Close to half of 129 new homes tested for air tightness across Australia’s capital cities have returned poor results, a new CSIRO study has shown, with the body recommending minimum air tightness standards be set in the National Construction Code.
Almost half of the houses tested scored above 15 air changes an hour at 50 Pascals pressure (15 ACH@50Pa), which is considered the upper limit for a newly constructed house in Australia, according to the CSIRO’s House Energy Efficiency Inspections Project.
While the National Construction Code does not set a maximum air leakage rate, it does require that “a building must have, to the degree necessary, a level of thermal performance to facilitate the efficient use of energy for artificial heating and cooling appropriate to the sealing of the building envelope against air leakage”.
Because most new houses are rated with NatHERS software to demonstrate compliance with the NCC’s energy efficiency requirements, results were compared with the level of air-tightness assumed by the NatHERS software – around 15 ACH@50Pa.
It was shown that around a third of homes had results lower than 10 ACH@50Pa, showing that well-sealed homes existed and were possible throughout Australia.
Almost half, however, had a rate above 15 ACH@50Pa, and several had rates of over 30 CH@50Pa, a figure common for old, draughty houses but regarded as unacceptable for new builds (all homes were three years old or less, except in Melbourne).
“Consequently, there is reason for concern about why so many houses recorded poor results,” the study authors said.
The study could not find conclusive reasons as to why some homes performed better than others, however general build quality and attention to detail seem to be significant factors.
No correlation could be found between poor weather sealing and high air change rates, nor with insulation quality, penetrations into the roof space by downlights and exhaust fans, gaps in subfloor systems, or house size.
“The differences may be due to factors that were not investigated during this study,” the study authors said. “This might include gaps around power points and light switches and also air return vents for heating/cooling systems. One potential cause might also be the quality of sealing between the window frame and the house frame.”
Interestingly, the more southern cities of Adelaide and Hobart recorded much better air tightness results than other major cities, with an average of 8.5 and 7.9 ACH@50Pa, respectively. The cause of the results could not be determined, with further investigation finding no specific differences from common building practice. However, a higher than usual proportion of architect- and custom-designed houses in both the Adelaide and Hobart cohort led to the conclusion that greater attention to build quality had occurred and this had resulted in better than average test results.
Push for specific requirements in the NCC
The authors concluded that consideration should be given to setting a specific air tightness requirement in the NCC, with 10 ACH@50Pa the recommended target, which would correspond to the minimum value required for houses in the UK.
“The results have shown that many houses are already achieving this goal with a third of the houses tested recording a value of 10 ACH@50Pa or less.”
The authors suggested that NatHERS could allow high-performing houses to receive higher star ratings by incorporating certified air pressure results into NatHERS calculations, which currently do not have any affect on NatHERS ratings.
“This would require ‘as designed’ and ‘as built’ NatHERS certification certificates to be issued with the ‘as built’ certificate only issued after verification of the house performance was established through testing. This could lead to the greater uptake of air pressure testing of new houses and help improve their performance and further reduce the energy requirements.”