A building tightness test under way (blower door)

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.”

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  1. Ronald is correct.

    When we think of the old IAQ saying “ventilate right, keep it tight”, why is all the focus on the tightness?

    Standards should be in place for building envelope leakage and testing SHOULD be the norm. This is something that I had been advocating in my previous role across Australia with associations, industry “leaders” and government for over FIVE years. Very few actually listened – Sean’s related article on TFE spoke to many of the barriers that I faced during my efforts…

    BUT, in doing so we cannot simply forget that our industry alo needs to ask itself some serious questions about ventilation rates and standards.

  2. The air tightness impact on occupant comfort and energy use is dramatically different for various types of construction in different climate zones.
    Leakage from a high thermal mass dwelling in a cool climate with good passive solar gain will result in a totally different situation to that of a lightweight neighbour with poor passive solar gain.
    Air leakage from lightweight structures in tropical climates is highly valued by designers striving to arrive at designs that do not need any mechanical cooling.
    Highly sealed buildings introduce air quality complications and the need for artificial/heat recovery ventilation and greater risks of condensation.
    Any discussion about air tightness which is made in isolation from all the myriad of other thermal performance attributes of building design will not arrive at the best conclusions.

  3. Air tightness has been recognised in Europe as one of the key causes of heat loss or gain in new and existing homes for 20 years. Passiv Haus certification requires .6 ACH@50PA. Compare this to 15ACH@50PA experienced in the average Australia home. At the end of the day great results as low as 3 ACH@50PA can be achieved through attention to detail in construction and the use of airtightness tapes and membrane in crucial areas of the home. Construction cost impacts are negligible and heating and cooling costs can be slashed.I have been in Passiv Haus certified homes in German with a summer winter temperature range of +30 and -15 C which has an annual heating/cooling bill ofjust 300 euro ($480). Time to catch up with the rest of the world.

  4. Have the authors considered the indoor air quality in tightly sealed houses, or is it assumed they are all air–conditioned.

    Climate variability is obviously a factor in thermal comfort issues across Australia.
    Maybe this is why the National Construction Code does not set a maximum air leakage rate,

    Houses are built for people.

    Energy saving is part of the process, not the end goal.

    1. Hi Ron: If an occupant desires fresh air, open a window. Better that the home owner be in control of their living environment, and not some substandard builder.

    2. Hi Ronald
      John M mentioned Passivhaus, a construction standard with occupant comfort AND energy efficiency its primary objectives. The stringent air tightness requirements are coupled with a heat recovery ventilation system in order to provide optimal indoor air quality as well as comfortable temperatures. One doesn’t work without the other.
      And the beauty of the Passivhaus standard is that it works in just about all climates. A house in Hobart will be built differently to one in Darwin, but the occupant comfort requirements remain the same. High energy efficiency construction will facilitate high levels of occupant comfort.