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A certified passive house, this Dunedin home stays at a comfortable temperature year-round while needing very little energy to operate. Kim Dungey reports. 

Architect Rafe Maclean and his family have taken a while to adjust to the warm, dry conditions inside their Dunedin passive house.

Before moving in, they spent two years in a cold rental property, where the heat pumps were on year-round and they slept with several blankets on each bed.

In their newly built Maori Hill home, the indoor temperature is always 20degC-23degC and, thanks to the extremely airtight building envelope and the high-spec triple-glazed windows, insulation and heat recovery ventilation system, their energy bills are low.

The family have swapped woollen tops for T-shirts and shorts, and often sleep under only a sheet.

"Living in the house, we've become quite sensitive to the temperature," Maclean says, smiling. "If it's sitting at 20 degrees, everyone's thinking it's a bit chilly and putting on jerseys."

Two asthmatic members of the family who are sensitive to mould, damp and pollen have not had any problems since moving in and all of them enjoy not having to think about fresh air, temperature and humidity.

"Here it's all controlled and we have much more energy to do other things. We're not scrambling over a fireplace or chopping wood or adjusting heaters."

Developed in Germany and applying to all kinds of buildings, not just houses, the passive house standard results in homes that use about 90% less heating energy than existing buildings and 75% less than an average new build.

Maclean, who designed the South Island's first certified passive house in Wanaka in 2015, says it is not only focused on energy efficiency. It also produces indoor environments that are quiet, comfortable and have excellent air quality.

Kowhai House, named after a native tree on the site, is perched high on a hillside overlooking Leith Stream.

No-one else was brave enough to build on the steeply sloping plot, which drops away about 50m from the top and has a no-build 5m-wide council wastewater easement running through the only flat area. But where others saw only pitfalls, Maclean saw potential: the section faced northeast, it was near his daughters' high school and his 20-plus years as an architect had given him the skills to address the site's challenges.

Because of the difficult access and marginal soil, the three-bedroom home was designed to be simple in form and "buildability". The shape is a gabled rectangle but with one face of the gable roof sloped up from the ridgeline, not down, to provide internal space for mezzanine beds.

The simple form also makes the home more thermally efficient: compared with a more complex design with lots of corners, there is less envelope surface area through which heat can escape.

With 70sq m on each of the two floors (including walls), the home is compact but big enough for the four family members to live together and still have their own space.

The inter-floor structure is exposed to give more height to the space under it, with wastewater lines and ventilation ducting carefully concealed behind a partial floating ceiling aligned with interior cabinetry.

Zincalume corrugate on the exterior provides a crisp contrast to the green surroundings, while yellow highlights inside and out are a visual salute to the kowhai that flowers outside the living area in spring.

Interior finishes are warm and welcoming, and the extensive use of pine plywood includes a pale painted floor.

"I'd always wanted a white floor but forgot we had a black dog," he jokes.

Prefabricated structural insulated panels (SIPs from NZSIPS) provided good insulation and reduced the time that builders Stevenson and Williams were on site.

The mechanical heat recovery ventilation system, which supplies fresh, pre-warmed air, is housed in a small utility room.

In theory, the family needs a heater of just under 1kW to heat the house on a cold day. In fact, they have two panel heaters - one upstairs and one downstairs to spread warmth throughout the home - and these are 1kW because they couldn't find smaller ones, Maclean says: "Dunedin just doesn't sell them, it would seem".

In one of the coldest months last winter the house used 540kWh, which was mostly for hot water, computers and appliances, not solely heating. The annual heating demand is 15.4kWh per square metre; installing photovoltaic panels on the roof would have offset this but the panels would have been difficult to access for cleaning.

The use of interconnected spreadsheets allows the performance of passive houses to be accurately modelled before construction and is based on climate data for each location. In Wanaka, where he also works, Maclean would typically specify more sun shading and more insulation.

While the passive house standard is mostly a voluntary one, a growing number of European cities and districts are requiring that all new buildings meet it.

