A bio-based product with a negative cradle-to-gate carbon figure has not removed carbon from the atmosphere for good. It is carrying carbon that the plant absorbed while growing, and under EN 15804+A2 that uptake is booked as −1 kg CO₂e per kg of CO₂ when the material enters the product system and +1 when it leaves, typically at end of life. Over the whole life cycle the biogenic part nets to about zero. What remains is the fossil part, which is the number that tells you whether one product is actually lower-carbon than another.
That is the whole rule, and most of the confusion around “carbon-negative” building materials comes from reading only the first half of it. This article explains how the standard splits the figures, shows the rule at work on a real, third-party-verified EPD, and says which number to compare at concept stage.
The short version
- EN 15804+A2 reports four climate indicators: GWP-fossil, GWP-biogenic, GWP-luluc and GWP-total (their sum). The old single GWP is gone. Mandatory for new EPDs since October 2022 — One Click LCA, “EN 15804+A2: What the changes mean for EPDs”.
- Biogenic CO₂ uptake is characterised as −1, release as +1 (the approach also recommended by EN 16485 and ISO 21930) — Hoxha et al., “Biogenic carbon in buildings: a critical overview of LCA methods”, Buildings & Cities, 2020.
- Stored biogenic carbon is released in module C (end of life) if the product is incinerated or landfilled, so a cradle-to-gate (A1–A3) negative GWP-total is a storage figure, not a permanent saving.
- Worked example below: a straw wall panel declares −101 kg CO₂e/m² for A1–A3 and about +118 kg CO₂e/m² in C3–C4. Its fossil footprint, +17 kg CO₂e/m², is the comparable number.
- EPDs must declare the biogenic carbon content of product and packaging in kg C (exempt below 5 % of mass). Multiply by 44/12 to get the CO₂ equivalent.
- All figures here are indicative, for early comparison, not for certification or compliance.
What changed in EN 15804+A2?
EN 15804 is the European core standard behind construction-product EPDs. Its second amendment, EN 15804:2012+A2:2019, was approved by CEN in July 2019 and became mandatory for new EPDs in October 2022 (One Click LCA). The change that matters here is to climate reporting. The single Global Warming Potential indicator was replaced by four:
| Indicator | What it counts |
|---|---|
| GWP-fossil | Emissions from fossil fuels and fossil-based materials |
| GWP-biogenic | Uptake (negative) and release (positive) of carbon from biomass |
| GWP-luluc | Emissions from land use and land-use change |
| GWP-total | The sum of the three |
The amendment also made it compulsory to declare the biogenic carbon content of the product and its packaging, in kilograms of carbon, unless biogenic material is under 5 % of the combined mass (One Click LCA, ibid.).
The intent is traceability. Before A2, a bio-based product could report one negative GWP number and leave the reader to guess how much of it was stored plant carbon and how much was a genuinely lower-emission process. Now the two are separate columns, and the storage has to be shown leaving the system as well as entering it.
Citation capsule: EN 15804+A2 (mandatory for new EPDs from October 2022) splits climate impact into GWP-fossil, GWP-biogenic, GWP-luluc and GWP-total, and requires the biogenic carbon content of product and packaging to be declared in kg C. Source: One Click LCA, “EN 15804+A2: What the changes mean for EPDs”.
How does the −1/+1 rule work?
When biomass grows it absorbs CO₂. When that biomass becomes a product, the carbon is inside the product, and the standard books the absorption as a negative emission: −1 kg CO₂e for every kg of CO₂ taken up. When the carbon leaves the system at end of life, released through incineration or decay or transferred to another product system through recycling, it is booked as +1 in module C. EN 15804+A2, EN 16485 and ISO 21930 all follow this −1/+1 approach (Hoxha et al., “Biogenic carbon in buildings: a critical overview of LCA methods”, Buildings & Cities, 2020).
Two consequences follow directly.
The negative number appears at the start. Uptake sits in module A1 (raw material supply), so the cradle-to-gate stages A1–A3 look best. A product can show a strongly negative GWP-total for A1–A3 while its fossil emissions are positive and perfectly ordinary.
The positive number appears at the end. If the product is burned or landfilled, the stored carbon is released in module C3 or C4 (One Click LCA, ibid.). Across A to C the biogenic column therefore sums to approximately zero. Hoxha et al. name the risk plainly: assessing only module A, with the uptake counted but the end-of-life release not reported, carries “a risk of biased and misleading results”.
The storage is real while the material exists. It is a delay of emissions, which has value, and it becomes a permanent benefit only if the material is kept or reused rather than burned. That is the point made qualitatively in bio-based insulation compared; this article is about how the standard makes it visible in numbers.
Citation capsule: Under EN 15804+A2 and ISO 21930, biogenic CO₂ uptake is characterised as −1 kg CO₂e per kg CO₂ and release as +1. Uptake appears in A1, release in C3/C4 when the product is incinerated or landfilled, so the biogenic contribution across modules A–C nets to approximately zero. A negative A1–A3 GWP-total indicates stored carbon, not a net removal.
What does it look like in a real EPD?
