Improving consistency in whole life carbon assessments

Whole life carbon assessments (WLCAs) of buildings are already used globally to direct investments, make design decisions and report company emissions, and are increasingly being adopted into regulation in some regions.

Notably, the revised Energy Performance of Buildings Directive requires EU member states to mandate WLCAs for construction activities and establish limits on carbon emissions. 

Therefore, comparability between assessments is fundamental if WLCA results are to support these functions. However, it's a known issue that WLCA comparison is fraught with unreliability due to the different ways that assessments can be conducted and reported.

It is often said that two individuals assessing the same project can easily come up with different results. This inconsistency undermines the efforts to decarbonise the built environment based on robust evidence, and risks hindering the adoption of WLCA across the industry.

Assessment alone is not sufficient to drive decarbonisation, but it is essential to this goal because you can't manage what you don't measure.

It is important, therefore, to understand the causes of inconsistency in WLCA results and how they may be addressed.

This article explains this inconsistency by looking into its potential sources across the assessment process, with the aim of helping decision-makers better understand this technical matter.

This distinction is not necessarily a difficult one to make, but it's useful to unpack what could drive a variation in results.

Several WLCA methodologies are used globally, with differences in terms of scope, level of detail, and degree of prescriptiveness.

• The international standard ISO 21931-2 is the broad framework for sustainability and environmental assessment of buildings.
• EN 15978 is the European standard that operationalises the principles of ISO 21931-2 and defines life cycle modules and general calculation rules. 

Both standards apply to all types of environmental impact, not just carbon emissions, and do not provide detailed rules.

Therefore, more specific methodologies have been developed by public organisations and professional bodies with the aim of improving methodological consistency and reporting transparency, such as France's RE2020 regulation and RICS' Whole life carbon assessment (WLCA) for the built environment professional standard.

If our hypothetical project is assessed with different methodologies, the results can be expected to vary significantly, even for the same project, due to factors such as the following.

• Different scopes: if one methodology excludes external works and another includes them, total emissions resulting from the second methodology are bound to be higher than those resulting from the first one.
• Different calculations: methodologies may apply different weighting or uncertainty factors to WLCA results – or no factors at all – with final emission results appearing different even if the underlying raw figures are the same.
• Different accounting and reporting structures: methodologies require aggregating or splitting emission figures into various categories, e.g. life cycle stages and building elements, so even if total carbon emissions are the same, they may appear different due to how they are accounted for. For example, some methodologies require biogenic carbon, e.g. emissions absorbed by timber, to be reported separately from other emissions. 

Clearly, any decision based on results produced with different methodologies will be unreliable unless it has been informed by a detailed understanding of the divergences produced.

Even when the same methodology has been used, results may still diverge due to that methodology's degree of prescriptiveness, as different assessors can make different choices based on their judgement.

Because of this, assessment results are more comparable, and therefore more useful, when complemented by clear information on all methodological choices that the assessors have made.

These differences could include the following.

• The values and assumptions used to model a building's activities and features, such as the frequency of material replacement during its life cycle, or the parameters used to predict energy use. Many of these values and assumptions are prescribed by the more specific WLCA methodologies to improve consistency, but at times they may be superseded by project-specific data.
• Data relating to quantities associated with project activities and materials, e.g. kilometres of material transport, kilowatt-hours of energy used or kilograms of cement used in construction. The source of information for these variables may change depending on the stage at which the project is assessed.

– The quantities of materials used are usually not known at the early design stage, so estimates or proxies from similar projects can be used. As such, results may diverge if different estimation methodologies or benchmarks are employed.

– At the late design stage, quantities should be derived from design documents and more precise estimates, so the specific methodologies used to quantify materials and activities can affect the resulting figures.

– At post-completion and during operations, quantities of materials and energy should be based on measurement, so divergence in results may be attributed to differences in the measurement methodology and its execution.

• The carbon factors used to convert quantities into resulting emissions, such as the kilograms of carbon released to generate and deliver 1 kilowatt-hour of energy use, or to transport 1 ton of material over 1km. Assessors access these factors via technical databases and are expected to select the most appropriate based on the project stage and how representative the carbon factors are of the quantified materials and activities. This involves professional judgement as well as the availability of appropriate data; therefore, it can greatly affect WLCA assessment results when two assessors make different choices and/or have access to different data sources. Broadly speaking, generic carbon factors that are representative of typical conditions should be used in the early design stages, as the details of products and energy sources are not yet known. As the design stage progresses and eventually the project moves to post-completion and the use stage, carbon factors that are representative of the materials and activities specific to the project should be used as more precise information becomes available. At any stage, when a precise match between materials and activities and carbon factors is not available, the assessors are left with a degree of freedom in their choice of data, which can produce divergent results.

Assessors can improve the consistency and usability of their results by applying methodologies correctly, aligning with established conventions on input values and maximising transparency in reporting.

By questioning the underlying methodologies and inputs, decision-makers can more confidently assess whether to trust the results of WLCAs. They can also help drive consistency by requesting assessments to be conducted with specific standards and default inputs.

Standard-setting organisations also have a role to play in ensuring that their methodologies are applied correctly. RICS members conducting WLCAs must follow RICS' WLCA standard, and RICS runs a software validation programme to verify that assessment tools implement RICS' WLCA standard accurately. 

More broadly, industry collaboration can drive consistency in WLCA at all levels. From a global perspective, different methodologies exist and will continue to do so, but their differences are not irreconcilable.

Frameworks designed to convert and harmonise assessments from different sources can improve comparability, while coordination between standard-setting organisations can foster progressive convergence between different methodologies.

The CLEAR initiative, founded by RICS, the Global Building Data Initiative and the World Business Council for Sustainable Development bring together major industry stakeholders for exactly this purpose.

In order to continue this trend towards alignment, each methodology can be further developed to reduce the degree of choice left to assessors, when it is justifiable to do so.

This entails refining the rules set by the methodologies, but also complementing them with sets of default values, assumptions, proxies and generic data to enable assessors to rely on standardised inputs when project-specific information is not available or applicable.

Initiatives such as Whole Life Carbon Conventions for New Homes by the Future Homes Hub and the Rail Infrastructure Carbon Conventions by the Rail Safety and Standards Board are positive steps in this direction, as they seek to build on RICS' WLCA standard to provide the UK construction sector with default values and generic data tailored to their specific sectors, such as housing and railways.

As discussed, fostering consistency entails establishing detailed rules and narrowing down the degree of choice left to assessors. However, assessments must reflect their own purpose and context, and the interpretation of results is a key aspect of the process.

The practice of WLCA is a technical skill that cannot be reduced to a mechanical process. Even with fully aligned methodologies and inputs, there is always potential for misunderstanding the rules and making genuine mistakes.

This is why the expertise of assessors is crucial, because they are ultimately responsible for interpreting requirements, applying professional judgement where guidance is not explicit, identifying anomalies in the data, and ensuring that results are transparent and technically robust.