How Passive House is working to close the performance gap
Willow Aliento | 13 April 2017
For the average homebuyer, it’s difficult to know whether a home with a 7 Star NatHERS rating is actually performing like one.
A new online tool from the Australian Passive House Association aims to close this knowledge gap and address the “performance gap” between design intent and built outcomes.
APHA chair Darren O’Dea says the standard Australian design and construction approach is delivering poorly performing buildings.
APHA looked at the federal government’s 7 Star NatHERS home designs in the Design for Place Guidelines, which aim to show that achieving a 7-7.5 Star home is possible, with floorplates and elevations provided to show how it can be done.
APHA members modelled the energy use and thermal comfort of the designs, finding that the homes were unlikely to perform as predicted.
O’Dea says there are concerns that the guidelines say single glazing is fine anywhere across Australia, and that “crappy aluminium” frames are fine in most climate zones.
From a building physics point of view, he says the 7 Star designs “simply won’t perform”.
Even homes designed for high NatHERS ratings are often using “more energy than they should”, no matter which tool the designer has applied, O’Dea says.
He recommends looking at “real world values” for building envelope elements, including walls, floors, windows, roofing, air tightness and thermal breaks.
One of the NatHERS designs was selected and the verified 7 Star specifications modelled for eight climate zones using the Passive House Planning Package (PHPP) V9.
O’Dea says around 33,000 calculations had to be run, and code and software developed to complete the tool.
The result is an online resource that shows the difference between designing for 7 Star NatHERS and designing for different degrees of Passive House.
Passive House for the masses
The user interface encompasses both the building fabric itself, and the quality of the building fabric with options ranging from minimum – 7 Star compliance – to excellent. It also shows how steps towards the Passive House standard will influence both thermal comfort and annual energy bills.
O’Dea says the building industry has become “skewed towards a measure of stars”.
This sees builders promoting 6 Star NatHERS homes as a bonus for buyers, rather than recognition that it is minimum compliance. It also means 6 Stars is where the majority aim.
O’Dea says being able to view how incremental improvements in the construction standard will improve outcomes gives consumers the knowledge to question a builder.
How it works
There are two levels of information delivered.
Users can see the simple outcome in terms of energy saved, increases in thermal comfort and utility bill reductions.
For example, a home in Melbourne where the windows are improved to “excellent”, the air tightness improved to 10 air changes per hour and the thermal bridging improved a little will likely, compared to 7 Star NatHERS, result in a 68 per cent energy saving, a 38 per cent increase in occupant comfort and a energy bill saving of around $3051 a year based on a 25 cent a kilowatt-hour tariff.
That’s without doing anything to improve the insulation in walls, floor or roof above the 7 Star specifications.
Then there is a more detailed second level of information that gives the engineering data in terms of factors such as air changes per hour, R values and percentage of hours annually occupants will experience temperatures over 25°C.
In Sydney, Canberra, Hobart, Adelaide, Perth and Brisbane the tool shows that incremental improvements deliver major savings and substantially increase comfort.
The one city where the tool does not show there are demonstrable and substantial benefits in terms of comfort from upgrading elements in line with degrees of Passive House is Darwin.
Improvements can yield energy savings, but do not vary the comfort level as substantially as they do for the other seven climate zones.
The tropics have their own challenges
Director of Brisbane consultancy Ecolateral John Moynihan, who is deputy chair of APHA, says the reason Darwin is different is due to the very tropical climate.
The NatHERS system, for a start, makes allowances for the climate by allowing four times higher energy use for 6 Star homes compared to the requirement for a home in Brisbane, due to the absolute need for mechanical cooling.
The PHPP software that informs the tool is, by comparison, more specific to the house itself, rather than the climate, Moynihan says.
Because the PHPP program was developed in Central Europe for cool and Mediterranean climates, it shows better results for Australian cities within those types of zones, such as Melbourne and Hobart.
Applying it to Darwin means the results appear to show a “no-win” situation, Moynihan says.
Humidity levels are part of the issue.
“From a Passive House point of view humidity is a very hard thing to try and control.”
To get water out of the air, mechanical cooling will cool the air down to six or seven degrees so the water condenses out, then the system will raise the air temperature back up to 23 degrees before sending in into the home.
With Passive House, however, the whole idea is not to use mechanical HVAC, and the systems generally used, such as heat recovery ventilation, cannot get large amounts of water out of the air.
To transfer water out, a home in Darwin looking to achieve Passive House would need to use an energy recovery ventilation system, Moynihan says.
“Then you start to get closer to an airconditioned environment.”
Other Passive House approaches such as double-glazing also show less beneficial impact in subtropical climates such as Brisbane when compared to a place like Canberra.
How to educate your builder
Moynihan says builders will generally talk about the cost of improving any building element.
The tool gives buyers information they can use to counter this minimum compliance approach.
“Education is the biggest missing link,” he says.
“The tool says that if [a buyer] gets medium quality or even really good work, it will pay off.”
Moynihan says it also shows that if the once-off investment of, say, $5000 results in savings of just $1000 a year, that still adds up to $20,000 over the average home’s 20-year lifespan.
“We are trying to open up people’s minds to the fact the result is the sum of all the individual parts.
“NatHERS has done a fabulous job, and as much as it could, but we could go beyond [just] compliance. We just need to look at the building elements.”
Why star ratings are not the only crucial element for energy bills
The focus on star ratings over recent years has led the construction industry to accept them as the “be all and end all”, Moynihan says.
But in terms of energy use, for a home in Queensland, for example, the HVAC only represents about 30 to 35 per cent of energy use.
In terms of NatHERS, when builders – and buyers – are talking about star ratings they are really only talking about heating and cooling costs, Moynihan says.
That’s why many buyers find that even a 8, 9 or 10 Star home can have bills about as high as they were in a previous, lower rated home.
Not only is a change in the building fabric not always enough to make a difference and reduce loads, 60 per cent or more of a bill is due to lifestyle factors, for example having three TVs, two fridges and lights left on constantly.
“It’s a bit like, ‘I have a very energy efficient radio in my car – but it still has an eight litre engine,’” Moynihan says.
- Access the Performance Gap tool