Biogenic Carbon Accounting
Biogenic carbon accounting is the set of rules for counting the carbon a plant pulled from the air and stored in a building’s timber or bio-based material, and for deciding when that carbon must be counted back out.
Also known as: Biogenic Carbon Modeling; Sequestered-Carbon Accounting; Stored-Carbon Reporting
A tree spends decades pulling carbon dioxide from the air and locking it into wood. Cut the tree, mill it, bolt it into a building, and that carbon now sits in the frame instead of the atmosphere. Every timber or bio-based carbon claim runs into one question: does the building get to count that stored carbon as a benefit, and must it give the credit back when the building comes down? Two competent assessors can answer differently and reach different footprints for the same building. Biogenic carbon accounting is the rulebook that decides which answer an assessment may use.
Understand This First
- Whole-Life Carbon Assessment — the accounting boundary biogenic carbon lives inside.
- Embodied Carbon (vs Operational Carbon) — the carbon categories this one sits beside.
This entry describes an accounting concept and the standards that codify it. It isn’t engineering, legal, financial, or planning advice. A qualified assessor must set the method, boundary, time horizon, and reporting duties for a specific project.
What It Is
Biogenic carbon is the carbon held in material of recent biological origin: the wood in a glulam beam or a cross-laminated panel, the hemp shiv in a hempcrete wall, the straw in a panel, the mycelium in a composite block. While that material stays in service, its carbon stays out of the atmosphere. Biogenic carbon accounting is the convention for representing that storage, and its eventual release, inside a whole-life carbon assessment.
The methods practitioners actually meet fall into three families.
| Method | How it treats uptake and release | Net effect over a typical building boundary |
|---|---|---|
| Static “0/0” | Ignores biogenic storage entirely; counts neither the uptake nor the release. | Storage is invisible. The timber carries only its fossil processing emissions. |
| Static “-1/+1” | Credits carbon uptake at the product stage (A1), then charges the same carbon back at end of life (Module C or D). | Net storage over the period is zero for a building assumed to be demolished and not reused. |
| Dynamic (GWPbio, dynamic LCA) | Weights a tonne of stored carbon by how long it stays out of the atmosphere, relative to the forest rotation and the chosen time horizon. | A partial credit that grows with storage duration; rarely used in real-world building assessments. |
The “-1/+1” approach is the one embedded in the standards most assessments follow: PAS 2050, ISO 21930, ISO 14067, EN 15804, and the draft prEN 18027. It books the uptake as a negative at A1 and the release as an equal positive at the building’s end of life, so a building that is demolished and not reused shows zero net biogenic benefit across the boundary. The dynamic methods try to reward storage duration directly, on the logic that a hundred years of carbon held out of the atmosphere is worth something even if the carbon is eventually released. They remain rare in practice, partly because they demand assumptions the static methods avoid.
Underneath the method sits a reporting discipline that keeps the answer honest. Stored biogenic carbon is reported as its own line, separate from fossil carbon, and never netted off the headline figure. Beyond-boundary recovery benefits, the credit for wood that is reused or burned for energy after the building’s life, go in Module D, where they sit apart from the upfront emissions they must not silently cancel.
Why It Matters
Without these rules, a timber building can be made to show almost any carbon number its promoter wants. Credit the uptake and ignore the release, and the frame looks net-negative. Apply the release and the same frame looks neutral. Choose a dynamic method with a generous time horizon and it lands somewhere in between. The practitioner who can name which method an assessment used is the one who can tell a defensible claim from an engineered one.
The choice is not cosmetic. Peer-reviewed life-cycle work on timber-frame buildings shows that the modelling choice, static “-1/+1” versus dynamic, can decide whether a timber building beats its reinforced-concrete alternative at all. The same building, same materials, same forest, comes out ahead under one convention and behind under another. A structural engineer who knows their load path but not their biogenic method cannot say whether their CLT frame may legitimately book a net-negative figure. A developer’s ESG officer cannot separate a real store-and-reuse claim from a greenwashed one without it.
The static accounting exists to expose a timing problem. Carbon stored in a building is borrowed, not banked, unless the material is recovered and kept in use. The Institution of Structural Engineers’ guidance mandates store-and-release for whole-life assessments precisely because, once release is assumed, much of the sequestered carbon is lost again within a century. Treating that temporary storage as permanent is the most common way a bio-based carbon claim goes wrong.
How to Recognize It
Ask which biogenic method an assessment applied, and how it reported the result.
- Which convention? Static “0/0”, static “-1/+1”, or a dynamic method (GWPbio or dynamic LCA). The “-1/+1” form is the default in EN 15804, ISO 21930, ISO 14067, and prEN 18027; a net-negative timber headline usually means something other than store-and-release is in play.
- Is uptake balanced by release? Under store-and-release, the A1 credit is matched by an end-of-life charge in Module C or D. A credit with no matching charge is a red flag unless the material’s continued reuse is genuinely secured.
