BRANZ: Reducing new-home emissions

By Dr Casimir MacGregor, Roman Jaques, and Dr Dave Dowdell, BRANZ

Designers, new homeowners and renovators can play a role in helping to reduce New Zealand’s greenhouse gas emissions. All it takes is critical thinking and taking some pertinent actions.

The Climate Change Commission’s recent report Ināia tonu nei: A low emissions future for Aotearoa noted, ‘the transition [to a zero-carbon future] can begin in earnest. The technology and tools needed to get there exist today – Aotearoa does not need to rely on future technologies.’

Reducing greenhouse gas emissions from the construction and occupation of our buildings will be a critically important step if we are to reduce the impact of climate change. As the Climate Change Commission report mentions, there are many things we can do now to help reduce our environmental impact in relation to how we design, construct and live in our buildings.

Reducing carbon emissions is imperative

Decarbonising our built environment is an important step for Aotearoa New Zealand if we are to operate within our carbon budget. Recent research by BRANZ and Massey University – Application of absolute sustainability assessment to New Zealand residential dwellings – estimates the impact of greenhouse gas emissions of the total stock from 2018–2050 to be around 170 million tonnes of CO₂ equivalent (MtCO₂eq).

The CO₂ equivalent metric provides a way to quantify the potential climate change impact of different greenhouse gases according to their potential to provide additional radiative forcing in the atmosphere, relative to carbon dioxide.

However, to ensure that global average temperatures do not rise above 1.5°C compared to pre-industrial levels, research shows total emissions from New Zealand residential buildings should be no more than 47 MtCO₂eq.

This means the total greenhouse gas emissions from our new and existing residential buildings from 2018 to 2050 are currently 3.5 times more than necessary. Substantial efforts are needed to reduce these by approximately 72% if our residential stock is to operate within a threshold consistent with achieving no more than a 1.5°C warming.

Six actions to focus on

There are six key areas to focus on to reduce the carbon footprint of a house (see Figure above). They are especially pertinent for those planning a new dwelling or considering a deep refurbishment.

1. Use low-carbon materials

Construction materials can have an enduring impact on the environment, from extraction/manufacture, ongoing maintenance and final disposal. For many materials, a large proportion of their emissions have been released before construction begins. Here are some ways to reduce the embodied carbon footprint of a house:

• Consider whether a material is needed – for example, keep suspended timber floors exposed rather than cover with carpets. If using a higher embodied carbon material such as concrete, consider whether it can serve dual purposes such as both a flooring and a thermal store for moderating indoor temperatures. When using concrete, specify concretes made with secondary cementitious materials (SCMs).

• Reuse materials or use materials with recycled content as much as possible.

• Use the right materials for what is needed. Where there is a choice, use lower embodied carbon materials.

• Minimise waste.

Decisions around materials usually involve balancing different factors. For example, adding more thermal insulation to walls and ceilings increases the material used in construction, but the extra carbon cost is low and can lead to reduced energy use and greenhouse gas emissions. See BRANZ free tools:

• BRANZ CO2NSTRUCT – dataset of embodied carbon of different materials. Visit www.branz.co.nz/co2nstruct.

• LCAQuick – estimates whole-of-life embodied carbon – including manufacture of materials, transport and installation, maintenance and replacement and end of life – and operational carbon from energy and water use. Visit www.branz.co.nz/lcaquick.

• NZGBC’s materials carbon calculator for Homestar v5 that BRANZ developed. It provides a free, user-friendly way to assess lifetime embodied energy for the main elements of a dwelling. See www.nzgbc.org.nz.

2. Design for thermal efficiency

If a house is well designed and built, little if any space heating or cooling is needed in many regions of New Zealand. This can significantly reduce the amount of energy required to heat and cool the home and therefore reduces the carbon footprint. For more, see www.level.org.nz.

There are several strategies that will help reduce a new house’s carbon footprint. One of the most important is to reduce house size and have a more compact design.

A smaller house means less materials, less maintenance and less space to heat and should be cheaper to build. Also use higher levels of insulation and ensure the house has a good orientation to make use of the sun and effective use of thermal mass.

When designing a new house, it is also important to be careful with window placement and size. Use high-quality double or triple glazing that is properly fitted and installed and have appropriate external shading.

3. Ensure energy efficiency

Energy consumption is estimated to contribute almost half of greenhouse gas emissions from new houses in 2018–2050. Even though New Zealand uses renewable energy sources for most of its electricity generation, the production of electricity still produces emissions from, for example, fossil fuels contributing to the grid and fugitive emissions from geothermal plants.

Key areas to address if you are seeking to reduce your house’s carbon footprint include energy-efficient appliances, lighting, energy efficient water heating and energy-efficient space heating.

4. Ensure water efficiency

Water consumption has a greenhouse gas impact from the supply, removal and treatment of water. Using water more efficiently is critical to preserving water supplies as well as mitigating climate change.

Use practices and technologies that cut water use including using a shower rose with a WELS star rating of 4 or more. Reducing shower times to 5 minutes can also help.

5. Minimise waste

Construction and demolition waste makes up 40–50% of New Zealand’s total waste going to landfill and cleanfill. Considerable amounts of greenhouses gases have been emitted in the manufacture and transport of this enormous volume of construction material. Reducing waste will reduce emissions. See REBRI, www.branz.co.nz/rebri.

6. Encourage zero-carbon living

To help reduce the impact on climate change, we need to introduce changes into how we live our lives. Changing what we do is often challenging.

There are a few easy steps that can be taken that will support a healthier home with a lower carbon footprint:

• Ventilate the house well.

• Don’t overheat a home – keep occupied spaces at a comfortable temperature of within 18–25°C.

• Make sure a home is well insulated to a level that is appropriate for the climate zone it is built in, and ensure the insulation is installed well.

• Only buy energy-efficient appliances. For more see www.energy-rating.gov.au/calculator.

• Consider if you need a second fridge, and if you do, ensure it is energy efficient.

• Shift to an all-electric household as old appliances wear out.

• Use hot water efficiently.

• Collect rainwater and use it to water the garden.

• Hang clothes outside to dry.

• Reduce food waste.

• Use green waste in the garden.

• Before renovating or building a new home, talk to an Eco Design Advisor, see www.ecodesignadvisor.org.nz.

First published August 2021 in build magazine, issue 185.