Reading the module breakdown in a whole life carbon assessment

A whole life carbon assessment reports its answer as a set of life cycle modules, and reading one properly means knowing which modules were included, which were excluded, and whether the figure on the front page is upfront carbon, embodied carbon or whole life carbon. Those three numbers are not interchangeable, and a submission that quotes one while labelling it as another is the most common reason an assessment gets sent back.

The module structure comes from the European standard for assessing the sustainability performance of buildings, and the RICS professional standard applies it to United Kingdom practice.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it.

Why carbon is split into modules

A building emits carbon at different times and for different reasons. Making the materials is one event. Transporting them is another. Replacing a roof covering in year thirty is a third. Demolishing the building and processing the waste is a fourth.

Lumping these together produces a single number that cannot be acted on. Splitting them into modules lets a design team see where the carbon actually sits, which is almost always in the structure and the substructure for a new build, and in replacement cycles for a long-lived building with short-lived finishes.

The split also makes assessments comparable. Two buildings reported as 800 kilogrammes of carbon dioxide equivalent per square metre are only comparable if both used the same modules, the same study period and the same scope of included elements.

Product and construction stages A1 to A5

Stage A covers everything up to practical completion.

A1 to A3 are usually reported together as the cradle to gate figure, because that is how environmental product declarations present it. A4 and A5 are site-specific and have to be calculated rather than taken from a database.

Upfront carbon means A1 to A5. It is the figure that matters most for a decision taken now, because those emissions happen during construction rather than being discounted across a sixty year study period. Planning policies that set a carbon target increasingly target upfront carbon for exactly that reason.

A5 deserves more attention than it gets. It splits into construction waste associated with the materials, and site activities such as plant, temporary works and welfare. Assessments that report A5 as a small percentage of A1 to A3 have often used a default rather than a calculation.

Use stage modules B1 to B7

Stage B covers the building in operation, over the study period.

B1 is use, covering emissions released by materials in place, such as carbonation of concrete, which is usually a small negative figure. B2 is maintenance. B3 is repair. B4 is replacement, which on most buildings is the dominant use stage module. B5 is refurbishment.

B6 and B7 are operational energy and operational water. These are the modules most often excluded from an embodied carbon assessment, and the exclusion needs to be stated, because a reader comparing a figure that includes B6 with one that does not is comparing nothing.

B4 is where the design decisions live. A facade system with a thirty year service life assessed over a sixty year study period is replaced once, which can double its contribution. Specifying a longer-lived material, or one that can be repaired rather than replaced, moves the number more than switching between two similar products at A1.

Service life assumptions therefore need stating element by element. An assessment that uses optimistic service lives without saying so produces a flattering B4 and a figure nobody can check.

End of life modules C1 to C4

Stage C covers what happens when the building reaches the end of the study period: C1 deconstruction and demolition, C2 transport of waste, C3 waste processing, and C4 disposal.

For most buildings stage C is a small proportion of the total, and it is modelled using current waste processing assumptions projected decades forward, which is an obvious source of uncertainty. That does not make it pointless. It makes the comparison between designs more useful than the absolute figure.

Whole life carbon means A, B and C together over the stated study period, normally sixty years for a building unless policy requires otherwise. Say which study period was used, because a figure calculated over a hundred and twenty years for infrastructure is not comparable with a sixty year building figure.

Module D and benefits beyond the boundary

Module D sits outside the system boundary and records benefits and loads beyond it: the value of materials recovered for reuse or recycling after the building is demolished, and energy exported from the site.

It is reported separately and must never be netted off the total. An assessment that subtracts module D from the whole life figure to show a lower number has misapplied the method, and it is the single thing most worth checking on a submitted report.

The reason for the separation is that module D records a benefit to a future system, realised by somebody else, decades away, under recycling assumptions that may not hold. It is useful for comparing designs on their recoverability. It is not a credit against the carbon the building emits now.

Treat a large module D figure as a prompt to look at design for disassembly rather than as an achievement, and read it alongside the pre-redevelopment audit content for material reuse, which deals with the same question at the start of a project rather than the end.

Benchmarks and what a good figure looks like

Benchmarks are published by professional bodies and by individual authorities, expressed in kilogrammes of carbon dioxide equivalent per square metre of gross internal area, usually for upfront carbon and separately for whole life carbon, and differentiated by building type.

Three cautions when using them:

• Check the scope. A benchmark covering substructure, superstructure and facade is not comparable with a figure that also includes fit-out and external works.
• Check the stage. A benchmark for upfront carbon cannot be compared with a whole life figure.
• Check the area basis. Gross internal area and net internal area give materially different results for the same building.

Where a local policy sets a target, read the policy's own definition rather than the benchmark's. Authorities differ on scope, and a scheme that meets a published industry benchmark can still miss a local target that counts more elements.

The useful discipline is to report the assessment at two points: an early design stage figure used to steer decisions, and a post-planning or as-built figure used to verify them. An assessment produced once, at submission, informs nothing.

Reporting consistently with the energy statement

The Whole Life Carbon Assessment and the Energy Statement are read together, and they contradict each other more often than they should.

The three recurring inconsistencies are the floor area used, the emissions factor applied to grid electricity, and whether operational carbon appears in both documents on the same basis. A carbon assessment using a decarbonising grid projection for B6 while the energy statement uses a fixed current factor will produce two different operational figures for the same building.

Fix it with a shared assumptions table: floor areas by measurement basis, study period, grid emissions factors and their source, service lives by element, and the scope of included elements. Reference it from both documents rather than restating it in each.

Where the scheme also carries a circular economy or sustainability submission, the same table should serve those too. Officers reading four documents that quote four different gross internal areas tend to ask about the areas rather than the carbon, which wastes a consultation round on something that was never the point. For the wider set of documents a scheme of this kind usually needs, the report index lists them by category.

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