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Project Report for Australian Building Codes Board
Effect of Thermal Bridging on Heat losses
of walls in Australian Houses
H. A. Trethowen
Aug 2004
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Project Report for ABCB
Effect of Thermal Bridging on Heat losses of walls in Australian Houses
H. A. Trethowen
August 2004
This study was undertaken in response to a concern of some stakeholders about whether
metal wall framing systems needed some additional insulation provisions in order to meet
the required thermal performance. The key findings are that the higher the thermal
performance required of a wall, the greater the need for a thermal break and this is
particularly important for a wall where the frame connects directly to the outer cladding
and inner lining.
As a result, Regulation Documents RD2004/01 and RD2004/02 proposed that both volumes
of the BCA in 2006 require that a thermal break be installed in situations where there is a
light-weight wall construction with a steel frame. Final adoption of any proposal is subject
to the regulatory change process.
This document has been prepared by an independent consultant, and as such, the views
within this document are those of the consultant and not the ABCB. No responsibility is
accepted by the ABCB for the views, content or intent within this document.
Method:
The method used is the “Isothermal Planes” method, carried out manually as described in
Ref. 2, “Calculating R-Values by the Isothermal Planes Method”. Some of the material
property data is also taken from that publication.
The validity of the methods outlined in the BRANZ Report is described in that Report, and
more fully in Ref. 3, “Validating the Isothermal Planes method for R-Value Predictions”.
These indicate that the method achieved predictions within 10 % of measured values, for
published results reported from several different countries for a set of 84 timber framed, metal
framed, and masonry walls.
The calculations allow for currently known variations in heat transfer properties, including
changes in reflectivity with trace levels of condensate, behaviour of small air cavities,
influence of edge gaps and fitting quality of insulants, as described in above publications.
To indicate how effectively any insulating materials are being used, use is made here of an
“insulating efficiency”. This term is not in general use; it has been taken to mean the ratio
of “Achieved R-Value”/ “Ideal R-Value if there were no thermal bridging”. This value can
exceed 100 % if the framing has lower heat transfer than the cavity, and this occurs for
instance with foil insulated cavity and timber framing.
Data:
The following data has been used:
Surface resistances: (inside + outside) - 0.12 m 2.o C/W
with typical thermal resistance values of:-
Plasterboard linings ( ~ 10 mm) - 0.04 m 2.o C/W
Fibre-cement board ( ~ 6 mm) - 0.03 m 2.o C/W
“ “ over RFL - 0.12 m 2.o C/W
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1 Timber weatherboard, no membrane - 0.14 m 2.o C/W
“ “ + nonrefl. paper - 0.23 m 2.o C/W
“ “ + RFL - 0.30 m 2.o C/W 2
1 Fibre-cementweatherboard, no membrane - 0.03 m 2.o C/W
“ “ + nonrefl. paper - 0.10 m 2.o C/W
“ “ “ + RFL - 0.11 m 2.o C/W 2
Brick (100 mm, 1920 kg/m 3 ) (“dry”) - 0.16 m 2.o C/W
Brick (100 mm, 1920 kg/m 3 ) (“wet”) - 0.14 m 2.o C/W
Concrete block ( 200 mm, 2200 kg/m 3 ) - 0.19 m 2.o C/W
“ , 1:3 cores grouted - 0.15 m 2.o C/W
1
2
These values have been precalculated using the same Isothermal Planes method,
but have also been observed during Guarded HotBox tests at BRANZ during
commercial tests.
These values are likely to drop to the “nonrefl. paper” value in hot humid outdoor
conditions.
Item 1 is a result of overlap by weatherboards, and there is some effective thickening of the
wood cladding. Where there is no foil or sheathing behind the weatherboard, that is the only
effect. But where there is foil or other sheathing, this forms a partial air gap as well, and
adds to the apparent R-Value.
Item 2 is a result of trace condensation on the foil (see Ref. 4). This has the effect of making
the foil behave as though black. Foils are sensistive to dew formation, even when the
amount is at trace levels that would not be visible even under good viewing conditions .
When the foil is on the (currently) warmer side of a cavity, it will tend to dry off and no loss
in performance is likely. However when it is on the cooler side, traces of condensate may
form, and it is necessary to assume that the reflectivity will be temporarily undermined.
