Study casts doubt on building industry’s ‘green certified’ ratings

Researchers from the University of Auckland are questioning the accuracy of some ‘green certified’ rating tools used by the building industry to measure the warmth and efficiency of New Zealand houses, after independent research failed to verify their claims.

Doctoral graduate Rochelle Ade, together with Dr Michael Rehm, from the Property Department at the Business School, tested some of the beneficial claims of Homestar, the national building ratings tool established by the New Zealand Green Building Council (NZGBC).

Homestar measures New Zealand houses on a scale from 6 to 10 with a 6-Homestar rating or higher said to provide assurance that a house will be better quality - warmer, drier, healthier and cost less to run - than a typical new house built to building code.

In a New Zealand first, Ade and Rehm, concurrently measured the quality of the indoor environment of not only older housing but also newly constructed 6-Homestar and code-compliant dwellings, to determine their relative performance against international healthiness thresholds.

Utilising a case study of 30 Auckland social houses, Ade and Rehm found that all the dwellings spent the majority of winter colder than the World Health Organisation’s healthy temperature of 18°C, with relative humidity outside the healthy range of 40-60%, over 80% of the time.

While there is already a strong existing narrative that New Zealand housing is cold and damp, this is typically discussed in the context of older vintage dwellings. Ade and Rehm’s study is the first empirical research showing that newly constructed code compliant and green certified homes can also be cold and damp.

Despite the NZGBC stating a “6-Homestar rating or higher provides assurance that a house will be better quality - warmer, drier, healthier and cost less to run - than a typical new house built to building code,” Ade and Rehm’s research found that newly built 6-Homestar rated case study dwellings still spent 56% of the time during winter colder than the World Health Organisation’s healthy temperature (18°C). In contrast, newly built code compliant case study dwellings spent 64% of the winter below this threshold. Although the certified case study homes outperformed, the difference was statistically insignificant.

Their research also found that 6-Homestar rated case study homes exceeded the World Health Organisation’s healthy temperature (24°C) threshold 75% of the time during summer. In other words, the studied 6-Homestar houses were chronically overheating. New, non-certified homes spent 58% of the time overheated, while older vintage homes fared significantly better by spending only 43% of their summer above the WHO healthy temperature threshold.

“The NZGBC promotes Homestar as a solution to cold and damp New Zealand homes,” says Ade. “If the actual performance of Homestar dwellings differs from what is claimed then this could be a breach of the Fair Trading Act and the industry and consumers need to know.”

This is the first independent academic research that has been conducted on Homestar. The case study evaluated social housing in Auckland, a category that houses vulnerable people frequently in fuel poverty. The Government has committed social housing provider Kāinga Ora to deliver 6,000 state and 2,000 transitional additional houses over the next three-four years. Currently these dwellings will be rated to 6-Homestar.

Rehm believes: “The most vulnerable people in our society need our greatest protection. Social housing provider Kāinga Ora has committed to delivering 6-Homestar certifications on all its new builds on the understanding that it will provide houses that are “warmer, drier, healthier and cost less to run. However the NZGBC has published no evidential basis for this claim and our research (whilst limited to 30 dwellings in Auckland), represents the first in-field site measurements of 6 Homestar certified dwellings, and has found evidence counter to these claims”.

Some banks have also partnered with the NZGBC and are mandating 6-Homestar certifications as the minimum requirement to access their healthy home loan packages. International standards with a strong evidential performance basis (like Passive House) are not eligible to access healthy home loan packages, despite international academic research confirming that the Passive House standard delivers warmer, drier, healthier homes that cost less to run.

Ade says “Developers shouldn’t be required to chase points in a rating tool, they should be encouraged to design and build to a standard that is independently verified to deliver healthy houses”.

Ade and Rehm’s research also demonstrates the necessity for ongoing performance monitoring of dwellings. Ade says “it isn’t good enough to evaluate the design and construction of dwellings, we must verify that they are healthy in operation”.

The International Energy Agency (IEQ) in 2017 recommended that an effective monitoring and enforcement regime be implemented for both tenancy legislation and building code with Ade and Rehm now calling for real time IEQ, energy and water performance monitoring to be implemented to satisfy this recommendation. Ade and Rehm utilised Tether EnviroQs in their research but say other alternatives are also available in New Zealand.

Ade and Rehm’s research also showed a 12% cost premium for 6-Homestar certification, comprising an 11% hard cost premium and 1% in additional soft costs, which is in stark contrast to the 1.5% to 2.2% increase in capital cost claimed by the NZGBC.

Further assertions by the NZGBC of Homestar’s 34% IRR and 2-year payback contrasted with the researchers’ findings of -23% IRR calculated with a 62-year payback (for houses in Auckland).

The researchers are calling for further independent evaluation of the claims of the NZGBC in relation to Homestar and for Kāinga Ora to implement not only internationally proven and recognised standards, but to also require ongoing performance monitoring of houses.

Doctoral candidate Rochelle Ade received assistance from the Building Research Levy and the St Cuthbert’s Old Girls’ Association Violet Wood Advanced Studies Grant.

Cold Comfort Link:

Summertime comparative evaluation of indoor temperature and comfort in Auckland New Zealand: a survey of green certified, code and older homes:

Buying Limes but getting lemons:

Reaching for the stars:

Hear Rochelle being interviewed on RNZ:
Research challenges warmth, efficiency of Homestar properties

Media queries

Miranda Playfair
Media adviser
Mob 021 0638393