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 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	2.00
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		2.00
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	1.71	2.00
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)	3.07	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
Roof Frame Type	Model-6 Solars pan panel roof + thermal bridging inclusion	-	3.93	63.6	30.9	94.5	6.0

Previous Next

Tech Article: Thermal bridging within residential building design

1 September 2020