Thus weatherboard claddings with foil sheathing should be slightly derated if used in hot
humid climates on airconditioned buildings. Non-airconditioned buildings are likely to get
some solar warming, even on cloudy days, and so are not likely to be affected. Similarly,
walls with timber frame and foil sheathings on the outer side should be derated for winter
conditions - see “Climate effects”.
The performance of insulants do not appear in the list above, because they have been
categorised by R-Values rather than by type or thickness. Different insulants will in fact alter
the wall R-value, even if the insulants have the same rating. If one has a different
conductivity than another of the same R-value, it will have a different thickness. This means
that the residual air space will have changed, and the wall R-value will change with that.
This variation is not examined in this report.
Construction Details:
(Note: In all cases where foil is used, it is on the outer side of frame).
Case 1, Weatherboard. 95 x 45 mm wood studs are taken at 600 mm pitch, with nogging at
600 mm. The linings are 10 - 12 mnm plasterboard. Weatherboards are 19 mm bevel backed
timber, and lapped ~30mm. Steel studs are 90 x 30 x 1.2 mm at 600 mm pitch.
Case 2, Sheet clad. Same framing and lining details as Case 1 weatherboards. Cladding is
fibre cement board with no laps.
Case 3. Brick veneer. Same framing details as Case 1 weatherboards. Bricks attached to
framing by ties at 350 mm centres on studs at 600 mm centres. Foil is more loosely attached
to frame, and so is given a higher contact resistance. Ties are 30 mm 2 cross section for steel,
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60 mm 2 for nylon. Drainage and venting openings in 50 mm veneer cavity allow some loss
of brick & veneer cavity insulation: 85% retention is taken.
Case 4. Masonry with sheet claddings over 70 x 35 mm wood battens, or 70 x 35 x 0.8 mm
steel battens.
Case 5. Internal brick finish. Brick is taken to have a higher R of 0.16 because it is drier.
Battens are 35 x 35 mm for both wood and steel, steel is 0.55 mm thick.,
Case 6. Double brick. Inner brick taken as R 0.16, outer as R 0.14. This case is for
comparing the relative effect of steel and nylon ties. The accuracy of this calculation is not
high. Battens are taken as 70 x 35 mm, with 0.8 mm steel.
RESULTS:
The results of the calculations are listed in Tables 1 - 3, and are shown graphically in Figures
1 to 5. These values are typical winter values - summer values would be slightly lower. The
“insulation efficiency” is also listed in Tables 1 - 3.
In these tables columns 3 - 8 list the R-Values for different values of cavity thermal
resistance. The value of 0.18 m 2.o C/W applies to an uninsulated cavity. The value 0.64
m
2.o C/W applies to a frame cavity with dry foil - this is a typical value but it does vary with
the temperature difference. The other values refer to the R rating of bulk insulant correctly
placed in the cavity.
Figure 6 shows the effect of adding thermal breaks to steel framed walls with weatherboard
cladding. Similar effects would be seen with the other cases of steel framed walls.
DISCUSSION:
General: There is a similarity of behaviour between the main construction types. The
“insulation efficiency” of timber framed walls falls moderately as insulation is added,
ranging from some 95% at low R-values to perhaps 75% at higher R-value ratings. For metal
framed structures, the insulation performance is often lower, and may require the inclusion
of some thermal break to provide similar performance as in timber, especially at higher
insulation values. The insulation efficiency with no thermal break ranges ~ 50 % - 90 %.
The conclusion is that steel framing can be thermally competitive with timber framing,
provided that adequate thermal breaks are used in board & sheet clad walls.
This shows a need to consider the possible effect of thermal breaks with steel (or other metal)
framing. This has been considered in Figure 6, which shows for example how a steel-framed
weatherboard wall R-value would change with varying quality of thermal break. It shows
that as a thermal break R-value increases, its effect is initially greater than its own R-value,
but the rewards get progressively less as the breaks are improved. The amount of
improvement possible also rises as the wall R-value itself rises. In the case shown, it would
seem that thermal breaks of about 0.4 m 2.o C/W would be practicable, as they give a
considerable improvement but are within the achievable range. There would thus be
opportunities for the engineering development of commercially attractive thermal break
systems.
