Air leakage through an unsealed building envelope accounts for 15−25 per cent of winter heat loss in Australian buildings according to the federal government’s Your Home Guide, yet there is no Building Codes of Australia (BCA) Standard for thermal breaches or airtightness of a house.
This troubles Bernward Bücheler of Passivhaus Australia who now lays claim to having built Australia’s first certified PassivHaus, a single storey partly underground residence in South Australia designed by Max Pritchard Architects.
PassivHaus is a German-born building performance standard that strives to achieve a very comfortable ultra-low energy building that requires little or no energy for space heating or cooling. This is achieved through a tightly sealed and insulated building envelope, and use of a whole house mechanical ventilation system.
“The three most important areas in Passivhaus construction are insulation, airtightness and a thermal breach free envelope,” explained Bücheler.
“There is no standard for thermal breaches or airtightness of a house in Australia but a 2013 CSIRO report sees 15 air exchanges per hour under 50 Pa pressure as acceptable average.”
This rate of air exchanges is nowhere near the acceptable rate for any project wishing to achieve Passivhaus certification, and the Passivhaus project at Pages Flat, South Australia achieved a miniscule 0.46 per hour at 50 Pa air exchange which is under the maximum 0.6 needed for PassivHaus certification.
‘Pages Flat House’ was designed by Max Pritchard Architects. The customer then contracted Passivhaus Australia to adapt the design to suit PassivHaus standards. Bucheler notes that a key provision of the brief was not to sacrifice any detail of the design proposed by Pritchard:
“Our brief was to change the design to Passivhaus standard without changing any of the visible features; no sacrifice of the style was allowed at all,” he emphasised.
“There is always a solution for any style of building, it might not always be the cheapest way to get the intended result, but it is certainly achievable as this project demonstrates.”
The house is a simple looking single storey home that sits on a 195sqm slab, slightly countersunk into the hill behind. The property is not connected to mains water and has 300,000 Litres of rainwater storage as well as 10 Kw electro voltaic power generation.
Material choice, application and convincing local engineers to accept his thermal breach-free and insulation proposals was not so simple however, and Bücheler said that he had to source products from overseas because Australian suppliers either didn’t stock them, or were more expensive than overseas competition.
“We always prefer to use Australian materials and products for our projects,” he said.
“But if necessary products are not available locally, or the quality to price relationship is noticeably superior overseas, we have to give our customer this option.”
“We think that Australian materials and products will only improve if a market is opened for new products through imports and comparisons of price and quality are possible.”
MATERIALS AND COMPOSITION
The slab caused the first hiccup for Bücheler when preparing his DA because Australian Standards classify insulation under foundations as a void, meaning the slab had to be strengthened and additional reinforcements added if he was to follow PassivHaus convention and insulate its underside. Passivhaus Australia chose to use hard foam Styrodur insulation anyway, and admits it did affect costs but not the design or insulation outcome; it achieved a U-value of 0.223.
All north facing walls of the home are heavily glazed with Interpane Iplus neutral E double glazed windows, imported from Germany. Also from Germany are the PURAL Eco fully thermally broken Aluminum frames that house the windows and the heavily insulated Hueck glass lift sliding doors. All glazing is also well shaded with Warema wind stable outside metal venetians:
“In Australia we have to look more at possible overheating through unshaded windows whereas in central Europe the emphasis is towards getting as much as possible out of the rare sunshine,” said Bücheler.
“An Australian design will clearly differ from a central European design with much more shading of the structure through roof overhangs and/or outside metal venetians.”
The internal wall is a render coated concrete block which abuts a concrete filled cavity. Another concrete block layer starts the external skin on the other side of the cavity which is then then protected by a waterproofing membrane. Finally a 160 mm Rockwool render baseboard exterior skin is added before it can be rendered and painted.
The construction of the curved roof from the outside:
Corrugated iron; 60 mm screened air gap; 2 layers of waterproofing membrane; fibrocement sheeting to add a fire barrier between the next layer of plywood and the outside environment to meets the site’s fire zone requirements; This is followed by plywood on top of the rafters.
280mm Gutex Thermoflex, a flexible wood fibre insulating board from Germany is laid in-between the 300mm rafters on top of an 19mm OSB (Oriented Strand Board) fixed to the underside of the rafters as airtight layer. All joints of the board sealed with airtight tape and the ceiling is then plaster boarded. An overall U-value of 0.158 was achieved.
The final component of a PassivHaus is its whole house mechanical ventilation system with highly efficient heat recovery which supplies fresh, thermally treated air to the interiors. Bücheler chose the Zehnder, ComfoAir 550 with balanced PH ventilation which is manufactured in the Netherlands.
Besides striving to achieve a high-performing, energy efficient project, Bücheler also noted that his choice of materials for the building was guided by Building Biology principles which saw formaldehyde free OSB, only solid timber and natural zero or low VOC materials used throughout.
Because PassivHaus is a building standard not a guide, the project’s certification means it has already proven its performance on some measures. Others such as heating demand and annual energy requirements are calculated according to the Passive House Planning Package (PHPP).
Here are its performance credentials:
Air tightness: n50 = 0.46/h press test value
Annual heating demand: 4 kWh /(m2a ) calculated according to PHPP
Heating load: 9 W/m2
Primary energy requirement: 120 kWh /(m2a ) on heating installation, domestic hot water, household electricity and auxiliary electricity calculated according to PHPP