2.00
5Plasterboard0.060.06
6Ext air0.040.04
7Indoor air0.120.12
Total2.022.31
Model TypesThickness
(mm)
Total
R-Value
Heating
(MJ/m2)
Cooling
(MJ/m2)
TotalStar
Model-1 Baseline Wall & Roof2312.5161.629.691.26.1
Model-2 Brick Veneer wall + thermal bridging inclusion2311.4670.931.1102.05.7
Model-3 Brick Veneer wall + Timber Stud frame + thermal bridging inclusion2312.0265.130.195.26.0

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 4ThicknessComponent
LayersInsulated Panel with R3.70 Rated ValueR-ValueR-Value
1140mm 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
3.073.70
2Capping with insulated tape (1mm)0.00
3Plasterboard0.060.06
4Ext air0.040.04
5Indoor air0.120.12
Total3.293.92

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 TypesThickness
(mm)
Total R-ValueHeating
(MJ/m2)
Cooling
(MJ/m2)
TotalStar
Model-1 Baseline Wall & Roof2312.5161.629.691.26.1
Model-2 Brick Veneer wall + thermal bridging inclusion2311.4670.931.1102.05.7
Model-3 Brick Veneer wall + Timber Stud frame + thermal bridging inclusion2312.0265.130.195.26.0
Model-4 Insulwall panel wall + thermal bridging inclusion1463.2960.231.892.06.1

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 5Thermal BridgedModel-1 Baseline Model
LayersConventional Metal Cladded, Metal Framed roomR-ValueR-Value
1Metal Sheet0.000.00
2Reflective Sarking0.000.00
3Metal frame + reflective roof space0.920.92
4Thermal break layer min, R0.22.030.20
590mm steel truss (12% F/W ratio, 0.5bmt, 39mm x 90mm)0.00
6165mm bulk insulation R3.00 (k=0.052)3.00
7Plasterboard0.060.06
8Ext air0.040.04
9Indoor air0.160.12
Total3.214.34
Model TypesThickness
(mm)
Total
R-Value
Heating
(MJ/m2)
Cooling
(MJ/m2)
TotalStar
Model-1 Baseline Wall & Roof-4.3461.629.691.26.1
Model-5 Metal framer roof + thermal bridging inclusion-3.2166.829.796.55.9

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 7Thermal BridgedComponent
LayersInsulated Panel with R3.90 Rated ValueR-ValueR-Value
1150mm insulated panel R3.9 + 1mm tape to end capping
web 0.5mm, width 150mm, total capping exposed 2% F/R ratio
3.673.90
2Capping with insulated tape (1mm)
3Plasterboard0.060.06
4Ext air0.040.04
5Indoor air0.160.12
Total3.934.12

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 TypesThickness
(mm)
Total
R-Value
Heating
(MJ/m2)
Cooling
(MJ/m2
TotalStar
Model-1 Baseline Wall & Roof-4.3461.629.691.26.1
Model-5 Metal frame roof + thermal bridging inclusion-3.2166.829.796.55.9
Model-6 Solarspan panel roof + thermal bridging inclusion-3.9363.630.994.56.0

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 ElementModel TypesThickness
(mm)
Total R-Value
Heating
(MJ/m2)
Cooling
(MJ/m2)
TotalStar
Baseline
Reference
Model-1 Baseline Wall & Roof231Wall
2.51
Roof
4.34
61.629.691.26.1
Wall Frame TypeModel-2 Brick Veneer wall + thermal bridging inclusion2311.4670.931.1102.05.7
Model-3 Brick Veneer wall + Timber Stud frame + thermal bridging inclusion2312.0265.130.195.26.0
Model-4 Insulwall panel wall + thermal bridging inclusion1463.2960.231.892.06.1
Roof Frame TypeModel-5 Metal frame roof + thermal bridging inclusion-3.2166.829.796.55.9
Model-6 Solars pan panel roof + thermal bridging inclusion-3.9363.630.994.56.0