Maclean says because the buildings use much less energy, it is one way to achieve climate change targets: "I think eventually all new buildings will have to be passive house or something similar but it's just a matter of time and education ... It's pretty exciting but very glacial in take-up speed."

Designing a passive house does take longer, the builders have to take extra care and, because most of the details are different from other builds, gaining council consent can be a "little more difficult".

Some people mistakenly think that the occupants can't open windows and that the houses overheat. Others say the New Zealand Building Code is good enough and there is no need for change.

"There are detractors ... but my challenge to them is if you can come up with something better to reduce energy use and increase interior comfort levels, let me know."

The Government needs to realise the big role that buildings play in energy emissions,"from fireplaces and gas heating to thermally inefficient building envelopes", he adds. "Housing should be considered as `infrastructure' and not a commodity."

"I'd also like to see more architects stepping up and designing much better buildings than they are at the moment, in terms of performance."

Of the 2023 registered architects in New Zealand, Maclean is one of only about a dozen who are certified passive house designers.

While not everyone building will aim for full passive house certification - which requires checks by an independent third party - talking to a passive house designer, using some of the methodologies and doing research online will make their houses much more energy efficient.

"We've got to step out of this `build cheap, sell, build cheap again' mentality because it's very hard to change the buildings' construction once they're built and the next generation is inheriting those buildings.

"There is an extra cost to building a passive house but long term, it's an investment. It's good for your health, good for your energy bills and good for the next generation."

Points of difference 

Eight things that make this certified passive house different from a New Zealand Building Code-compliant house:

1. There is much more insulation. Structural insulated panels (SIPs) were used for the walls and roof and there is wool insulation under the floor.

2. Finding the right balance of letting solar heat in but not too much, was key, while also maintaining views and natural daylight.

3. Through careful detailing of construction junctions and pre-construction computer modelling, thermal bridges - where heat "bridges" or passes through construction - were identified and designed out. For example, a steel column inside an external wall can conduct heat through the building and, in turn, can cause a dew point surface to form and increase the risk of condensation.

4. The highly-airtight building envelope eliminates draughts and reduces heating demand. Airtight construction prevents air flowing through the construction of a building, including the seals around window and door openings. It reduces air (both warm and cold) passing through the building envelope, which reduces heat loss on cold days and lowers heat gain on hot days.

To verify airtightness levels, pressure tests are performed using a fan to pressurise or depressurise a building. The passive house requirement is less than 0.6 air changes per hour at an indoor/outdoor pressure difference of 50 Pascal (the equivalent of a 32kmh wind blowing against the building). The result at Rafe Maclean's house was 0.1% of its volume leaking out. "If you measured a villa, it would probably be 15 to 20 air changes and a new code-compliant house would be around seven or eight," he says.

5. Most new houses have aluminium-framed windows which conduct heat and cold. In this house, the windows (made by ThermaDura in Mosgiel), have low heat-conducting spruce frames and German triple-glazing while the front door is like a big airtight fridge door; 65mm thick, double-sealed and insulated.

6. A ventilation system with heat recovery extracts stale, moist air from the kitchen and bathrooms and supplies fresh air to other areas. The occupants can open windows without affecting the building's performance but don't have to open them to air out rooms. The unit transfers up to 90% of the heat from the outgoing air to the incoming air, without the fresh and used air ever mixing. It uses a maximum of 50 watts of power on boost function and has filters to remove pollen and pollutants such as coal dust, which Maclean says he can smell when he goes outside in winter. After a year of living in the house, he found the previously-white filter had turned black: "We filter our drinking water but we don't really think about the air too much, I guess because we don't see it."

With such a high airtightness result and low air volume inside the house, the interior is sensitive to overheating. This has been successfully mitigated by adjusting the ventilation rates and by occasional night time "flushing" through opening windows for short periods.

7. The clothes dryer and kitchen extractor fan are not ducted to the outside. Instead, the energy they create is kept inside the house. The kitchen fan has carbon filters to capture particulates and remove cooking smells.

8. A hot water heat pump is located under the house for low-energy hot water heating.