Weatherboard & Sheet clad walls: Weatherboard walls have better insulation value than
sheet clad, mainly because the weatherboards are thicker and have lower conductivity, and
they are lapped. Fibre cement weatherboards are not included, if they were they would have
similar values to sheet claddings. Wood frames give higher R-Values than steel, but if the
steel frames are fitted with thermal breaks they can equal wood frames - a case is shown
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where the addition of a thermal break of only R 0.2 m 2.o C/W brings the performance to almost
the same as for wood frame. The tightness of fit of linings and claddings on steel frames
makes a significant difference, and some results are shown with a range of resluts to reflect
this.
Brick Veneer walls with foil on the outside of timber framing has been measured by Robin
Clarke (1987?), who found that the thermal performance varied with the amount of wind
penetration into the veneer cavity. This form of construction tends to offer some degree of
natural thermal break as the veneer brick is less tightly bonded to the frame. Foil, where
used, is also less tightly bonded to the frame, and this makes a considerable difference to
results for steel framed walls, which also appear in measurements to benefit from the absence
of trace moisture on the foil in cold conditions.
Because of these effects, Table 2 shows that the performance of steel framed walls quite
close to that of wood frames. They can be improved futher with thermal breaks.
Masonry and internal brick: In their simplest forms these offer lower thermal performance,
but could be modified to produce much higher ratings.
Ties: It has not been possible to predict the effect of ties with good reliabitiy here. The
methods available are not powerful enough (it would need 3-D modelling &/or direct
measurement), but it does seem that there may not be need to. Steel ties are likely to have
only a few percent effect except at higher insulation levels, and nylon ones hardly noticeable.
Minimising Bridging: It is possible to design and build frame walls which have little
susceptibility to thermal bridging. This can be done, for instance, by using brick veneer. It
can also be done with insulating sheathings such as EPS or XEPS, and this increases the
thermal storage of the wall - usually an advantage as it reduces the daily indoor temperature
swings.
Miscellaneous: Ref 3 gives an outline of how sensitive certain factors are to detail
differences, such as steel thickness, edge gaps, etc. Some of these effects are also
mentioned in Ref 2.
CLIMATIC EFFECTS:
The thermal performance of most materials degrades as the mean temperature rises. A typical
rate is at ~ 0.7%/ o C, so for a mean temperature of say 25 o C the R-Value of a wall would be
~ 7 % lower than at 15 o C. This is why there are sometimes “summer” and “winter” ratings.
The direction of heat flow also has a marked effect with reflective systems. Reflective
insulation gives excellent insulation performance for downward heat flow (~ 1.5 to 2
m
2.o C/W), but only moderate performance (~0.5 to 0.6 m 2.o C/W) for upward or horizontal
heat flow. This is why foil is so effective in roofs in Australian summer conditions, and for
suspended floors in winter.
In winter conditions, timber framed walls with foil outside frame can display more-or-less
sudden temporary drop in R-value when outside conditions get cold enough. This arises
principally from moisture from the frame (which is warmer than the foil sheathing) moving
into the cavity space. It the foil gets cold enough, some of this moisture may begin to
condense on the foil, which then behaves as though black. The risk depends on many details
of the walls and conditions, but an initial assessment can be made from the moisture content
of the framing and the outside temperature. The risk decreases as the framing becomes drier,
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and where the cladding system comprises a larger fraction of the whole wall thermal
resistance.
Thus the no-foil cases (*.*.1) will not show any such effect at all, and the foil-only cases
(*.*.2) may or may not show this effect in Australian conditions. The foil + bulk-insulant
cases (*.*.3) may show a drop towards the *.*.1 values if the framing moisture content is
high - say 16 - 18 % m.c. - and the outdoor temperature drops below 8 - 10 o C. But as the
timber framing dries towards a more likely final value, say 12 % - 14 % m.c., such a drop
might not occur unless the outdoor temperature stays below about 6 o C or less. These wall
frames will also become drier in normal service because of their higher insulation.
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REFERENCES:
1. ABCB Brief; “To quantify the impact of thermal bridging on the BCA housing
provisions for walls”. ABCB. 3 February 2004.
2. “Calculating R-Values by the Isothermal Planes Method”, BRANZ, June 1998.
