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Passive Houses: Raising the bar on efficiency and comfort

Passive Houses: Raising the bar on efficiency and comfort

New Zealand homes have improved in the last decade: from cold, damp, mouldy and leaky homes, evolving to more comfortable, efficient and durable buildings. But there is still a lot of space for improvement. Leaky buildings 2.0 has been brought up in recent years, and more extreme climate events have reminded us of the need to be prepared for extreme temperature changes. Coupled with energy prices getting higher and higher, it is a good idea to rethink how to keep a house comfortable without relying only on electricity and thinking of the economical sustainability and the environmental impact of a building.

In this context, the Passive House standard started to be applied in New Zealand since 2012, with a growing number of houses getting certified since then. This building certification scheme focuses on low energy buildings with a high level of comfort. The idea of passive solar design is very old, but it has been taken to a higher level for the development of this standard, created in Germany in the 1990s. The idea was to construct very well insulated and airtight buildings that would require almost no active heating. Internal heat gains (lights, appliances and even heat from the occupants) and solar heat gains would be sufficient to heat the buildings, with a minimal requirement for active heating. A key to this model is a heat recovery ventilation system, which brings fresh air to the building, but uses the internal heat to warm up this external air – basically “recycling” the heat generated indoors. Another important principle is the absence of “thermal bridges” – there is a need for a continuous layer of insulation separating the inside to the outside. This is also very beneficial for protecting the structure of the house, since it stops condensation from occurring in those spots where you would have thermal bridges in traditional construction, improving the durability of these homes.

Throughout the years, the standard was tested and applied in many different countries and climates, built and verified in places such as Antarctica to Qatar. A rigorous post-occupancy evaluation was carried out to check that the concept actually works in reality, without just relying on assumptions. In the beginning, this concept faced some scepticism in NZ, as people didn’t like the idea of living in an airtight building, “sealed” and isolated from the outdoors. But there’s a lot of misunderstanding: in summer, a Passive House can be opened up and get natural ventilation without any issues. And in the winter months – when windows are usually closed in any house – it allows houses to be constantly ventilated without opening windows. So, in a Passive House, there would be constant fresh air provided by the heat recovery ventilation, without losing heat or getting cold drafts.

During my Master of Architecture in Sustainable Design, I wanted to research about the Passive Houses already built in NZ and check how they perform in reality – I was a little bit sceptical myself. I have installed sensors to monitor the indoor conditions in two of these houses during one year: one in Auckland and one in Whanganui. The results were amazing and made me believe that this is the future of house design and construction. Both houses have kept the internal temperatures between 19 and 26oC during the whole year. They are expected to maintain temperatures no lower than 20oC, but in this case, it was a choice of the occupants not to use the allowed heating. Relative humidity was well controlled and maintained between 30-70%, while the outside numbers were up to 100%. Homeowners were extremely happy living in these homes. In one of the houses, one of the kids used to suffer from pneumonia and chest infection in previous houses they lived – but had no symptoms since they moved to the Passive House.

I could say a lot more, but words and numbers really can’t explain the feeling of entering such a nice and comfortable place. When I first installed the sensors in the House in Auckland was the coldest day of that winter, there was frost on the grass and external temperature was around 5C in that morning. I entered this cosy house, and I instantly felt the difference compared to entering a room that is artificially warm because of a heater or heat pump – the Passive House one felt naturally warm. There were no cold rooms, no cold spots near the windows. The whole space felt like a nice summer day.

The downside? The upfront cost is higher. Rough calculations showed that in NZ the average payback time is 7 years when your savings on energy bills overcome the extra cost to build the house to this standard. But if analysing the whole lifecycle of the building, the house will have been more economical than a normal house. It is the same idea as buying LED lights – it costs more at the start, but after some time you are actually saving money. Slowly, the market should start to give value to this kind of building, as it has happened in other countries; hopefully, in the future there will be some subsidies to help with the extra upfront cost. Meanwhile, the solution for those who don’t have any extra budget is to build a smaller house – focusing on quality rather than quantity. PHINZ ( http://phinz.org.nz/) has the latest news and research about these homes in NZ.

At Woods, we are interested in taking designs to this higher level of comfort and efficiency, especially multi unit developments, where the extra cost might be lower, compared to stand alone houses.

Priscila Besen

Sustainability Specialist