- Is the stored carbon a separate line? Sequestered biogenic carbon should be reported on its own, never blended into a single net headline. The RICS whole-life carbon standard requires this transparent, separate reporting for biogenic materials.
- Where do recovery benefits sit? Reuse, recycling, and energy-recovery credits belong in Module D, outside the A-to-C totals, so they cannot quietly offset upfront emissions.
- How is the stored quantity derived? For wood, EN 16485 and the EN 16449 equation derive stored carbon from the material’s carbon fraction, density, moisture content, and volume, rather than from a round-number claim.
EN 16485 sets the product category rules for round and sawn timber Environmental Product Declarations, requiring stored biogenic carbon to be quantified and reported separately from fossil carbon. EN 15804 carries the underlying “-1/+1” convention for construction-product declarations, and the draft prEN 18027 extends the same treatment.
How It Plays Out
A design team submits a mass-timber office and reports it as net-negative on upfront carbon. The number comes from crediting the wood’s biogenic uptake at A1 without any matching end-of-life charge. An assessor reapplies the store-and-release convention the relevant standards mandate, moves the release into Module C, and the headline rises to roughly neutral on the structural frame. The timber still avoids the embodied carbon of a steel or concrete alternative, which is a real benefit, but the “net-negative” claim was an artifact of an incomplete accounting, not a property of the building.
A developer weighs a CLT frame against a reinforced-concrete one and asks for the carbon case. The assessment runs both the static “-1/+1” method and a dynamic GWPbio variant. Under one, the timber option wins comfortably; under the other, the margin narrows or reverses. The useful output isn’t a single verdict but the visible sensitivity: the team learns that the timber advantage on this project depends on a contested modelling choice, and that the defensible claim is the conservative one.
A manufacturer prepares an Environmental Product Declaration for a hempcrete wall system. Following the product category rules, they quantify the stored biogenic carbon from the material’s carbon fraction and mass, report it as a distinct line, and apply the end-of-life release. The declaration lets a specifier see the genuine storage without mistaking it for a permanent offset, and lets the certification body credit it under whatever rule its scheme applies.
Caveats and Open Questions
The standards do not agree, and the field knows it. A critical overview of the LCA literature catalogs the divergence in how biogenic carbon is handled across methods and argues for harmonization that has not yet arrived. Until it does, two compliant assessments can reach different timber footprints honestly, each following a different sanctioned convention.
The dynamic methods are theoretically attractive and practically scarce. GWPbio and full dynamic LCA reward storage duration in a way the static methods cannot, but they require a time horizon, a forest-rotation assumption, and a discounting logic that the static “-1/+1” approach sidesteps. Whether the extra realism justifies the extra contestability is unsettled.
The deepest open question is behavioral, not methodological: store-and-release assumes the wood is released, and store-and-reuse assumes it is recovered, but the accounting can’t make either happen. The credit a building may legitimately claim depends on recovery infrastructure and disassembly decisions that sit outside the carbon model entirely.
Consequences
Benefits: Biogenic carbon accounting gives a timber or bio-based claim a defensible basis instead of a marketing number. It forces stored carbon to be reported transparently and separately, so a reader can see the genuine storage without mistaking borrowed carbon for banked carbon. It connects the carbon case for circular, bio-based materials to the same standards-based boundary that governs the rest of a whole-life assessment, and it makes the timing of storage and release visible rather than hidden.
Liabilities: The method choice is contested, so an assessment that does not name its convention is hard to trust and hard to compare. Stored biogenic carbon held loosely becomes a greenwashing vector: book temporary storage as permanent, or net it off the headline, and a building can manufacture a net-negative figure it has not earned. The accounting decides what a bio-based claim may count, not whether the material is good. A building can store real carbon and still be greenwashed, and a conservative biogenic number does not by itself make a material the right structural or fire-safety choice.
Related Articles
Sources
- The Buildings and Cities critical overview, Biogenic carbon in buildings: a critical overview of LCA methods, catalogs the divergence among LCA standards on biogenic carbon and argues for harmonization.
- Ramboll’s Biogenic carbon in timber buildings — how should it be considered? frames the practitioner choice between store-and-release and store-and-reuse and notes how much sequestered carbon is lost once release is assumed.
- The European Commission DG CLIMA expert paper Biogenic carbon storage in buildings supplies the EU policy framing for how stored biogenic carbon should be treated.
- The Carbon Leadership Forum’s Biogenic Carbon Accounting in Wood Environmental Product Declarations explains how the accounting is applied in wood EPD practice.
- Peer-reviewed timber-frame LCA research in Journal of Cleaner Production on the climate impacts of timber-frame buildings under static and dynamic biogenic-carbon modelling shows how the modelling choice changes whether a timber building beats its concrete alternative.
- The CEN standard EN 16485 sets the EPD product category rules for round and sawn timber, requiring stored biogenic carbon to be quantified and reported separately from fossil carbon.