A new study from the University of Adelaide indicates that some of the baseline assumptions underlying the Nationwide House Energy Scheme (NatHERS) do not fully correspond to actual behaviours in certain types of low-energy dwellings, leading to significant overestimates of their power consumption.

The study, entitled House energy rating schemes and low energy dwellings: The impact of occupant behaviours, by Lyrian Daniel, Veronica Soebarto and Terence Williamson from the University of Adelaide’s School of Architecture and Built Environment looked at the assumptions used by NatHERS for assessing the energy efficiency of house designs, with a particular focus on the extent to which they reflect behaviour in homes that make use of earth construction components.

According to the study’s conclusion, NatHERS’ current assumptions about the heating or cooling of such homes fail to adequately reflect the real-world behaviour of their occupants, and as a consequence provide inaccurate assessments of their efficiency.

The study authors surveyed a total of 175 households around Australia that incorporated earth construction components and found that the heating and cooling practices of such dwellings differed significantly from those that  NatHERS assumes are in use during the assessment process.

When researchers modified these assumptions to reflect differences in occupant behaviour for earth construction dwellings, they found that predictions for heating and cooling loads produced by software simulations became far more accurate.

“This reveals a considerable over-estimation of the predicted heating and cooling energy loads in the regulatory mode of the software,” said the study. “It is clear…that current static occupant related settings within the Nationwide house energy rating scheme thermal performance simulation software do not adequately reflect actual practices within a cohort of low energy households in Australia.”

The study further states that these in-built assumptions could be deterring the use of alternative design solutions for the creation of high-efficiency dwellings by underestimating their performance and necessitating the adoption of superfluous measures at additional cost for the improvement of energy ratings.

“A house design that is initially assessed with NatHERS and achieves less than 6 Star rating will have to be improved, perhaps by adding more thermal insulation or using better glazing, in order to meet this minimum requirement,” said the study. “If a more realistic assumption of user pattern and behaviour is used, it is likely that this house design can achieve higher than 6 Star rating without requiring such improvements that will increase the construction cost.

“[There is] a distinct disadvantage to those wishing to build in this manner, furthering the suggestion that this type of housing may need to be assessed in a different manner to support equitable assessment.”