A higher NatHERS star rating: Is it the holy grail of low energy and heat resilient residential buildings?

LinkedIn / Anir Upadhyay

The National Construction Code (NCC) of Australia has mandated new residential buildings to meet the minimum thermal performance standard for building fabric since the start of 21st century. Over the years, stringency has increased to ensure houses are thermally more comfortable without using significant amount of energy for space heating and cooling. 

The NCC allows using multiple methods to demonstrate thermal performance of buildings. However, for residential construction, thermal simulation using the Nationwide House Energy Rating Scheme (NatHERS) accredited software packages is the most popular method to satisfy the NCC requirement. The NatHERS software packages report heating and cooling loads, and star rating (a scale of 0 to 10 stars) of a dwelling based on the user inputs on building design and material properties; software-controlled pre-defined parameters such as, occupancy profile, user behaviour and thermostat settings for heating and cooling to maintain indoor thermal comfort. The software automatically assigns a relevant climate zone, based on the address of the building, which provides a climatic context to perform thermal simulations for each hour of a typical year. Therefore, climate files are important in any thermal performance simulation tools. 

NatHERS climate files currently being used to demonstrate building energy efficiency portray a climate of the late last century, i.e., the average of 1967 to 2004. The irony is that the National Construction Code is trying to achieve building energy efficiency in the first quarter of 21st century by using outdated climate data sets. Recently, the Australian Building Codes Board (ABCB) has proposed a significant update of the National Construction Code for 2022. One of the major changes is to increase the building thermal performance standard of houses to achieve a NatHERS 7-star from the current 6-star rating. It is a significant step to ensure houses will be more resilient and therefore would require less energy for space heating and cooling. However, this good intention is overshadowed by the proposed updated (but already outdated) climate files which are prepared using weather data from 1990 to 2015.

In the last decade (2011 to 2020), Australia experienced a significant increase in temperature together with gradual increase in the frequency of heatwaves . If the houses that are designed and built in 2021 cannot withstand the current climate, then how can we ensure that these houses will save energy, as well as offer heat resilience during blackouts and heat wave conditions in future. Houses which achieve a higher NatHERS star rating, using the outdated climate files are therefore more vulnerable during heat waves as they are specifically designed to perform well in much cooler climate. These houses perform badly during heat waves as they cannot effectively purge heat trapped inside the house. Unfortunately, homeowners and builders who aspire to achieve higher NatHERS ratings for their houses may not be aware of the unintended consequences of achieving higher star rating using the outdated climate files.

The regulatory authorities need to ensure NatHERS climate files represent a realistic climatic context that reflects current as well as future climate scenarios predicted by the latest climate models. If the regulatory authorities keep using outdated climate files in the NatHERS accredited software packages, it is likely that the houses will keep getting designed for historical irrelevant climate, requiring the houses to use significant amounts of energy for cooling. Therefore, Australian dwellings will neither be energy efficient nor heat resilient.  

This article first appeared on LinkedIn.