Carbon budget for NZ housing

This is a part of the Net-zero carbon buildings feature

By Professor Sarah Mclaren - 1 February 2021, Build 182

Research shows New Zealand needs to massively reduce the carbon footprint of our residential housing stock to stay within the international climate target of 1.5°C warming by 2050.

ON 2 DECEMBER 2020, the New Zealand government declared a climate emergency along with 32 other countries. This is significant in steering the direction of future policy initiatives because it was accompanied by a commitment to make the government and public sector carbon neutral by 2025.

Achieving carbon neutrality requires us to measure our greenhouse gas (GHG) emissions and sinks, reduce the emissions as much as possible and offset the remaining emissions by investing in carbon sequestration activities, such as planting trees.

A related approach that has also gained traction recently is to calculate the so-called carbon budget available for different activities in the economy, such as electricity use, travel and diets.

These carbon budgets are usually derived from the amount of additional GHG emissions that can be released into the atmosphere up to 2050 and still avoid exceeding either a 1.5°C or 2.0°C global average temperature increase.

Carbon budgets for the sector

What does a carbon budget approach imply for the building and construction sector?

Dr David Dowdell, Principal Scientist Sustainability at BRANZ, has discussed the application of carbon budgets to residential buildings in New Zealand – see Build 176 Cutting carbon is a material issue and Build 177 Design to cut carbon – the time is now. These articles draw on collaborative research by Massey University and BRANZ over the last few years.

In this article, I discuss the research on application of carbon budgets to the entire residential sector in New Zealand for the period 2018 to 2050.

Carbon footprint of all housing stock up to 2050

Firstly, we calculated the carbon footprint of the entire stock of New Zealand residential dwellings between 2018 and 2050. This was done by projecting the total amount of pre-existing plus new-build dwellings over this period, accounting for demolition of old dwellings.

The carbon footprint was then calculated for the three main types of dwellings – detached, medium-density housing and apartments. This included construction, use and final demolition where relevant.

Using these values and accounting for variability in operational energy use in the different climate regions of New Zealand, the total carbon footprint of the residential housing stock up to 2050 was calculated. See Figure 1, which also shows how the different life cycle stages of the dwelling types and typologies contribute to the total carbon footprint:

Pre-existing residential dwellings contribute 63% of the total carbon footprint, whereas the new-build buildings contribute 37% of the impact.
By building typologies, the largest contributor of the total climate impact is detached houses (77%), followed by medium-density housing (14%) and then apartments (9%).
By individual life cycle stages, operational energy use is the largest contributor of the total carbon footprint of the residential buildings (59%), followed by the product stage (16%), i.e. the materials and products used in construction of the buildings. The third-largest contributor to the climate impact of the residential buildings is the maintenance and replacement stage (13%).

Figure 1: Carbon footprint of all New Zealand residential buildings up to the year 2050.

Exceeds climate target by 3.6 times

Secondly, we calculated the carbon budget for the 1.5°C global climate target for the total stock of New Zealand residential buildings up to 2050, calculating it to be 47 MtCO₂eq. As the stock’s climate impact is 170 MtCO₂eq for this time period, it is projected to exceed its climate target by a factor of 3.6.

In other words, New Zealand’s residential building stock needs to reduce its carbon footprint by 72% to perform within the 1.5°C global climate target.

Prioritise retrofitting and refurbishment

This suggests a step change is required in our efforts to reduce the carbon footprint of the residential sector – incremental improvements will not get us there.

These efforts need to prioritise retrofitting and refurbishment of pre-existing buildings to reduce operational energy use (from Figure 1).

For new-build dwellings, efforts should focus on both reducing operational energy use and utilising materials with low carbon footprints – biogenic carbon storage in construction materials is likely to be particularly relevant here.

Obviously, the numbers used in this analysis are approximate, and more data is needed to verify the results. However, overall, this type of approach has potential to contribute to future-proofing the building sector and supporting the commitments of a growing number of countries across the world to achieving net-zero carbon status between 2030 and 2050.

Note

This study was supported by BRANZ. The research was presented at the recent BEYOND2020 World Sustainable Built Environment Conference in November 2020 where it received Best Paper award. The conference paper is available at https://iopscience.iop.org/article/10.1088/1755-1315/588/2/022064/pdf.