3. “Validating the Isothermal Planes method for R-Value Predictions”. ASHRAE
Transactions, Vol. 101, Part 2, pp 755 - 765.
4. “The effect of condensation on the emittance of reflective insulation. M.R.Basset &
H.A.Trethowen, Journal of Thermal Insulation, Vol. 8, Oct 1984, pp 127 - 135.
Bibliography:
One bridge is too many. Journal of Thermal Insulation of Australia. Melbourne (p. 6-9). An early
attempt to show that thermal bridging is a real effect.
R-Values that are made-to-measure. ASHRAE transactions Vol 91, Part 2, p36-47 1985. 1985 (also as
BRANZ reprint No. 45). Compares measured and calculated thermal performances of ~30 structures.
Thermal Insulation and Contact Resistance in Metal Framed Panels. ASHRAE transactions Vol 94, part
2 p 1802-1816 1988. Gives measured thermal performance of about 10 metal framed wall structures,
with full construction details.
Sensitivity of Insulated Wall and Ceiling cavities to Workmanship. Journal of Thermal Insulation, Vol.
15 October 1991. (also available as BRANZ Reprint 109). Describes a laboratory measurement study
on the effects of edge gaps (another type of thermal bridge) on the R-Value of cavities.
.
The thermal insulation performance of light-weight steel-framed external wall elements. Heavy
Engineering Research Association (N.Z.). Report R4-72 - January 1993. (joint paper with Carson W.J.,
Clifton G.S.). The feasible performance of about 13 steel framed walls, and several thermal breaks.
Validating the Isothermal Planes method for R-Value Predictions. ASHRAE Transactions, Vol. 101, Part
2, pp 755 - 765. Describes a set of 84 internationally published results with measured and calculated
thermal performances.
Contact resistance in steel framed walls. Journal of Thermal Insulation, Vol 20, pp 132 - 143. Oct.
1996. The contact resistance is shown by measurement to be a real physical factor, which varies with
the tightness of fit.
How are U on your R’s?. IRHACE Annual Conference, Napier, May 1996. Also as BRANZ
Conference Paper 34. A shorter and less formal presentation of Ref. 2.
Placement of Wall Insulation. Australian Institute of Refrigeration, Air conditioning, Heating
Engineers Conference, Sydney. April, 1998. This examines edge gaps round inserted insulation
materials. It shows dominant effects of size and orientation (wall or ceiling).
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Cavity or Added R-Value
0.18 0.64 1.00 1.50 2.00 2.50
Weatherboard Description R total (m 2.o C/W)
1.1.1 expected R Wood frame 1.24 1.58 1.87 2.12
ideal R Bulk ins, no foil 1.30 1.80 2.30 2.80
efficiency % 95 88 81 76
1.1.2 expected R Wood frame 0.75 1.20
ideal R Foil only 0.73 1.19
efficiency % 103 101
1.1.3 expected R Wood frame 1.49 1.83 2.12 2.37
ideal R Foil + bulk ins. 1.55 2.05 2.55 3.05
efficiency % 96 89 83 77
1.2.1 expected R Steel frame 0.95 - 1.02 1.12 - 1.22 1.24 - 1.38 1.34 - 1.50
ideal R Bulk ins, no foil 1.30 1.80 2.30 2.80
efficiency % 73 - 78 62 - 68 54 - 60 48 - 54
(1.2.1 expected R
ideal R
(with thermal break
R = 0.2 m 2.o C/W)
1.18
1.30
1.51
1.80
1.79
2.30
2.03)
2.80)
efficiency % 90 84 78 72 )
1.2.2 expected R Steel frame 0.72 1.04 - 1.07
ideal R Foil only 0.73 1.19
efficiency % 99 87 - 90
1.2.3 expected R Steel frame 1.20 - 1.27 1.37 - 1.47 1.49 - 1.63 1.59 - 1.75
ideal R Foil + bulk ins. 1.55 2.05 2.55 3.05
efficiency % 78 - 82 67 - 72 59 - 64 52 - 57
Sheet Cladding
2.1.1 expected R Wood frame 1.13 1.47 1.76 2.01
ideal R Bulk ins, no foil 1.19 1.69 2.19 2.69
efficiency % 95 87 80 75
2.1.2 expected R Wood frame 0.48 0.93
ideal R Foil only 0.46 0.92
efficiency % 105 101
2.1.3 expected R Wood frame 1.22 1.56 1.85 2.10
ideal R Foil + bulk ins. 1.28 1.78 2.28 2.78
efficiency % 95 88 81 75
2.2.1 expected R Steel frame 0.84 - 0.91 1.01 - 1.11 1.13 - 2.19 1.23 - 1.39
ideal R Bulk ins, no foil 1.19 1.69 2.19 2.69
efficiency % 71 - 76 60 - 66 52 - 58 46 - 52
2.2.2 expected R Steel frame 0.45 0.77 - 0.88
ideal R Foil only 0.46 0.92
efficiency % 98 83 - 87
2.2.3 expected R Steel frame 0.93 - 1.00 1.10 - 1.20 1.22 - 1.36 1.32 - 1.48
ideal R Foil + bulk ins. 1.28 1.78 2.28 2.78
efficiency % 73 - 78 62 - 68 54 - 60 47 - 53
TABLE 1. Thermal Insulation Value of Walls v insulation R-value.
