thermal bridging - calculations and impacts · •thermal bridging impacts energy use, comfort and...
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Thermal Bridging - Calculations and Impacts
Erica Kenna
Department of Environment and Energy
Lynley Boland
Energy Action
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Why does thermal bridging matter?
Image: Building Resarch Establishment (BRE)Image: Texas thermal imaging
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So why haven’t we been paying more attention?
It’s hard to see… and hard to calculate…
Image: morrison hershfield Image: BRE
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What do our regulations say?
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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
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Calculating the impact of thermal bridging using NZS 4214
Isotherms
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Calculating the impact of thermal bridging using NZS 4214
f1
f2
𝑅𝑏 =1
𝑓1𝑅1+𝑓2𝑅2+⋯
𝑓𝑛𝑅𝑛
R1 R2
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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
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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
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Different concepts of heat transfer thru the metal framed wall
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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).
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Outcome
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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
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ISO 6946: Rmin and Rmax concepts
RminRmax
99.6%0.4%
99.6%0.4%
𝑅𝑏 =1
𝑓1𝑅1+𝑓2𝑅2+⋯
𝑓𝑛𝑅𝑛
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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
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Some more examples: 78 mm frame, 400 mm stud spacing
Air gap
gets very
shallow
Insulation is compressed
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Some more examples: 78 mm frame, 600 mm stud spacing
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Some more examples: metal framed walls with furring channel
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More examples: metal roof
View 1:
Parallel
to the
purlins
View 2:
Perpendicular to the
purlins
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Some more examples: metal roof
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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.