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by Dr Jez Weston, Senior Policy Analyst, Royal Society of New Zealand

As someone who works in climate change policy, I am acutely aware of the need to reduce greenhouse gas emissions. I’d even say that we are beginning to realise that emissions of long-lived greenhouse gases need to be cut to zero to avoid the worst possible outcomes from climate change.

Equally, as someone who pays attention to local research, I find it shocking that New Zealand’s cold and damp houses are estimated to kill over a thousand people per year through the ill-health that they cause.

Thus, with my partner Andrea Knox, we found ourselves wanting something very simple — a warm and healthy house with low lifetime greenhouse gas emissions. Sadly, we couldn’t find a house for sale in Wellington that met these criteria and so we ended up taking the long and hard route to building a place for ourselves that met our own standards. We’re finally there and in this article I’d like to talk more about the decisions we made and why we ended up with what we did.

Firstly, the house is small, or rather, we have the all the space that we need. Bigger means more cost and environmental impact to build and more to run (and more to clean). At 150 square metres, the house measures only 75% of the average house being built today. Despite that, we can hang from a trapeze in our living room — but more about that later.

credit: Jez Weston

credit: Jez Weston

Heating accounts for about a third of the energy demand from a typical house. However, in Wellington it is actually feasible to build a house that barely needs additional heating input. We chose to use passive solar design, with almost all of the windows facing north to gather free heat from the sun. Of course, that heat input varies throughout the day, week, and season, so we have plenty of thermal mass (thick internal concrete internal walls and a heavy and exposed earth floor) to soak up and store that heat. Double glazing, blinds, and plenty of insulation keep heat in the house.

In the summer, when the sun is high in the sky, the position of windows and eaves keeps much of the sun out and avoids overheating; in the winter the sun reaches deep into the house for maximum heat gain. Should there be a prolonged cloudy winter period, there is a wood-burner for backup.

credit: Jez Weston

credit: Jez Weston

Similarly, water heating makes up another third of the energy use of a house – solar hot water heating covers hot water, with a large tank for plenty of storage and gas backup if needed.

Combine solar heating with good natural light, energy efficient bulbs, and an efficient fridge and our use of electricity has become very small. It is dominated by the energy used for cooking, to the point where we can tell from our electricity bill the nights when we ate in town, rather than cooking at home.

We look at the house as part of our lives, to support active and low-carbon lifestyles. We did not want to build in a location that required us to use a car. Commuting by car is a colossal waste of personal time and money, so the land we found for the house is close to public transport and within cycling distance of work, with bicycle storage just inside the front door. As we are both physical performers (as well as being Wellington policy wonks), the lounge space is six metres high, so we can rig trapezes and fabric to train and develop performances without having to regularly travel to use facilities elsewhere.

credit: Jez Weston

credit: Jez Weston

The energy used by a house isn’t just the energy used for living. There is also the energy tied up in making the house itself, called embodied energy. Some materials, such as wood, require very little energy to create; others, such as concrete and steel, are far more energy intensive. We wanted to use as little embodied energy as possible over the life of the house so we used as much wood as possible for the structure of the house. However, because of our location right next to one of Wellington’s fault lines, we had to include a large amount of concrete and reinforcing steel for the foundations and lower parts of the house. While this increases the embodied energy needed to build the house, we are building for the long term and, if that extra concrete and steel means the house can survive the earthquakes expected in Wellington over the next few hundred years, then that extra embodied energy is a good investment in durability.

For the most environmental bang-for-the-buck, we’d point to the insulation. NZ houses remain chronically under-insulated, resulting in huge heating bills for those who can afford heating and sickness for those who cannot. Insulation is a cheap, permanent and passive investment in keeping a house warmer and the recent evaluation of the Government’s home insulation scheme by Philippa Howden-Chapman of the NZ Centre for Sustainable Cities shows that the main benefit of upgrading insulation is in improving people’s health.

In terms of the least environmental benefit per dollar (or hour of labour), we would point to the green roof or the bottle wall. A roof covered in plants is beautiful, durable, provides thermal insulation and mass, replaces the natural habitat lost to the footprint of the house, and makes obvious our intent to build a greener house. However in terms of low greenhouse gas emissions, we could have just added thicker ceiling insulation for much lower cost. Similarly, the bottle wall by the stairs reuses glass bottles that have a very high embodied energy and allows natural light to penetrate deep into the house, but really we did it because it looks beautiful.

credit: Jez Weston

credit: Jez Weston

As with any project in the real world, it is rarely enough to just optimise for low greenhouse gas emissions. There are always multiple goals to achieve and with this house we feel we’ve gone a long way toward demonstrating that houses in New Zealand can be designed and built to be healthy for both people and the planet.

“Our builder was Cliff Glass, from Envirobuild. His vast experience, creative problem-solving, and helpful guidance made this project possible.”