1. The “expected” value is calculated to represent the likely value that would be measured over a
representative area of wall. The “ideal” value is calculated ignoring all thermal bridging. The
insulation “efficiency” is the ratio of the two.
2. R-values are reported to 2 places, only so that comparisons can be made. They are not accurate to that
degree.
3. The range shown for steel frames is for different tightness of fit to the steel frame.
(4. Cases 1.x.2, 1.x.3, 2.x.2, & 2.x.3 with foil outside frames are not accepted in New Zealand because of
risk of excessive condensate formation on foil. Cases 1.x.2 and 2.x.2 are also excluded for lack of
insulation value leading to bands of mould formimg above noggings).
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Cavity or Added R-Value
0.18 0.64 1.00 1.50 2.00 2.50
Brick Veneer Description R total (m 2.o C/W)
3.1.1 expected R Wood frame 1.25 1.62 1.95 2.22
ideal R Bulk ins, no foil 1.27 1.77 2.27 2.77
efficiency % 98 92 86 80
3.1.2 expected R Wood frame 1.02 1.47
ideal R Foil only 1.00 1.46
efficiency % 102 101
3.1.3 expected R Wood frame 1.76 2.10 2.39 2.63
ideal R Foil + bulk ins. 1.82 2.32 2.82 3.32
efficiency % 97 91 85 79
3.2.1 expected R Steel frame 1.10 1.39 1.63 1.82
ideal R Bulk ins, no foil 1.27 1.77 2.27 2.78
efficiency % 86 78 71 65
3.2.2 expected R Steel frame 0.98 1.26
ideal R Foil only 1.00 1.46
efficiency % 99 86
3.2.3 expected R Steel frame 1.53 - 1.65 1.74 - 1.94 1.90 - 2.18 2.02 - 2.38
ideal R Foil + bulk ins. 1.82 2.32 2.82 3.32
efficiency % 84 - 91 75 - 84 67 - 77 61 - 72
Masonry
4.1.1 expected R Wood frame 1.02 1.19
ideal R Bulk ins, no foil 1.31 1.81
efficiency % 78 66
4.1.2 expected R Wood frame 0.50 0.82
ideal R Foil only 0.49 0.91
efficiency % 102 90
4.2.1 expected R Steel frame 0.93 1.07
ideal R Bulk ins, no foil 1.31 1.81
efficiency % 71 59
4.2.2 expected R Steel frame 0.48 0.76
ideal R Foil only 0.49 0.91
efficiency % 98 83
Internal brick
5.1.1 expected R Wood frame 1.14 1.48
ideal R Bulk ins, no foil 1.23 1.73
efficiency % 93 85
5.1.2 expected R Wood frame 0.42 0.82
ideal R Foil only 0.41 0.83
efficiency % 104 99
5.2.1 expected R Steel frame 0.78 0.89
ideal R Bulk ins, no foil 1.23 1.73
efficiency % 63 51
5.2.2 expected R Steel frame 0.40 0.64
ideal R Foil only 0.41 0.83
efficiency % 98 78
TABLE 2. Thermal Insulation Value of Walls v insulation R-value.
