2.7. Topic 4: The modelling method includes requirements that are unclear or outdated
Discussion topic on whether the modelling method used in the Building Code H1 Energy Efficiency requirements is unclear or outdated.
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2.7.1. Reasons for the change
The modelling method is based on computer simulation of the thermal and energy performance of the proposed building, and of a theoretical reference building which acts as a compliance benchmark. The simulations rely on several modelling inputs and assumptions that influence the modelling results and compliance outcomes. Verification Method H1/VM1 specifies modelling requirements and default assumptions that aim to achieve consistency in how buildings are simulated, and certainty that a building has enough insulation to comply.
Feedback from sector technical experts have helped MBIE identify some areas in the modelling method where current requirements and assumptions are unclear or outdated. This includes:
- Uncertainty about what climate data best represents the climate at a building site.
All building modelling software requires climate information for the location of the proposed building, usually in the form of weather files.
There is a wide range of weather files from various sources that either come with relevant modelling software or can be downloaded online. However, not all weather files are robust and up to date. This can affect the accuracy of the modelling and how much insulation is required for a building to comply.
- Uncertainty about what solar heat gain coefficient should be modelled for glazing in the reference building.
The solar heat gain coefficient is a measure for how easily heat from solar radiation can pass through glazing into the building. It can vary widely for different glazing products and can have a big impact on the thermal and energy performance of a building. Currently H1/VM1 does not specify the solar heat gain coefficient that modellers should assume for the theoretical reference building. This results in inconsistent compliance outcomes.
- The current natural ventilation setpoint for housing does not adequately assess a building’s ability to cool itself via open windows.
The natural ventilation setpoint is the indoor temperature at which the modelling software assumes that windows in the simulated building get opened to passively cool down the building.
In H1/VM1, it is currently set at 24°C for housing, just one degree below the 25°C active cooling setpoint at which windows are simulated to close. This does not allow enough ‘room’ for natural ventilation to have much cooling effect. It contributes to cooling energy use results in the modelling method that may be unrealistically high and that distort compliance outcomes.
- The current default modelling assumptions for internal heat gains from electrical plug loads and occupants are out of date and too high.
These are assumptions for how much heat is released inside a building from electrical appliances and the body heat from people.
The current assumptions are out of date. They do not reflect the improved energy efficiency of modern appliances, and that new homes tend to be larger with fewer occupants than was historically the case. This affects the accuracy of modelling results and can distort compliance outcomes.
- Deviations from default modelling assumptions are not always explained in building consent applications.
Deviation makes it more difficult for Building Consent Authorities to check if the modelling method was used correctly, and if a building complies.
2.7.2. Proposed changes
The proposed changes involve amendments to Verification Method H1/VM1 Energy Efficiency for all housing, and buildings up to 300m2. They include:
- Prescribing the use of the most recent NIWA weather files.
NIWA have recently produced updated weather files for use with building energy modelling software. These files include information about the present climate of the different parts of Aotearoa New Zealand and can be freely downloaded from MBIE’s Building Performance website.
Weather files for Aotearoa New Zealand(external link) – Building.govt.nz
MBIE proposes to prescribe the use of the new NIWA weather files for the present climate when using the modelling method, either directly or as climate data that have been converted from these weather files into the format required by the modelling software.
- Specifying the solar heat gain coefficient to be modelled for glazing in the theoretical reference building.
MBIE proposes to add a new requirement that glazing in the reference building needs to be modelled with a solar heat gain coefficient of no less than 0.55 and no more than 0.6. These values correspond to the most used double-glazing products in new housing, with a Low E coating and a Ug-value of 1.1.
This proposed change would better define the glazing properties in the theoretical reference building model but would not prescribe the type of glazing that can be used in the actual proposed building.
- Reducing the natural ventilation setpoint for passive cooling from 24°C to 22°C for housing to better assess a building’s ability to cool itself via open windows.
- Adjusting default modelling assumptions for internal heat gains from electrical plug loads and occupants.
For buildings classified as housing, MBIE proposes to reduce the default power density for internal gains from plug loads from currently 24.5 W/m2 to 13.5 W/m2.
In combination with the existing H1/VM1 default percentages of maximum plug load for different times of day, the proposed new value better represents current typical electricity use and associated heat release from electrical appliances in Aotearoa New Zealand households. It is based on analysis of data from EECA’s Energy End Use Database and comparisons to international building modelling standards.
MBIE also proposes to simplify and reduce the default power density for internal gains from occupants (heat release from people’s bodies). The current default assumption is 150 W for the first 50m2 floor area, plus 3 W/m2 beyond that.;
The proposed new default assumption is 2.5 W/m2, which equates to about one occupant for each 30m2 floor area. MBIE also proposes to make adjustments to account for the reduced amount of body heat that people release while sleeping at night.
- Adding a new requirement for modellers to document and justify any deviations from default assumptions. This is to improve transparency and make it easier for Building Consent Authorities to assess building consent applications that are based on the modelling method.
For more details of the proposed wording in H1/VM1, refer to Appendix B.
Appendix B: Proposed changes to Verification Method H1/VM1 [PDF, 3.3 MB]
2.7.3. Analysis of the proposed changes
The primary objective of the proposed changes is to support consistency of how the modelling method is applied 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).
For this issue, MBIE considers that the proposed changes to the modelling method will best achieve this objective.
MBIE expects that the impacts of this proposal include:
- Improved accuracy of modelling results that better reflect how buildings perform from updated modelling inputs and assumptions.
- Better consistency and certainty of compliance from clearer requirements.
- Fewer delays in the building consenting process from more transparent building consent documentation.
Upskilling required by building modellers to adopt the proposed changes in their modelling practices. - Costs to some software providers to incorporate the proposed changes in their building modelling software. Whilst most modelling software is generic, some tools have been specifically built to incorporate the H1/VM1 modelling method and would require updating.
On balance, MBIE considers that the benefits of the proposed exemption outweigh the costs.
2.7.4. 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:
- Allowing the use of weather files with future-projected climate data in the modelling method.
Whilst MBIE has confidence in the robustness of the future-projected climate versions of NIWA’s weather files[1], there has not been enough time since completion of these files to assess the potential impacts of their use in the modelling method.
There is also great uncertainty about which climate change scenario may be most appropriate to use. MBIE may consider this option for a future update of H1/VM1.
- Specifying a single value for the solar heat gain coefficient for glazing in the theoretical reference building.
MBIE considers that providing a narrow range of permitted values as proposed is more reasonable. This is because the solar heat gain coefficients of the most used double-glazing products in new housing vary slightly between suppliers.
Whilst the proposed change only affects the theoretical reference building, the narrow range of permitted values will enable more modellers to use identical solar heat gain coefficients in both the proposed and reference building models. This saves time and costs in setting up these models, whilst still adequately achieving the objective of consistency.
We determined that the proposed approach of amending Verification Method H1/VM1 to clarify and update modelling method requirements is the most reasonable and effective option for achieving the objective.
2.7.5. Questions for the consultation Topic 4
4-1. Do you support amending Verification Method H1/VM1 as proposed to clarify and update requirements for the modelling method?
- Yes, I support it.
- Yes, with changes.
- No, I don’t support it.
- Not sure/no preference.
4-2. Please explain your views.
Footnote
[1]Also available on MBIE’s Building Performance website – Weather files for Aotearoa New Zealan(external link)d – building.govt.nz