Sequestered carbon in timber: reporting it honestly
A whole life carbon assessment showing a negative embodied carbon total has usually dropped the end of life release. Here is how to report timber sequestration so a reviewer can verify it.
Report biogenic carbon separately, and never net it off the headline embodied carbon figure. That single rule settles most of the arguments about biogenic carbon timber reporting in a whole life carbon assessment. The carbon a tree absorbed while growing is recorded as a negative entry in the product stage, it comes back out at end of life, and across a cradle to grave boundary the two entries cancel. A assessment that shows a timber building with a negative embodied carbon total has almost always dropped the second entry.
Planning officers and their advisers now spot this. So do the reviewers appointed by authorities with mandatory whole life carbon policies.
Why sequestration is reported on its own line
The European standards that govern the calculation treat biogenic carbon as a flow rather than a saving. Carbon taken up by the growing tree enters the inventory when the product is made, and leaves it when the product is burned, decays or is landfilled. The standard requires both halves to be reported so the inventory balances.
Presenting only the uptake half produces a figure that is not wrong so much as incomplete, and incomplete in the direction that flatters the scheme. That is why the current industry methodology for whole life carbon assessment asks for sequestration to be shown as a separate, clearly labelled line rather than folded into the total.
There is a second reason. Sequestration is not a mitigation in the same sense as using less steel. The carbon is stored temporarily, and the storage period depends on decisions made decades from now by people who are not in the room.
The product stage treatment
In module A1 to A3, biogenic carbon in the timber product is entered as a negative value equal to the carbon content of the wood. It depends on the mass of timber, its species and its moisture content, which is why a credible assessment states all three rather than applying a generic factor.
Two practical points for anyone assembling the model:
- Use product-specific environmental product declarations where they exist, and state the declaration reference. Generic datasets vary widely for engineered timber.
- Keep the biogenic and fossil components of module A1 to A3 separate inside the model, and not merely in the way the results are presented. Combining them early makes the end of life balancing entry close to impossible to reconcile later.
Transport and manufacturing emissions for engineered timber are not trivial, and glulam and cross-laminated products carry more processing energy than sawn timber. The sequestration line does not excuse them.
The end of life release that balances it
Whatever is recorded as uptake has to reappear in the end of life modules. How it reappears depends on the assumed fate: incineration with energy recovery releases it as carbon dioxide in module C, landfilling releases part of it over time including a methane fraction, and reuse or recycling moves the burden into the next product system.
That last route is reported in module D, outside the cradle to grave total. Module D is where a genuine advantage for timber can appear, and it is also where assessments become unverifiable, because it rests on an assumption about a market that may not exist in eighty years.
State the assumed end of life scenario explicitly. An assessment that assumes full reuse without saying so is making the strongest possible claim silently.
Sourcing evidence and chain of custody
A sequestration entry is only defensible if the timber came from a forest that is being replanted. The standards make sustainable sourcing a condition of claiming the uptake at all, which in practice means certification through a recognised forest certification scheme and a chain of custody that follows the material to the site.
Record the certificate references in the assessment. Where the procurement route is not yet fixed at planning stage, say so and flag it as an assumption that a condition or a later submission will need to close. Authorities with whole life carbon policies increasingly ask for a post-construction assessment, and an unsupported sourcing assumption is exactly what that stage will test.
Presenting a net figure without overstating it
The clearest presentation separates four numbers: fossil embodied carbon for modules A to C, the biogenic uptake entry, the biogenic release entry, and module D shown outside the total. A reader can then see the effect of the timber without being told a conclusion.
Avoid the two common distortions. Headline figures quoted as negative. And comparisons against a concrete or steel alternative that has not been modelled to the same boundary, with the same service life and the same end of life assumptions. If you are making a substitution argument, model both options properly, which is the same discipline described in modern methods of construction and the carbon case.
A whole life carbon assessment is a report councils expect a suitably qualified author to prepare, and professional review before submission is strongly advised where a policy target has to be met.
How reviewers test a timber carbon claim
The questions come in a predictable order, and they are easy to pass if the model was built cleanly.
| Question | What the reviewer is looking for |
|---|---|
| What mass of timber, at what moisture content? | A stated quantity that ties to the structural drawings |
| Which environmental product declarations were used? | Product-specific references, not generic averages |
| Where is the module C release entry? | A positive entry that mirrors the uptake |
| What end of life scenario is assumed? | An explicit statement, not an implied one |
| Is module D inside or outside the headline? | Outside, and labelled |
| What is the sourcing evidence? | Certification and chain of custody, or a flagged assumption |
If any of those cannot be answered from the submitted document, expect a request for further information rather than a refusal. The delay is usually four to six weeks, which matters more on a committee timetable than the work itself does.
Design decisions that change the answer
Sequestration accounting rewards quantity of timber. Good design rewards something else: using timber where it displaces a high-carbon material, detailing it so it can be taken apart, and protecting it so the service life is long.
Three decisions move the number more than the accounting does. Whether the frame is timber or the frame is concrete with timber infill. Whether connections are bolted and demountable or glued and permanent. Whether the building is designed for a 60 year or a 120 year service life, because the replacement cycles in module B are often larger than the whole product stage.
The infrastructure a scheme has to build sits outside the building model and is frequently forgotten, and carbon in the infrastructure a scheme has to build explains why that omission distorts comparisons. Where the authority also asks for a broader sustainability narrative, the sustainability statement is the place to set the carbon strategy in context rather than repeating the calculation.
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Frequently asked questions
Can a timber building have negative embodied carbon?
Not across a cradle to grave boundary. The biogenic uptake recorded in the product stage is matched by a release in the end of life modules, so the two cancel. A negative headline figure nearly always means the release entry has been omitted. Report the uptake and the release on separate, clearly labelled lines.
Where does biogenic carbon sit in the modules?
The uptake is entered as a negative value in modules A1 to A3, based on the mass, species and moisture content of the timber. The release appears in module C, with the exact treatment depending on whether incineration, landfill or recycling is assumed. Benefits from reuse in a later product system belong in module D, outside the headline total.
Do I need certification to claim sequestration?
In effect, yes. The standards make sustainable sourcing a condition of claiming biogenic uptake, which means certification through a recognised forest scheme and a chain of custody that follows the material to site. Where procurement is not fixed at planning stage, record it as an assumption that a later submission will need to close.
Should module D be included in the reported total?
No. Module D sits outside the cradle to grave boundary and should be shown separately and labelled. It is where a real advantage for demountable timber can appear, and it is also the least verifiable part of the assessment because it depends on an assumed future market for reclaimed material.
What do reviewers check first?
The timber quantity and moisture content against the structural drawings, whether product-specific environmental product declarations were used, whether a module C release entry mirrors the uptake, what end of life scenario is assumed, and whether module D is inside or outside the headline. Missing answers usually bring a request for further information rather than a refusal.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.