Cradle-to-gate or cradle-to-cradle? How to navigate the complexities of calculating embodied carbon
As the operational efficiency of buildings improves, the relative proportion of embodied carbon within the total emissions is rising. As a result, embodied carbon is coming under greater scrutiny as part of the drive to meet government targets for reducing CO2 emissions by 80% by 2050.
Know your definitions
Embodied carbon has become synonymous with the term carbon footprint. This refers to the lifecycle greenhouse gas emissions during manufacture and transport of construction materials and components, plus the construction process and end-of-life aspects of the building. It is expressed as carbon dioxide equivalents – CO2e – and is separate to operational carbon, which is the CO2 emitted during a building’s operational phase, such as from heating, cooling, ventilation and lighting. A building’s total emissions are a combination of embodied and operational carbon.
It is important to be clear on popular jargon such as cradle-to-cradle – also sometimes referred to as cradle-to-grave – which includes end of life stages of demolition and recycling. Unlike these, cradle-to-gate only covers extraction and manufacturing processes.
Compare like with like
Lifecycle assessment (LCA) should be used to determine the embodied carbon impact of construction products. Preferably, it should be cradle-to-cradle and follow all the lifecycle stages set out in BS EN15804. However, some manufacturers’ data only considers impacts from the extraction and manufacturing process and not end of life aspects as well. The significant difference between these two measurements for most materials means it is important that any comparative analysis uses like for like information to avoid flawed conclusions.
To ensure data is creditable and robust, designers should check with manufacturers how their data has been derived and, if not explicitly stated, whether it includes all lifecycle stages.
Most embodied carbon impacts are measured using rates of kgCO2e/kg. However, direct material comparisons should be avoided, as different materials aren’t used in the same quantities to deliver the same performance. Instead, a kgCO2e/m2 should be used for the various options to reflect the different amounts used when built.
Make use of new end of life datasets
A major issue when calculating embodied carbon is that data has been more widely available in cradle-to-gate rather than cradle-to-cradle formats. As a result, even those who have wanted to consider the latter have often been unable to do so. Where manufacturers do not give end of life data, designers can now plug the gap by using new embodied carbon data for commonly-used framing materials produced by PE International, a strategic consultancy specialising in sustainability. This data set was overseen by Jane Anderson, lead author of the BRE’s Green Guides to Specification. The table below is an extract from the PE International data, covering demolition and recycling impacts for common construction materials (BS EN15804 modules C and D) and includes robust comparative data for extraction and manufacturing lifecycle stages too.
Calculate embodied carbon footprints online
Tata Steel and the BCSA have developed an online tool to assist in estimating the embodied carbon footprint for a multi-storey superstructure as part of a new guide on the subject. The tool can auto generate a CO2e figure using algorithms developed by the Steel Construction Institute, or can be used with manual inputs specific to the designer’s building. This manual option also enables a comparison to be made between the impact of a steel and concrete framed building.
The carbon emissions rates used in the tool can alternatively be incorporated into the designer’s own spreadsheet.
John Dowling is sustainability manager of the British Constructional Steelwork Association and author of the Tata Steel and British Constructional Steelwork Association’s new guide to calculating embodied carbon.
Steel Construction: Embodied Carbon is available to download at www.steelconstruction.info
An extended version of the table below is also available online.
Case studies
Steel Construction: Embodied Carbon includes an analysis of the embodied carbon impacts of four case study buildings, calculating comparative data for different framing options. Data includes cost and programme analysis and was produced independently either by Gardiner & Theobald, Peter Brett Associates and Mace or AECOM, Sweett Group and the Steel Construction Institute.
In all cases, standard steel framed buildings outperformed standard concrete framed buildings. For Building 1 – a typical business park office building with a gross internal area of 3,200m2 – embodied carbon for the total structure was 180kgCO2e/m² for steel composite compared with 267kgCO2e/m², 268kgCO2e/m² and 328kgCO2e/m² respectively for post-tensioned flat slab, steel precast and concrete flat slab frames. For Building 2, a typical 16,500m2 city centre office building, a composite steel frame option had around 11% less embodied carbon than the post-tensioned frame.
At the 10 storey office building of One Kingdom Street in London, the total building impact of the post-tensioned concrete option was 12% greater than the composite steel option.
Cradle-to-cradle research data for the 17 storey Holiday Inn hotel and office tower at MediaCityUK in Salford found that the total building impact of the concrete flat slab option was 18% greater than the composite steel option.
Full details of the relative embodied carbon and cost comparisons can be found in Steel Construction: Embodied Carbon.
pecifics, mindsets, and a holistic approach
Architects specialising in sustainability discuss some of the issues they encounter when calculating embodied carbon.
Stewart Dodd, managing director, Satellite Architects
Embodied energy is a minefield. There are so many bits of data out there that contradict each other. The major problem is that it’s so subjective you rarely end up with any true information. BRE databases can give you a broad brush idea but you really need an analysis of your particular building to get the true picture, and that’s where the cost comes in.
Duncan Baker-Brown, director, BBM Sustainable Design
You can’t analyse embodied carbon without looking at things holistically. Architects and specifiers need to understand the relative embodied carbon values of the most commonly used materials in principle, but then it’s all about how they detail and construct a building so that any components that might have high embodied carbon can be easily re-used.
Anna Woodeson, head of sustainability, Wilkinson Eyre
As an industry we’ve just about cracked calculating embodied energy, although it’s clear there’s no one single approach. What’s interesting is how we then use that information to adapt our designs. We’ve started analysing embodied carbon in detail on some of our projects and as soon as you start questioning it, you begin to get a different mindset and start to pare back and simplify the design and the use of materials.