Passive Design

Designing the building and the spaces within it to benefit from natural light, ventilation and even temperatures.

A carbon budget for New Zealand houses

What volume of emissions can a house be responsible for while the country moves towards its 2050 net-zero carbon goal? The “carbon budget” project calculated this figure – and found that typical new-built detached houses today exceed the carbon budget by several multiples.

New Zealand has set in law a target to reduce net emissions of greenhouse gases (except methane from plants and animals) to zero by 2050.

The building industry will need to be part of the change. Starting with stand-alone houses, we can calculate a “carbon budget” to ensure that any emissions are consistent with working towards the 2050 goal.

Part of the calculation is around emissions from the building operations (space and water heating and so on) and part comes from materials-related emissions.

Scientists from BRANZ and Massey University developed a method for calculating a carbon budget. It takes a top-down approach that assigns a share of the global carbon budget for 2018–2050 to a country, then to its construction sector, then to each life cycle stage of a building. The typical house was taken as 198 m2 gross floor area (from Stats NZ data).

The life cycle stages assessed were:

  • product – embodied emissions of materials
  • construction process – emissions from materials transportation, assembly and energy for construction machinery
  • maintenance – emissions from activities such as repainting
  • replacement – emissions from replacing materials
  • operational energy use – emissions from heating, cooking, water heating and so on
  • operational water use – emissions from pumping/treating water
  • end-of-life – emissions from demolition activities

Emissions were modelled over a 90-year service life. The house was simulated to maintain indoor temperatures of 18–25°C.

The carbon budget

The budget represents the level of emissions that are “allowable” for the transition period before we need to be net zero-carbon. For the target of limiting global temperature rises to 1.5°C above pre-industrial levels – the target referred to in New Zealand law – the carbon budget for the life cycle of our new houses is 35 tonnes CO2 equivalent.

To see how this compares to houses being built today, BRANZ researchers calculated the carbon footprints of 10 real New Zealand houses. Some have been designed to just comply with New Zealand Building Code requirements and some designed to exceed it. Carbon footprints were calculated with the BRANZ tool LCAQuick. Each house was simulated to maintain indoor temperatures of 18–25°C in Auckland, Wellington and Christchurch. The total carbon footprints obtained were then averaged.

There is a wide gap between what we are building and what we need to be building – we are substantially “over budget” in greenhouse gas emissions.

Figure 1 Total global warming potential (tonnes CO2 equivalent) over 90 years calculated for 10 stand-alone houses.

 

Higher-performance houses may have lower space heating needs but they are not necessarily low-carbon houses – they may have significant energy demand for plug-in appliances and hot water.

Material selection and house design

Energy used in operating our houses accounts for the biggest contribution to greenhouse gas emissions over the 90-year service life of a house. However, measuring emissions just up to 2050, the role of materials increases significantly, going from just over one-quarter to be closer to one-half.

 

Figure 2 The contribution of materials, energy use and water consumption to a new house’s greenhouse gas emissions over different periods of time.

 

Using timber has a big positive impact because it captures and stores atmospheric carbon dioxide while it is growing. When one of the new-built houses was credited for the biogenic carbon (such as timber) in its materials, the excess of actual emissions over the carbon budget for product stage emissions dropped by about half, and in the construction process by one quarter.

The carbon budget figures will change as input data is updated.

 

Updated: 28 June 2020