1. The “expected” value is calculated to represent the likely value that would be measured over a
representative area of wall. The “ideal” value is calculated ignoring all thermal bridging. The insulation
“efficiency is the ratio of the two.
2. The range shown for steel framed brick veneer is for whether or not trace condensate forms on foil (most
likely not). Timber frames are likely to have trace condensate if outdoor temperature < say 5 .o C
9

Double Brick
Cavity or Added R-Value
0.18 0.64 1.00 1.50 2.00 2.50
R total (m 2.o C/W)
6.1 expected R Steel ties 0.6 ± .0.1 1.0 ± 0.1 1.3 ± 0.15 1.7 ± 0.3
efficiency % >98% ~95 % ~90 % ~85 %
6.2 expected R Nylon ties 0.6 1.1 1.4 1.9
efficiency % ~ 100 % ~ 100 % ~ 100 % ~ 100 %
TABLE 3. Thermal Insulation Value of Walls v insulation R-value.
1. The “expected” value is calculated to represent the likely value that would be measured over a
representative area of wall. The “ideal” value is calculated ignoring all thermal bridging. The insulation
“efficiency is the ratio of the two.
2. The accuracy of these results is low, and will depend among other things on the moisture level in the
bricks.
10

Weatherboard
3
1.1.1
(Bulk insul only)
Expected R (m 2.o C/W)
2
1
1.1.2
1.1.3
1.2.1
1.2.2
1.2.3
1.2.1+
0.2 th br
(Foil only)
(Foil + bulk ins)
(Bulk insul only)
(Foil only)
(Foil + bulk ins)
(With thermal break)
0
0 1 2 3
No bridging R (m 2.o C/W)
Figure 1. Effect of thermal bridging on framed weatherboard walls
Sheet cladding
3
Expected R (m 2.o C/W)
2
1
2.1.1
2.1.2
2.1.3
2.2.1
2.2.2
2.2.3
(Bulk insul only)
(Foil only)
(Foil + bulk ins)
(Bulk insul only)
(Foil only)
(Foil + bulk ins)
0
0 1 2 3
No bridging R (m 2.o C/W)
Figure 2. Effect of thermal bridging on framed sheet-clad walls
Brick Veneer
3
Expected R (m 2.o C/W)
2
1
3.1.1
3.1.2
3.1.3
3.2.1
3.2.2
3.2.3
(Bulk insul only)
(Foil only)
(Foil + bulk ins)
(Bulk insul only)
(Foil only)
(Foil + bulk ins)
0
0 1 2 3
No bridging R (m 2.o C/W)
Figure 3. Effect of thermal bridging on framed brick veneer walls
11

Masonry
3
Expected R (m 2.o C/W)
2
1
4.1.1
4.1.2
4.2.1
4.2.2
(Bulk insul only)
(Foil only)
(Bulk insul only)
(Foil only)
0
0 1 2 3
No bridging R (m 2.o C/W)
Figure 4. Effect of thermal bridging on masonry walls
Interna l Brick
3
Expected R (m 2.o C/W)
2
1
5.1.1
5.1.2
5.2.1
5.2.2
(Bulk insul only)
(Foil only)
(Bulk insul only)
(Foil only)
0
0 1 2 3
No bridging R (m 2.o C/W)
Figure 5. Effect of thermal bridging on framed internal brick-lined walls
12

Weatherboard
stee l frame
Thermal break
R-Value
3
Expected R (m 2.o C/W)
2
1
0.6
0.4
0.2
0.1
0.0
0
0 1 2 3
No bridging R (m 2.o C/W)
(a)
Weatherboard
steel frame
3
Expected R (m C/W)
2.o
2
1
2.5
2.0
1.5
1.0
Bulk
R-Value
0.0
0
0.0 0.5 1.0
Thermal break R (m 2.o C/W)
Figure 6 Effect of adding thermal breaks to framed weatherboard wall 1.2.3
(b)
The data in (a) and (b) is the same, only replotted to show a different aspect. The
diagonal line in (a) indicates the line of ideal performance, ie, if there had been no
thermal bridging. The diagonal line in (b) indicates the zone where the increase in wall
R becomes equal to the increase in thermal break R.
13