3.9. Topic 17: Thermal bridging from framing in walls is not adequately considered
Proposed changes to the values for construction of framed walls.
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3.9.1. Reason for change
As discussed for housing and small buildings in subsection 2.8 above, current requirements for determining the R-value of framed walls in the H1 acceptable solutions and verification methods significantly overestimate R-values for framed walls. There is also great uncertainty about the amount of framing that should be assumed when determining framed wall R-values. The same issues exist for large buildings within the scope of H1/AS2 and H1/VM2.
3.9.2. Proposed changes
The proposed changes involve amendments to Acceptable Solution H1/AS2 and Verification Method H1/VM2 Energy Efficiency for buildings greater than 300m2. Similar amendments are proposed for Acceptable Solution H1/AS1 and Verification Method H1/VM1 Energy Efficiency for all housing, and buildings up to 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-values across the six climate zones for the theoretical reference building of the calculation and modelling methods to compensate so that the introduction of the default 38% framing fraction would not result in additional costs. Equally, if MBIE was not to proceed with the proposed removal of the schedule method discussed in subsection 2.3 above, the minimum R-values for walls would be reduced in the schedule method.
For more details of the proposed wording, please refer to Appendix C for H1/AS2 and Appendix D for H1/VM2.
Appendix C: Proposed changes to Acceptable Solution H1/AS2 [PDF, 4.3 MB]
Appendix D: Proposed changes to Verification Method H1/VM2 [PDF, 4 MB]
3.9.3. 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 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.
3.9.4. 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-values 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/AS2 and Verification Method H1/VM2 to better consider thermal bridging in framed walls is the most reasonable and effective option for achieving the objectives.
3.9.5. Questions for the consultation Topic 17
17-1. Do you support amending Acceptable Solution H1/AS2 and Verification Method H1/VM2 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.
Please explain your views.