2.8. Topic 5: Thermal bridging from framing in walls is not adequately considered
Discussion topic on thermal bridging from framing in walls which is not adequately considered.
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2.8.1. Reasons for the change
Many buildings in Aotearoa New Zealand have walls where thermal insulation is installed between timber or steel framing. The framing members act as thermal bridges. This is because heat can pass more easily through the framing materials than through the insulation. The more framing a wall has, the greater the thermal bridging effect that worsens the thermal resistance (R-value) of the wall.
With all compliance pathways for the H1 energy efficiency insulation provisions, designers need to determine the thermal resistance (R-value) of the proposed building elements that form part of a building’s thermal envelope. Designers can either calculate the R-value or use tools with pre-calculated values such as the BRANZ House Insulation Guide.
For determining the R-value of framed walls, current requirements in Acceptable Solution H1/AS1 and Verification Method H1/VM1 require consideration of the effects of certain framing members but allow ignoring the effects of additional framing members at corners, junctions and around window and door openings. This significantly overestimates the R-value of framed walls.
In addition, designers will generally not know how much framing a building’s walls require to meet structural and buildability requirements. This is because the required amount of framing in walls is generally determined by pre-nailed frame and truss manufacturers during the construction-stage of a building, which is after building consent. This means, with the current requirements there is great uncertainty about the amount of framing that should be assumed.
Research by Beacon Pathway from 2018[1] and 2021[2] suggests that industry commonly assume that framing accounts for 14 to 18% of a wall’s area, whereas the actual timber framing fraction observed in a sample of 47 new detached houses ranged from 24 to 57%, with an average of 34%. It found that the observed amount of framing is generally necessary to meet structural and weathertightness requirements, and not excessive.
More recent data from PlaceMakers Manufacturing for a much larger random sample of 452 new residential and commercial buildings had an average framing percentage of 38% of the net wall area, ranging from 23 to 64%. Whilst framing percentages in this sample vary between individual buildings, there was no significant difference in average framing percentages for different building types, or for different spacings of regular studs and dwangs. MBIE considers this the most robust information on wall framing fractions currently available in Aotearoa New Zealand.
The big difference in commonly assumed and observed actual wall framing percentages means that wall R-values determined for demonstrating compliance with the H1 energy efficiency insulation provisions greatly overestimate actual wall R-values. For example, a timber-framed wall insulated to achieve a wall R-value of R2.0 using an assumed 18% framing percentage only achieves R1.6 if a more realistic framing percentage of 38% is considered.
2.8.2. Proposed changes
The proposed changes involve amendments to Acceptable Solution H1/AS1 and Verification Method H1/VM1 Energy Efficiency for all housing, and buildings up to 300m2. Similar amendments are proposed for Acceptable Solution H1/AS2 and Verification Method H1/VM2 Energy Efficiency for buildings greater than 300m2. They include:
- Requiring a framing fraction of no less than 38% to be assumed when determining the construction R-value of framed walls, unless a designer can demonstrate that a lower framing fraction is justified. This would replace the current provision which requires consideration of the effects of certain framing members but not others.
- Reducing the wall R-value in the theoretical reference building of the calculation and modelling methods from R2.0 to R1.6. Equally, if MBIE was not to proceed with the proposed removal of the schedule method discussed in subsection 2.3 above, the minimum R-value for walls would be reduced from R2.0 to R1.6 in the schedule method.
For more details of the proposed wording, please refer to Appendix A for H1/AS1 and Appendix B for H1/VM1.
Appendix A: Proposed changes to Acceptable Solution H1/AS1 [PDF, 4.4 MB]
Appendix B: Proposed changes to Verification Method H1/VM1 [PDF, 3.3 MB]
2.8.3. Analysis of the proposed changes
For this issue, the primary objective of the proposed changes is to support better consistency of accuracy between R-values of framed and non-framed walls and provide certainty for designers, Building Consent Authorities and building users that buildings have sufficient insulation for achieving Objective H1.1 of the Building Code, Functional requirement H1.2(a) and Performance H1.3.1(a).
A secondary objective is to avoid additional costs.
MBIE considers that the proposed changes to requirements for determining framed wall R-values will best achieve these objectives. MBIE expects that the impacts of this proposal include:
- Better consistency and certainty of compliance from clearer requirements.
- Improved accuracy of calculation and modelling results used for establishing compliance that better reflect how buildings perform. This is because of more accurate wall R-value inputs.
- A more level playing field between buildings with framed and non-framed walls.
The currently permitted practice of overestimating R-values for framed walls puts buildings with non-framed walls, such as those made of structural insulated panels or with external insulation, at a disadvantage.
- No additional work or costs for designers and Building Consent Authorities.
MBIE expects that designers will adopt the proposed default framing fraction of 38% in most situations and simply use this instead of their current lower framing fraction assumptions.
Alternatively, where designers wish to use a lower value, they could request framing fraction information for their proposed building from a frame and truss manufacturer and provide it as justification with the building consent application.
MBIE is aware of one frame and truss manufacturer offering this service already pre-consent and expects that other manufacturers would follow if the proposed changes were implemented.
- No additional building costs. The proposed adjustment of reference building wall R-values from R2.0 to R1.6 would ensure that the change in framing fraction would not result in required changes to the construction of buildings, or different specifications of required insulation products to achieve compliance.
On balance, MBIE considers that the benefits of the proposed changes outweigh the costs.
2.8.4. Other options MBIE considered
As part of the analysis, we also considered other options that were not further pursued on the basis that the proposed changes were considered to address the issue more effectively.
These discounted options included:
- Requiring designers to use the actual framing fraction when determining the construction R-value of framed walls.
Whilst this option would be most accurate, MBIE does not consider it reasonably practicable. It could result in considerable additional work, delays and costs if frame and truss manufacturers had to provide information on actual framing fractions to designers pre-consent for every building with framed walls.
- Not reducing the wall R-value in the theoretical reference building of the calculation and modelling methods to compensate for the proposed higher framing fraction assumption.
This option would require additional insulation and construction changes and not meet MBIE’s objective of avoiding additional costs.
We determined that the proposed approach of amending Acceptable Solution H1/AS1 and Verification Method H1/VM1 to better consider thermal bridging in framed walls is the most reasonable and effective option for achieving the objectives.
2.8.5. Questions for the consultation Topic 5
5-1. Do you support amending Acceptable Solution H1/AS1 and Verification Method H1/VM1 as proposed to better consider thermal bridging in framed walls?
- Yes, I support it.
- Yes, with changes.
- No, I don’t support it.
- Not sure/no preference.
5-2. Please explain your views.
Footnotes
[1] Ryan, V., Penny, G., Cuming, J., Baker, G and Mayes, I. (2019). Measuring the Extent of Thermal Bridging in External Timber-Framed Walls in New Zealand. Final Report – Building Levy Project LR11092. Report Wall/3 from Beacon Pathway Inc.
[2]Ryan, V., Penny, G., Cuming, J., Riley, M. (2021). Thermal Bridging in External Walls: Stage Two. Report by Beacon Pathway.