What is net zero carbon?

24 December 2020

Architectural Copyright FCBSStudios


Achieving net zero carbon in our designs means balancing operational carbon, energy generation from renewables, embodied carbon and carbon sequestration on every project.

It means having a light touch on the planet and making decisions that go beyond the specific projects we are working on today. We aim to achieve net zero operational carbon in all our projects on site by 2025.

Operational Energy

We follow the UKGBC definition of zero operational energy. “When the amount of carbon emissions associated with the building’s operational energy on an annual basis is zero or negative. A net zero carbon building is highly energy-efficient and powered from on-site and/or off-site renewable energy sources, with any remaining carbon balance offset.”  (ref.1)

Operational carbon relates to the in use emissions of a building, from the lighting, heating, cooling and appliances – our fridges, cookers, fans, computers etc. FCBStudios have long been focussed on delivering low energy buildings, with particular attention to closing the performance gap between the expected energy use and the actual use once the building is in operation.

We follow a fabric first approach, lowering the energy demands of the building through insulation, airtightness and an optimised glazing ratio to achieve low U-values. These approaches will have benefits that outlast any new technology installed. Efficient electric heating and hot water (using ground or air source heat pump) make the most of a rapidly decarbonising electrical grid.

On site renewable energy generation can reduce the net operational energy, but a reduction in energy demand should be prioritised over adding renewables to generate energy.

Embodied Carbon

UKGBC define net zero embodied carbon as "When the amount of carbon emissions associated with a building’s product and construction stages up to practical completion is zero or negative, through the use of offsets or the net export of on-site renewable energy."

Through the Carbon Counts exhibition, its related events and FCBS Carbon we have explored the relative carbon impact of materials and provided a tool to give an indicative carbon calculation for early-stage projects. These provide the best possible start for a low or zero carbon buildings, but this momentum needs to be continued throughout the life of the project.

To truly understand the embodied carbon of a building, it should be assessed using a robust framework. British Standard BS EN 15978:2011 (fig 2) is the definitive summary of a building's emissions throughout its life cycle. Across the modules, it looks at: cradle to gate production (A1 - A3); transport to site and construction (A4-A5); operation, including replacement and refurbishment (B1-B5); and end of life (C1-C4). Module D deals with the future of the materials in the building: the potential for their recycling or recovery.

Data is available from many sources, but the best sources are the ICE database from Circular Ecology (Ref.2), and specific Environmental Product Declarations (EPDs) available direct from manufacturers. EPDs will always be more accurate than the ICE database, and should be used as soon as the specific product is known. There are currently only a limited number in the UK, but the more we ask for EPDs as an industry, the more will be made available by manufacturers.

Embodied carbon reporting is often split into two sections: cradle to handover (modules A1-5), and whole life embodied carbon (assuming a 60-year life and including modules A1-B5, & C1-4)This promotes thinking about not only the immediate emissions from creating a building, but also around longevity and maintainability.

Using any construction material emits some carbon. Some natural building materials sequester carbon as they grow, locking in carbon from the atmosphere, which can often be more than emitted through their use. These materials can be treated as zero carbon, but only when the end-of-life emissions are factored in.

An embodied carbon calculation takes into account the ‘cradle to grave’ emissions of all the materials that go into the construction of the building. This can also include sequestration from natural materials and the benefits of the circular economy. Seen as a whole, this gives the net embodied carbon.

Whole Life Carbon

For us, this embraces 60 years of operational and embodied carbon emissions, including the construction, replacement and maintenance cycles and what happens to the building at the end of its life.

In a project that starts off zero embodied carbon on occupation, and continues to be net zero in its operational carbon, with low impact maintenance and replacement cycles, then it is possible that it could continue to be net zero carbon throughout its life. These projects we call regenerative.

In order to achieve net zero carbon, the detail matters. However, if we adhere to six main principles in our projects, we will reach a starting point for calculating the carbon emissions that is closer to zero.

Build less, build light, build wise, build low carbon, build for the future and build collaboratively. (ref. 3)

Dr Joe Jack Williams

Joe is a Partner at FCBStudios and alongside Ian Taylor leads our Research and Innovation. He sits on the LETI Steering Group, is part of core research groups with Architects Declare and CIBSE and is part of a network of sustainability professionals in the UK Architecture industry. With Joe Taylor, he has developed FCBS Carbon.

A note on Zero carbon vs Net zero carbon

At FCBStudios, we talk about net zero carbon, rather than zero carbon, and this is important. Zero operational carbon refers to the generation of enough energy on-site over the course of the year to power the building. Net zero operational carbon is connected to the grid, so can feed energy to the grid during times of oversupply, and draw energy when the supply is lower (eg, in winter when the demand for energy is higher, but the generation of solar energy is lower). Net zero carbon is much easier to achieve.


UKGBC : Net zero carbon buildings, a framework definition  ICE Database available from: https://circularecology.com/ LETI embodied carbon primer

Further reading: RIBA whole life carbon assessment for architectsRICS whole life carbon assessment for the built environment


Top: Croft Gardens for Kings College Cambridge is designed to Passivhaus Standards with a 100 year design life

1. British Standard for Embodied Carbon

2. University of Staffordshire Woodland Nursery\, Net zero carbon. (c) Daniel Hopkinson

3. Croft Gardens for Kings College Cambridge (c) James Newton

4. University of Staffordshire Woodland Nursery\, Net zero carbon. (c) Daniel Hopkinson