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Houses with well sealed uPVC windows perform better
There are widespread concerns about the actual performance of Australian homes that are supposed to be achieving minimum energy efficiency standards in the National Construction Code (NCC).
While 6 stars NatHERS energy rating* is the minimum requirement for design of new houses in Australia, how those houses actually perform is another matter, depending on the quality of the build, products and materials, compliance and crucially, how well sealed the building envelope is to retain thermal efficiency.
CSIRO completed a report in December 2015 for the Department of Industry, Innovation and Science (whhich administers NatHERS) investigating air-tightness, insulation and heating/cooling ductwork aspects of new house construction in capital cities around Australia. Apart from some older (10 years) houses, most of the houses surveyed were assumed to have been built within the last 3 years to the 6 star NatHERS standard.
A key finding of the report was that houses with uPVC window frames recorded much lower air change rates than most other houses.
Physically measuring Air Changes per Hour at 50 Pascals pressure (ACH@50Pa) at the houses, a wide range of air tightness results were found by the CSIRO researchers with an average of 15.4 ACH@50Pa. Almost half the houses were above 15 ACH@50Pa suggesting they are not particularly well sealed from an energy efficiency perspective. About a third were less than 10 ACH@50Pa, with the top performing house which had a specific design objective for air tightness, at 1.4. There’s clearly great room for improvement for the bulk of Australian housing.
The report suggests that the quality of window sealing may be one of the factors having an impact on air tightness. Many uPVC window systems have built-in sealing systems so that there is a tight seal between the window frame and wall. Houses with uPVC windows surveyed by CSIRO were found, with one exception, to have lower air change rates than most other houses.
The report authors concluded that consideration should be given to setting specific air tightness requirements in the NCC (next due for revision in 2019), recommending a value of no more than 10 ACH@50Pa be adopted.
Furthermore, they suggested that NatHERS allow incorporation of certified air pressure results into NatHERS calculations, so that high performing houses like those using uPVC windows, can receive higher star ratings. At the moment a 6 star NatHERS rating at design stage is no guarantee the house will be energy efficient once built.
Read the full report: House Energy Efficiency Inspections Project.
* NatHERS: Nationwide House Energy Rating Scheme