Though common in Northern Europe, the Passive House standard is still uncommon in Australia.
To date, just two Passive Houses are listed in the Passive House Database: a partly underground house in South Australia, designed by architect Max Pritchard, and a modernist house in Victoria by McCabe Architects. One of the five Passive Houses in New Zealand, designed by Jessop Architects, was a finalist in the 2014 International Passive House Awards.
Why have so few Passive Houses been built in Australia, and is the standard appropriate for a warm climate, such as Australia’s?
The PassivHaus Institute (PHI) in Darmstadt, Germany, says the standard works anywhere and everywhere, but the Passive House Institute US, based in Chicago, acknowledges that the standard must be adjusted for different climates. The two groups certify the Passive House standard, and were affiliated until they cut ties over this disagreement.
It is important to ask why to bother with the standard at all. The Passive House standard is all about energy efficiency; it does not address the sustainability of the structure, such as the materials used. It’s rigorous but the structure will use 80 to 90 per cent less energy than a standard building. The standard addresses only three things, sometimes called the “three hurdles”:
- Maximum heating or cooling energy up to 15 kWh/m2 per year.
- Maximum total source energy of 120 kWh/m2 per year. (“Source energy” by definition includes the energy required to produce and deliver the energy to the site, and can be offset with solar thermal and other measures. Photovoltaics cannot be used to offset this energy, but are recognized, at this time.)
- Maximum air leakage equivalent to 0.6 air changes per hour at 50 Pascals (ACH50), (~0.03 ACHNAT).
Here are the design principles typically used to meet the standard:
- Superinsulate the building envelope
- Eliminate thermal bridging
- Make it airtight
- Install an energy- or heat-recovery ventilator
- Use high-performance windows and doors
- Optimize passive solar where appropriate
- Model energy gains and losses using the Passive House Planning Package (PHPP), a software tool.
More than 3100 Passive House buildings have been certified worldwide. The majority are located in Europe, and the standard is well understood and effective in that climate. In the US, with its more varied climate, more than 100 certified projects have been built, but most are located in the cooler areas of the country.
A notable exception is the humid eastern US, where architect and builder Adam Cohen has now built several residential and commercial Passive House structures. Cohen has also achieved cost parity with standard building methods, and a home he designed and built in Thaxton, Virginia, received the GreenBuilder Home of the Year Award for 2013 in the Best Mainstream Green category. That home cost about $150/square foot, which is nearly the same as a standard home in the US, on average. The secret, Cohen said, was handling all aspects of the home, including design, construction, commissioning, and monitoring.
The PHIUS group, however, has been working since 2011 on revising the standard for different climates. Partnering with Building Science Corporation, they’ve studied buildings in different areas of North America, and recently released a draft report titled Climate-Specific Passive Building Standards, with recommendations for adjusting the standard. The recommendations aim to establish a global model for building efficiency guided by cost-effectiveness and building science.
In the report, authors Betsy Pettit, FAIA, G. Wright (PHIUS), and K. Klingenberg (PHIUS) note that after certifying many homes in the U.S. to the Passive House standard, “By 2011 it became clear that the space-conditioning criteria needed some climate-dependent adjustment, if the standard was to deliver on the promise of deep energy savings cost-optimally (or at least cost-competitively).”
In designing buildings to meet the standard, the architects used the Passive House Planning Package (PHPP) to calculate the design requirements of the house, such as thickness of insulation. Many homes ended up over-insulated and over-cost.
“PHIUS+ certification that uses European energy metrics and specific standards as written has resulted in (broadly speaking) passive-solar-esque designs with a tendency to overheating, and discouragingly high cost premiums,” the report states. “Adjustments to the criteria are necessary to redeem the promises of the passive building standard for North America.”
The recommendations for adapting the standard affect all three areas and are technical, but the focus on the three hurdles remains. As the report says, “The designer’s attention is directed first to reducing heating and cooling energy use by passive means (including some mechanical devices,) then to reducing total energy demand by efficient equipment (and some renewables,) and finally to net zero by more renewable generation.”
The PassivHaus Institute, in contrast, has not changed the Passive House standard, but has issued guidelines for homes in tropical climates, including:
- Protecting the home from solar loads, such as by using fixed shading devices for windows.
- Using reflective, cool colours.
- Low-e solar windows with high selectivity.
- Moderate levels of insulation.
For Australia, time will tell which approach architects and builders choose for their Passive House projects. Cohen has shown what can be done under the current standard, but the PHIUS report demonstrates that the group has substantial data to support their contention that building standards must be regionally optimised.