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22 July 2019

Homestar Ratings: Moving Beyond 6 Stars

calculating thermal performance

In my April 2019 blog post, "ANZ Bank Incentivises Homestar Ratings," I referred to the need to meet certain mandatory limits before a dwelling would be eligible for the minimum 6 star Homestar rating. These relate to thermal comfort, space heating, ventilation, internal moisture, and maximum water-flows for showers and WCs. To reach seven-star, and then eight-stars and beyond, the thermal and moisture requirements are more onerous. Of course, the final rating is subject to sufficient points being collected from a range of all the available Homestar Categories, each of which have particular credits for which points are gained.

In moving beyond 6 stars, the moisture requirements are an uncomplicated matter of using appropriate standard construction details, but to gain an increase in thermal performance there are a variety of interrelated factors which can be combined in various ways, and which do not need to restrict the architectural design solutions for the home and site. As I discussed in my December 2014 blog post, "Insulation, Thermal Mass and Glazing: The Juggling Game", there are important variables to be considered. Even only taking the tight inter-relationship of these three primary ones (insulation, thermal mass and glazing), the selection process is too complex to be analysed manually, which is why Homestar requires energy modelling to demonstrate compliance for the higher star ratings. I further discuss the emphasis on just increasing thermal insulation in my June 2018 blog, “Can Thermal Performance Be Measured Objectively?

To determine the energy demand for 6 stars, the simplified process of the Homestar spreadsheet ‘Tool’ is sufficient. BRANZ’s ‘ALF calculator’ can be used to calculate the heating load up to seven stars, but beyond that, energy modelling must be used. Of course, modelling can be applied for 6 Homestar dwellings and above, especially if these analyses are being applied to the design for other than Homestar purposes. In these circumstances, a house which can only achieve a 6 Homestar rating due to insufficient points from other Homestar Categories, can maximise points for its EHC-1 (Thermal Comfort) credit, even tipping it into 7 stars.

The Energy Modelling pathway of EHC-1 considers both the heating and cooling energy necessary to boost the passive solar heating gains, and the natural ventilation cooling needed to overcome this deficit. The thermal simulation software we use, as brought to New Zealand in the 2000s for the government’s Home Energy Rating Scheme, is accepted for this thermal energy modelling analysis.

Government developed the HERS (NZ Home Energy Rating Scheme) through EECA (Energy Efficiency & Conservation Authority). The scheme uses thermal simulation software, (still used in Australia) from CSIRO Australia (adapted to 18 climate zones throughout NZ), as a means of evaluating the thermal performance of our housing. The scheme was put into abeyance around 2010 primarily due to the complexity of its use and the public disinterest in star-ratings. Also at the time, the Homestar rating scheme was being developed by the NZ Green Building Council as a broad measure of the sustainability of our housing stock. Even so, the HERS analysis is still valid and accepted by Councils to show NZBC-H1 compliance by the modelling method.

Since 2010 I have been using the HERS software analysis to assist architects, ADNZ designers, developers and homeowners to refine their designs through the exploration of plan, elevation, orientation and material options using the objective results from the analyses so as to demonstrate NZBC-H1 compliance, and to maximise Homestar points for the EHC-1 Credit. These have shown that apartments and medium-density housing developments can quite easily reach high levels of passive solar thermal performance when care is taken with the construction and fenestration detailing so that the EHC-1 (Thermal Comfort) credit Homestar points can be maximised.

For further explanation, I can be contacted at

Photo by Helloquence on Unsplash

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