DECARBONISING BUILDINGS 101: Backgrounder to building decarbonisation

In an emerging field like decarbonising construction, many concepts are unfamiliar, and seemingly simple ones can prove more complex than expected. Here we try to demystify some basics.

Terminology

“Greenhouse gas” GHG.

A gas that allows short wavelength radiant heat to pass (e.g. from the sun) but resists long wavelength heat reradiating to space from Earth (so contributing to global overheating). See CO2e for examples of gases.

“Carbon” and its derivatives:

“Carbon”: an element forming, with oxygen, the gas carbon dioxide, “carbon” is used loosely in global warming terms, including often referring to carbon dioxide. A notable exception is regarding carbon stored in timber (see “Counting carbon in timber products”).

“Carbon dioxide,” CO2: a major, long-lived greenhouse gas, the product of much combustion (especially fossil fuels). Its “global warming potential” (GWP) of 1 is used for comparison between greenhouse gases (see “CO2e”).

“CO2e”: several gases contribute to global warming, with widely varying impacts. To compare and combine these impacts, the GWP of GHG may be expressed as “Carbon dioxide equivalent,” or CO2e.

Major gases and their 100-year GWPs include carbon dioxide (1), methane (27), nitrous oxide (273), hydrofluorocarbon R-32 refrigerant (675), sulphur hexafluoride (in high voltage equipment - 23,500) – and no, that’s not a typo: releasing just 1 kg of SF6 has almost the climate impact of two people flying around the world.

“Decarbonise”: refers to actions to reduce the climate impacts of greenhouse gases, through reducing emissions (e.g. minimising high-emissions products like steel and concrete), or removing GHG from the atmosphere, notably by sustainable forestry, and maximising long-term carbon storage in timber products.

Embodied and operational emissions:

Note: Clearcut calculates embodied but not operational emissions.

It is common to separate emissions associated with creating an object from those due to operating it. A battery electric car has higher embodied emissions than a similar fossil fuel car, but much lower operational emissions. Over their lives the total emissions from an EV are thus dramatically lower than for a fossil fuelled car.

Embodied emissions come from the manufacture, transport, and installation of components, plus “end of life” processes. Materials that take a lot of energy to produce typically have high embodied emissions (e.g. steel or aluminium). Concrete is unusual as over half of its emissions come from the chemistry of its manufacture.

“If the cement industry were a country, it would be the third largest emitter in the world.” ( carbonbrief ).

Embodied emissions aren’t all during construction: a small portion come from repair, maintenance, and demolition.

Operational emissions are caused by processes needed to run a building. Energy is the main cause, but water supply and wastewater treatment also create emissions.

Operational emissions traditionally make up 2/3 of New Zealand’s building-related emissions, but as energy supplies decarbonise and buildings become more energy efficient, embodied carbon will become relatively more important.

MBIE explains its strategic approach to managing operational emissions in “Transforming operational efficiency” at MBIE/operational, and will regulate embodied emissions via its “Whole of life embodied carbon assessment technical methodology”, currently in draft form at  MBIE/embodied .

Assessing embodied carbon: methodology, LCA, modules, and data sources.

Calculating emissions associated with any product is complex, and methods vary across industries. The construction sector, creating large, complex, bespoke structures, has a methodology of its own founded in European standards covering wide environmental impacts, of which greenhouse gas emissions are just part.

These standards underpin the process of “life cycle assessment”, LCA, and BRANZ have a useful explainer at BRANZ/BU608-LCA . This includes the well-known diagram below, showing the stages and modules used as a basis of the assessment (also used in MBIE’s methodology, excluding B6 and B7, in the red box, which are operational emissions).

Tools like Clearcut need access to robust product data for their calculations.

The development of such data can be expensive, and MBIE’s methodology outlines a “data hierarchy” which prioritises well-validated data while not precluding less robust information when only that is available.

Their first preference is data from an “Environmental Product Declaration”, EPD, These are prepared by skilled practitioners following well-defined processes, and may be for, say, modules A1-A3 (before delivery to a construction site). A raft of other data thus may be needed to complete an assessment, and there is currently (November 2023) discussion about establishing a related national database to promote consistency across all users.