3.4. Topic 13: The calculation method does not provide flexibility for roof, skylight and floor R-values, limiting opportunities for optimising insulation
Allowing flexibility for the R-values of all building elements in the calculation method.
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3.4.1. Reason for the change
The calculation method uses simple equations and allows a designer to customise the insulation levels between different building elements.
A proposed building must not exceed the calculated heat loss of a theoretical reference building that is insulated with R-values that match those of the current schedule method.
The calculation method offers more flexibility but currently allows designers to vary the R-values of the walls, doors and windows only. For the roof, skylights and floor, the calculation method for large buildings has fixed minimum R-values.
Industry feedback suggests that the current inflexibility for roofs, skylights and floors results in unnecessarily costly and complex construction in some buildings.
A common example is a large commercial warehouse where the only heated and cooled space is a small office area. Here, achieving the minimum R-values for the office floor can be difficult and costly because its area is so small.[1]
Whilst designers can choose to use the modelling method in such situations (which provides full flexibility for the R-values of all building elements), this is more time-consuming and requires access to modelling tools and specialist technical skills that not all designers have.
3.4.2. Proposed change: Allow flexibility for the R-values of all building elements in the calculation method
The proposed change includes adjusting the heat loss equations for the proposed and theoretical reference buildings in the calculation method to allow flexibility for the R-values of all building elements that form part of a building’s thermal envelope.
This proposed change involves amendments to Acceptable Solution H1/AS2 Energy Efficiency for buildings greater than 300m2. For more details of the proposed wording in H1/AS2, please refer to Appendix C.
Appendix C: Proposed changes to Acceptable Solution H1/AS2 [PDF, 4.3 MB]
3.4.3. Analysis of the proposed change
The primary objective of this proposal is to reduce upfront building costs and improve the cost-effectiveness of the insulation required for achieving Objective H1.1 of the Building Code, Functional requirement H1.2(a) and Performance H1.3.1(a).
MBIE considers that allowing flexibility for the R-values of all building elements in the calculation method will best achieve this objective. The proposed change will enable building designers avoid complex and costly constructions in the situation described in subsection 3.4.1 above, without having to use the more time-consuming and complex modelling method.
Compared to the status quo, MBIE considers that this proposal will reduce upfront costs for some buildings, whilst still achieving ‘adequate thermal resistance’ as required by Building Code clause H1.3.1(a).
MBIE expects that the impacts of allowing flexibility for the R-values of all building elements in the calculation method as proposed include:
- Lower upfront building costs.
Building designers using the calculation method will be better able to optimise insulation for each individual building and avoid complex and costly constructions, for example in large buildings with small heated or cooled spaces.
- Less work for designers and Building Consent Authorities when establishing compliance.
Designers wanting to avoid complex and costly insulation solutions will be able to use the calculation method, rather than the more time-consuming and complex modelling method.
- No significant change to a building’s energy usage (running costs and carbon emissions).
Whilst the proposed change will increase the flexibility of the calculation method, it will not change the required overall thermal performance of the building.
The maximum permitted calculated heat loss of a proposed building will remain unchanged and continue to be based on the calculated heat loss of a theoretical reference building.
Where a designer reduces roof, skylight or floor R-values as a result of the proposed added flexibility, this will need to be compensated for in other parts of a building’s thermal envelope.
- More innovation.
Increased flexibility of the calculation method could encourage innovation within the industry as practitioners explore new ways of achieving compliance. This could lead to increased development and uptake of innovative products, technologies or design methods that improve building performance.
On balance, MBIE considers that the benefits of adjusting the minimum possible R-values in the calculation method as proposed outweigh the costs.
3.4.4. Other options MBIE considered
Apart from the retaining the status quo MBIE did not consider any other options.
3.4.5. Questions for the consultaion Topic 13
13-1. Do you support amending Acceptable Solution H1/AS2 to allow flexibility for the R-values of all building elements in the calculation method as proposed?
- Yes, I support it.
- Yes, with changes.
- No, I don’t support it.
- Not sure/no preference.
Please explain your views.
[1] This is because the achieved R-value depends on the ratio between the area of the slab-on-ground floor, and its perimeter. For this, only the parts of the floor under spaces that can be heated or cooled are considered. The lower the area-to-perimeter ratio, the lower the achieved R-value of a slab-on-ground floor of a particular construction and insulation.
Footnote
[1]This is because the achieved R-value depends on the ratio between the area of the slab-on-ground floor, and its perimeter. For this, only the parts of the floor under spaces that can be heated or cooled are considered. The lower the area-to-perimeter ratio, the lower the achieved R-value of a slab-on-ground floor of a particular construction and insulation.