Overlooking St Clair beach, Dunedin’s popular surfing spot, this modern black pavilion is a sustainable certified passive house with a sunny outdoor living room that is cantilevered over a steep and challenging site.

Dissatisfied with the quality of many of the houses they found on the market in Dunedin, a family who had previously lived in a well-insulated, centrally heated home in the UK were eager to build a passive house that would be comfortable, healthy and energy efficient year round.

From the start of the design process, the architects faced a number of challenges that implicated each other. They needed to capture views over St Clair beach to the east, make the most of the solar gains from the north, and work around a steep site set close to the road. The sunniest part of the site, at the northern corner, was the most obvious place for outdoor living but it was also the most sensible place for cars to access the property and situated right below the street, leaving little privacy.

“It was a little tricky to make the design work,” explains Architype’s Tim Ross. “There is also a very tall house to the north which shaded the house for much of the day. We decided to raise the house as high on the site as possible to allow the main outdoor living space to look down on the street. We added movable shutters to create a private yet sunny outdoor living deck that captures the the sun and views throughout the day, without interference from cars accessing the property.”

Along with a cantilevered outdoor area that hangs out over the bush, the height of the foundations and retaining walls have been minimised to create a relatively economical solution on this very steep site. “With a passive house, ideally you want to keep the thermal envelope of the building reasonably compact. This doesn't mean you have to end up with a boring building form though. Often, you can play with those parts of the building that are outside the thermal envelope (garages, decks, etc) to articulate the building form and, from a thermal massing point of view, a compact form makes it easier to build and to heat,” says Tim. 

However, the overall result is a decent-sized four-bedroomed family home that nods to mid-century modern designs with its black pavilion form, exposed rafters, cantilevered terrace and low roof profile that draws in the western afternoon sun. The main living areas are located on the upper level, with a guest bedroom, service areas and garage downstairs. And, because street parking in the area is limited, extra parking has been added on the upper level.

To be certified as a Passive House, the building needed to have a maximum heating demand of 15kW per m2, per annum. This requires a clever balancing act during the design stage to keeping the house well-insulated and draft-proof, with quiet mechanical ventilation drawing in fresh air and expelling warm, stale air to create an even temperature of 20-22 degrees celsius all-year-round.

When considering building a passive home, potential homeowners are often confused about the idea of ‘airtightness’, which makes the house sound like it will be stuffy. But the principal is to basically to eliminate the leaks and gaps around a house, where warm air escapes, and to control the rate of ventilation to ensure a warm and comfortable home. “It’s not a hermetically sealed box – you can still open the windows on a nice day,” says Tim.

“The system is more about being draft-proof rather than airtight,” says Tim “If you have a house that leaks air, when the wind is blowing outside, then you can’t control the leaks. With a passive home, you actually prevent any leaks. The warm air is expelled and the heat recovery ventilation system essentially jumps the heat over into the incoming fresh air to create an even temperature. But, it’s not like a ducted heat pump system that moves huge volumes of air; with this system, you can’t hear it or feel it.”

“We do our own testing for draftproofness,” explains Tim. “During the build, once the envelope is completed, we attach a large fan to the front door and pressurise and depressurise the house to 50Pa and, then, monitor any leaks so the builder can address them. It’s much easier to do this on a cold day because you can run your hand around the junctions of materials or services penetrations and feel the jets of cold air on your hand where leaks are present.” To create the draft-free building envelope, the architects used a special membrane and tapes, along with triple glazing and thermally broken windows.

To model the energy performance of the house, the architects used the Passive House Planning Package, which runs as an Excel spreadsheet and is able to accurately predict the energy consumption and comfort of the house depending on the design, materials and product specifications you enter. It factors in the performance data, such as occupied floor areas, insulation, window sizes, types of glazing, thermal bridging and so on.

“It’s a great tool,” says Tim. “As you work through the process, it tells you if you will achieve the standard and is the backbone to the entire certification process. It is a great quality-control process and ensure that you can't fudge your numbers. The builder also has to photograph key aspects of their work throughout the build process and sign a declaration, which is a legal document, that helps protect the integrity of the certification.”

In many countries, passive house certification is becoming more and more popular because this scientific building process, which originates from Germany, ensures the health and comfort of the tenants, as well as reducing the energy costs of a home. Overseas, the data suggests that passive houses can sell for five to ten per cent more than a regular house, justifying the extra investment from the start.

“For the owners of St Clair House, they were mostly looking for comfort and quality,” explains Tim. “There is a level of comfort that you can achieve with a Passive House that you just can’t achieve with a code house, no matter how much heat you pump into it. Here, the temperatures are very even with no cold spots, which can result in condensation and mould.”

In the future, low-energy houses like St Clair House will have to become the norm in New Zealand, as our electricity supplies are leaned on more for infrastructure and industry and, also, to meet our national zero-carbon targets. “New homes like this are relatively easy to design to high energy standards, whereas completely retro-fitting houses is more challenging” says Tim. “We cannot continue to build homes that place huge demands on our winter electricity supply. In the summer, we are okay but, in the winter, we will struggle to decarbonise our grid if we don’t reduce energy consumption from our buildings.”