In the February’s issue of Guidelines, (the 9th item), BRANZ reminded readers that its ALF software is not suitable for determining compliance with the New Zealand Building Code – Clause H1 (Energy Efficiency) – where the dwelling has a party-wall as part of its thermal envelope.  This would also apply to party-floors/ceilings in multi-storey apartments.  ALF was developed to analyse detached dwellings for the purpose of showing compliance with NZBC-H1 where the Schedule Method is not applicable.

The BRANZ guideline advises that when the Modelling Method is used to calculate the BPI (Building Performance Index) of a non-detached dwelling, so as to demonstrate H1 compliance, thermal simulation software such as AccuRateNZ can be used.  AccuRateNZ was brought to, and adapted for, New Zealand, for EECA’s Home Energy Rating Scheme (HERS).  The reference to MBIE’s “Codewords  issue 32” refers to a research report that used both ALF and AccuRateNZ to calculate the heating energy use for 14 different house designs when they were each located in 16 different climate zones. 

The results showed “a very good correlation between the two different programmes”, concluding that “AccuRateNZ can be used to calculate heating energy and BPI.”  When the BPI formula is used to demonstrate compliance with NZBC-H1 a correlation constant is applied, which has since been incorporated into AccuRateNZ’s calculations.

ALF simplifies the form of the dwelling, (for ease of data input and because it  is a compliance tool), to just give a heating energy figure, whereas AccuRateNZ models the whole house as designed and gives the cooling as well as heating energy shortfall.  It is important to recognise that the energy results from both are based on climate data, not weather reports, and are numbers for use within the design process.  They are not predictors of performance on any particular day of the year.  (If the kids leave the back door open on a winter’s evening then the power bill will be higher.)

Following the NZBC-H1 result, the AccuRateNZ analysis can – because the whole interior and exterior of the building has been input – be effectively used to give objective data for use in the design and documentation phases of a project.  The objective information allows for comparisons to be made between differing: site/plan/design arrangements; elevation/window alternatives; materials;  construction methods; increases or decreases in thermal mass, insulation and/or glazing; and so on.

While architects and designers can intuitively design for thermal performance, the AccuRateNZ analysis is able to give objective support and conformation so as to optimise thermal performance.  This is especially useful for building alteration and extension projects that require an awareness of the final cost and important for the thermal aspects of the project.  There is little point, nor economic sense, to further increase the performance of the new instead of first upgrading the existing profile to match.