2.4. Topic 2: The calculation method contains restrictions to the flexibility of roof, wall and floor R-values that can lead to unnecessarily costly and complex construction in some buildings

2.4.1. Reasons 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 a lot of flexibility but does not allow the R-values of the roof, wall and floor of a proposed building to be reduced below 50% of the corresponding R-values in the reference building. 

Industry feedback and recent BRANZ analysis suggest that the current minimum possible R-values for roof, walls and floors in the calculation method are too restrictive, resulting in unnecessarily costly and complex construction in some buildings. These concerns are primarily about the minimum R-values for roofs and slab-on-ground floors. 

For roofs, insulation products required to achieve the current minimum possible R-value of R3.3^[1] can be too thick to find adequate space in some low pitch roofs or skillion roofs. Creating adequate space may require costly and complex solutions such as raised heel roof trusses. Apart from the additional construction costs, the additional height required by a raised heel truss may make it more difficult to stay within the recession plane where a building is close to a boundary.

For slab-on-ground floors, achieving the required minimum possible R-value (ranging from R0.75 to R0.85^[2]depending on the climate zone) can be difficult and costly where the area of heated and cooled space directly above the floor is very small.^[3] An example would be a multi-storey house where the only space that may be heated or cooled on the ground floor are the bottom of a stairwell and a small bathroom next to a large garage. 

Whilst designers can choose to use the modelling method in such situations (which does not have these minimum R-value requirements), this is more time-consuming and requires access to modelling tools and specialist technical skills that not all designers have.

2.4.2. Proposed change: Adjust the minimum possible R-values in the calculation method

The proposed change includes adjusting the minimum possible R-values for the roof, walls and floor of the calculation method as shown in Table 2-1. To improve clarity, MBIE proposes to specify the new, adjusted minimum R-values directly instead of a percentage of reference building R-values. 

This proposed change involves amendments to Acceptable Solution H1/AS1 Energy Efficiency for all housing, and buildings up to 300m2. For more details of the proposed wording in H1/AS1, please refer to Appendix A. 

Table 2-1: Calculation method – Minimum possible R-values (status quo and proposed) 

Notes:

(1) Based on 50% of R6.6, which is the roof R-value in the reference building equations for all climate zones.

(2) Based on 50% of R2.0, which is the wall R-value in the reference building equations for all climate zones.

(3) Based on 50% of R1.5, R1.6 and R1.7, which are the slab-on-ground floor R-values in the reference building equations for the respective climate zones.

(4) Based on 50% of R2.5, R2.8 and R3.0, which are the R-values for other floors in the reference building equations for the respective climate zones.

2.4.3. 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 adjusting the minimum possible R-values in the calculation method will best achieve this objective. The proposed change will enable building designers avoid complex and costly constructions in the situations described in subsection 2.2.3.1 above, without having to use the more time-consuming and complex modelling method. 

Compared to the status quo, MBIE considers that adjusting the minimum possible R-values as proposed 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 adjusting the minimum possible R-values in the calculation method as proposed include:

• Lower upfront building costs

Building designers using the calculation method will be able to avoid complex and costly constructions in buildings with low-pitch roofs, skillion roofs and small slab-on-ground floors.

• Less work for designers and Building Consent Authorities when establishing compliance. 

Designers wanting to avoid complex and costly insulation solutions for low-pitch roofs, skillion roofs and small slab-on-ground floors will be able to use the calculation method, rather than the more time-consuming and complex modelling method.

• Less thermal comfort for building occupants 

Where people are in proximity to building elements with less insulation, they may experience some thermal discomfort during hot or cold weather. However, MBIE considers the proposed adjustments of the minimum possible R-values to have a minimal impact on thermal comfort. 

• 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. This means where a designer makes use of the lower minimum R-values, this will need to be compensated for in other parts of a building’s thermal envelope.

• No change to internal moisture risks 

Buildings classified as housing and communal residential buildings will still be required to satisfy the Building Code performance requirement E3.3.1 for internal moisture. For this, Acceptable solution E3/AS1 Internal Moisture specifies minimum R-values for walls, roofs and ceilings which are not affected by this proposed change. 

For light timber frame wall or other framed wall constructions with cavities, the E3/AS1 minimum R-value of R1.5 requires more insulation than the minimum R-value of the H1/AS1 calculation method. 

On balance, MBIE considers that the benefits of adjusting the minimum possible R-values in the calculation method as proposed outweigh the costs.

2.4.4. Other option MBIE considered

As part of the analysis, we also considered the option of removing the minimum R-values in the calculation method. However, this option was not further pursued on the basis that the proposed option was considered to address the issue more effectively. Removing the minimum R-values altogether would not likely provide significant additional upfront cost savings over the proposed option but could undermine achieving the ‘adequate thermal resistance’ required by Building Code clause H1.3.1(a). 

Whilst the modelling method does not have minimum R-values, it uses advanced computer tools to comprehensively assess a building’s thermal performance. In contrast, the calculation method’s simple equations are less accurate and minimum R-values for the roof, walls and floor are a necessary component of ensuring ‘adequate thermal resistance’.

We determined that the proposed approach of amending Acceptable Solution H1/AS1 to adjust the minimum R-values in the calculation method is the most reasonable and effective option for achieving the objective.  

2.4.5. Questions for the consultation Topic 2

2-1. Do you support amending Acceptable Solution H1/AS1 to adjust the minimum possible R-values in the calculation method as proposed?

• Yes, I support it.
• Yes, with changes.
• No, I don’t support it.
• Not sure/no preference.

2-2.  Please explain your views.

 
[1] Which is 50% of the R6.6 roof R-value in the reference building heat loss equation. 
[2] Which is 50% of the slab-on-ground floor R-values in the reference building heat loss equations.
[3] 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.

Footnotes

^[1]Which is 50% of the R6.6 roof R-value in the reference building heat loss equation.

^[2] Which is 50% of the slab-on-ground floor R-values in the reference building heat loss equations.

^[3]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.

< 2.3 Topic 1: The schedule method may lead to higher upfront costs | 2.5 Topic 3: Where underfloor heating is only used in bathrooms, the minimum R-values for heated floors may cause unreasonable upfront costs >