Design to cut carbon – the time is now

By - , Build 177

Part two of our series on the carbon impact of stand-alone houses looks at how to design houses to reduce their greenhouse gas emissions. This is crucial to helping New Zealand meet its environmental commitments.

Figure 1 Relationship between current stand-alone house size and carbon footprint.

IN BUILD 176, the article Cutting carbon is a material issue introduced the concept of a carbon budget for our new stand-alone houses. This represents an allowable greenhouse gas emission from stand-alone houses constructed before 2050.

New house carbon footprint too high

A new stand-alone house with cumulative greenhouse gas emissions over its life cycle within the calculated carbon budget will contribute to meeting New Zealand’s commitment enshrined in the Climate Change Response (Zero Carbon) Amendment Act. This commitment is to cut New Zealand’s greenhouse gas emissions to limit global average temperature rise to no more than 1.5°C above pre-industrial levels.

Based on Massey University and BRANZ research to date, to meet this commitment, the available carbon budget for each new stand-alone house is currently calculated to be 39 tonnes CO2eq.

In comparison, a typically sized stand-alone house – just less than 200 m2 gross floor area – designed to be just compliant with the New Zealand Building Code currently can have a carbon footprint almost seven times this carbon budget.

The time to act is now

There are implications from the current high carbon footprint for stand-alone houses and any delay reducing it:

  • By taking a business-as-usual approach to construction and operation of all our new houses in the coming years, our new standalone houses will exceed their allocated carbon allowance. This means that other New Zealand sectors such as transport and agriculture will need to do more to cut their greenhouse gas emissions to make up for this exceedance.
  • Developing and accelerating a retrofit programme for existing stand-alone houses so they use less energy (in real terms) means these greenhouse gas savings can compensate for the additional emissions from new builds. In implementing such a programme, low embodied carbon materials will need to be used, otherwise the emissions from the manufacture of the materials used will erode the benefit from the resulting energy savings.
  • Occupancy per stand-alone house is also important. Currently, the occupancy in stand-alone houses is 2.8 – based on Census 2013 data. The carbon budget can also be calculated on an occupancy basis rather than a floor area basis. On an occupancy basis, a family of five living in a 200 m2 house will have a larger carbon budget than a couple living in the same sized house. On a floor area basis, both would have the same carbon budget.
  • There is a time imperative for us to begin building stand-alone houses with less greenhouse gas emissions. The longer we delay, the bigger the challenge and the smaller the remaining carbon budget available for each newly constructed stand-alone house.

Ways to reduce greenhouse emissions of houses

How do we reduce the greenhouse gas emissions of our stand-alone houses? The following are some steps we can take.

1. Reduce house size

Based on QV data, the average house size in New Zealand has increased from 113 m² to 205 m² in the last 70 years. Larger houses require more materials, meaning more embodied carbon due to their manufacture, transport and installation, as well as ongoing maintenance. They will typically need more heating (and cooling) to maintain a comfortable temperature, meaning they use more energy (with associated greenhouse gas emissions).

Figure 1 illustrates the strong relationship between house size and carbon footprint for the 10 houses BRANZ has carbon footprinted – almost 82% of the carbon footprint of a stand-alone house can be attributed to the gross floor area.

Thus, reducing house size – while maintaining or increasing occupancy – appears to be an effective way of reducing the carbon footprint of our houses.

Figure 1 Relationship between current stand-alone house size and carbon footprint.

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2. Design for low carbon and calculate it

Designing a stand-alone house to be low carbon means considering carbon emissions from operational energy use (see 3 and 4 below) as well as the materials used in the design (see 5). Figure 1 includes six high-performance houses designed to (at least) exceed the minimum requirements in clause H1 Energy efficiency of the Building Code using, for example, passive solar design, minimising thermal bridging and increased insulation levels.

Despite incorporating features such as these, the high-performance houses still exceed the carbon budget. This may be because of the house size and/or choice of materials.

LCAQuick v3.4 – available at – can help to quantify the greenhouse gas emissions associated with a design and compare it to the carbon budget.

3. Reduce plug loads

Only buy energy-using appliances and devices that are really needed. When considering a purchase, choose the most energy-efficient appliances you can within your budget. For more information, refer to EECA’s EnergyWise programme

Ensure appliances and devices are turned off when not in use.

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4. Reduce energy needed for heating water

Greater use of hot water – for example, for showers and baths – means more energy is needed to heat the water, with associated greenhouse gas emissions. Limiting hot water use therefore saves energy in addition to water. BRANZ has just started a research project looking at innovative water heating technologies, and this will include consideration of their associated greenhouse gas emissions. More advice will be available at the end of this project.

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5. Reduce embodied carbon

Preferably use lower embodied carbon materials and minimise their wastage. BRANZ CO2NSTRUCT provides embodied carbon data for a range of materials, and this will be expanded in the future.

Industry initiatives are already under way to begin the process of decarbonising materials. For example, Concrete NZ is receiving Building Research Levy funding to develop a classification system for secondary cementitious materials (SCMs) to make their use in concrete easier and more predictable. SCMs help reduce the carbon footprint of concrete by reducing the amount of cement used.

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For more

LCAQuick v3.4 is available at

BRANZ CO2NSTRUCT is available at

Download the PDF

More articles about these topics

Articles are correct at the time of publication but may have since become outdated.

Figure 1 Relationship between current stand-alone house size and carbon footprint.