Thermal Bridging - Calculations and Impacts

Erica Kenna

Department of Environment and Energy

Lynley Boland

Energy Action

Why does thermal bridging matter?

Image: Building Resarch Establishment (BRE)Image: Texas thermal imaging

So why haven’t we been paying more attention?

It’s hard to see… and hard to calculate…

Image: morrison hershfield Image: BRE

What do our regulations say?

NZS 4214 (2006) “Methods of determining the total thermal resistance of parts of buildings”

AS/NZS 4859.1: Materials for the insulation of thermal insulation of buildings– General criteria and tech provisions

Calculating the impact of thermal bridging using NZS 4214

Isotherms

Calculating the impact of thermal bridging using NZS 4214

f1

f2

𝑅𝑏 =1

𝑓1𝑅1+𝑓2𝑅2+⋯

𝑓𝑛𝑅𝑛

R1 R2

NZS 4214: Dealing with metal frame elements

Image: Standards NZ

𝑅𝑒𝑞 =𝑙𝑓𝑟𝑎𝑚𝑒

𝑘𝑒𝑞

𝑘𝑒𝑞 = 𝑘𝑓𝑟𝑎𝑚𝑒 ×𝑤𝑒𝑏 𝐵𝑀𝑇

𝑓𝑙𝑎𝑛𝑔𝑒 𝑤𝑖𝑑𝑡ℎ

Web BMT

½ BMT

flange

width

l.frame

Example

Layer Description Depth (mm) Layer k-value, W/m-K

Layer R-value, m²-K/W

External surface film n/a n/a 0.04Metal cladding 0.7 mm 47.5 Approx 0R0.25 thermal break (sheath-type) on metal frame/nogging

8.5 mm 0.034 0.25

Fibre glass type insulation batts bridged by metal frame

Frame: 92 mm Insulation: 92 mm

Frame: 47.5Insulation: 0.04

Frame: 0.061Insulation: 2.5

Plasterboard 10 mm 0.170 0.0588Internal surface film n/a n/a 0.12

Contact R 0.03 m²-K/W

Different concepts of heat transfer thru the metal framed wall

Results

• Equivalent rectangle 47mm x 92 mm, R eq 0.091 m²/K-W

• Frame path R1 increases to 0.401 with thermal break resistance and contact resistance

• Path 1 area fraction, 0.17; Path 2 area fraction, 0.83

• Outcome R = 1.55 (compared with R2.5 nominal value on insulation bag).

Outcome

Comparison with ISO 6946

𝑅𝑇 = 𝑝𝑅𝑚𝑎𝑥 + 1 − 𝑝 𝑅𝑚𝑖𝑛

where p is found as :

• 𝑝 = 0.8𝑅𝑚𝑖𝑛

𝑅𝑚𝑎𝑥+ 0.44 − 0.1

𝐹𝑙𝑎𝑛𝑔𝑒 𝑤𝑖𝑑𝑡ℎ

40−

0.2600

𝑠𝑡𝑢𝑑 𝑠𝑝𝑎𝑐𝑖𝑛𝑔− 0.04[

𝑓𝑟𝑎𝑚𝑒 𝑤𝑒𝑏 𝑑𝑒𝑝𝑡ℎ

100]

• 𝑅𝑇 =𝑅𝑚𝑖𝑛+𝑅𝑚𝑎𝑥

2

ISO 6946: Rmin and Rmax concepts

RminRmax

99.6%0.4%

99.6%0.4%

𝑅𝑏 =1

𝑓1𝑅1+𝑓2𝑅2+⋯

𝑓𝑛𝑅𝑛

Comparison: metal framed wall

Method R-value, m²-K/W

Simple unbridged 2.77Nominal “on the bag” 2.5

NZS 4214 1.56ISO 6946 1.21

Some more examples: 78 mm frame, 400 mm stud spacing

Air gap

gets very

shallow

Insulation is compressed

Some more examples: 78 mm frame, 600 mm stud spacing

Some more examples: metal framed walls with furring channel

More examples: metal roof

View 1:

Parallel

to the

purlins

View 2:

Perpendicular to the

purlins

Some more examples: metal roof

Conclusions

• Thermal bridging impacts energy use, comfort and IAQ.

• Proposed changes to the NCC in 2019 will require more detailed consideration of thermal bridging.

• The ABCB has commissioned nomograms to help designers assess thermal bridging impacts through common wall and roof configurations.