Take a third-party-verified EPD for a bio-based wall element: EcoCocon’s straw-and-timber wall panel, declared per 1 m² as a weighted average of six panel types, registered with Stichting MRPI as 1.1.00555.2024, verified by an independent third party (Tim Mol, EcoReview) and calculated to EN 15804+A2 (EcoCocon EPD, MRPI 1.1.00555.2024). The climate rows read:
| Module | GWP-total | GWP-fossil | GWP-biogenic | GWP-luluc |
|---|---|---|---|---|
| A1–A3 (production) | −101 | +17.4 | −119 | +0.09 |
| A5 (installation) | +3.9 | +1.6 | +2.2 | 0.00 |
| C3 (waste processing) | +112 | +0.6 | +111 | 0.00 |
| C4 (disposal) | +6.2 | +0.04 | +6.1 | 0.00 |
| D (beyond the system) | −4.9 | −4.8 | 0.00 | −0.04 |
All values in kg CO₂e per m², rounded from the EPD’s scientific notation. Biogenic carbon content of the product: 31.5 kg C per m², which at 44/12 is roughly 116 kg CO₂, matching the −119 kg uptake (the remainder is packaging).
Read the biogenic column top to bottom: −119 in, +2 during installation waste, +111 and +6 out at end of life. Net across A–C: about zero. Read the total column top to bottom: −101 at the factory gate, +112 at the incinerator. Net across A–C: about +23, almost all of it the fossil +17 plus transport and installation.
So the honest, comparable statement about this product is: around 17 kg CO₂e/m² of fossil emissions, plus around 116 kg CO₂ of biogenic carbon stored for as long as the panel exists. Both parts are true. “−101 kg CO₂e/m²” on its own is only the first.
None of this is a criticism of the product. Its fossil footprint is low, and the EPD does exactly what A2 asks: it declares the storage and shows it leaving. The criticism is of reading the top-left cell of the table and stopping.
Citation capsule: In EcoCocon’s MRPI-registered EPD 1.1.00555.2024 (EN 15804+A2, third-party verified), the straw wall panel declares GWP-total −101 kg CO₂e/m² for A1–A3, of which GWP-fossil is +17.4 and GWP-biogenic −119. Module C3 releases +111 kg CO₂e/m² biogenic, so the biogenic balance across A–C is approximately zero and the net life-cycle fossil footprint is around +17 kg CO₂e/m².
Which number should you compare at concept stage?
GWP-fossil, first. It is the part that does not cancel out, and it is comparable across bio-based and mineral products on the same basis. If a straw panel, a hemp-lime block and a gypsum board are all on the table for an internal wall, their fossil columns tell you which process actually emitted less.
GWP-biogenic, second, as a separate line. The storage matters, but its value depends on what happens at end of life, which at concept stage you do not know. Elementa’s circularity pathways are the right frame: a bio-based element that is retained or reused keeps its carbon stored; one that is replaced and burned does not. Report the stored quantity, keep it apart from the fossil figure, and let the pathway decide what it is worth.
GWP-total, with care. It is the headline number, and for A1–A3 on a bio-based product it is the least informative of the four, because it is dominated by the temporary storage. Use GWP-total only across the full A–C scope, or not at all.
Module D, separately and never summed. Module D reports benefits and loads beyond the system boundary, such as avoided emissions from energy recovery. EN 15804 requires it to be declared apart from A–C, and it should never be added to them.
This is how Elementa treats bio-based materials in pre-assessment: each layer is priced from the generic ÖKOBAUDAT dataset (EN 15804+A2) pinned to it, for the product stage A1–A3 only. A negative value there is therefore stored carbon, not a saving; the release in C3/C4 is not modelled. Where a dataset lists the same timber twice, once excluding and once including stored carbon, those are two accounting conventions for one material, not two materials, and Elementa never proposes a substitution across that boundary. It is decision support for early comparison, not a certified LCA.
Citation capsule: At concept stage, compare bio-based and mineral products on GWP-fossil, report GWP-biogenic separately as stored carbon whose permanence depends on the end-of-life pathway, use GWP-total only across the full A–C scope, and never add module D to modules A–C.
Frequently asked questions
Does a negative GWP mean the product is carbon-negative?
Not over its life cycle. Under EN 15804+A2 the negative value in A1–A3 is stored biogenic carbon booked as −1. If the product is incinerated or landfilled, the same carbon is booked as +1 in module C, and the biogenic balance across A–C is approximately zero. The remaining net emissions are the fossil ones.
Where is the biogenic carbon released in the EPD?
In module C3 (waste processing) for incineration and C4 (disposal) for landfill, according to the end-of-life scenario the EPD assumes. Check the scenario: a Dutch or German default scenario for bio-based waste typically assumes incineration with energy recovery, which puts the release in C3 and a small credit in module D.
How do I convert declared biogenic carbon (kg C) to CO₂?
Multiply by 44/12, roughly 3.67. A product declaring 31.5 kg C per m² is storing about 116 kg CO₂ per m². EN 16449 gives the calculation for wood-based products.
Why do hemp-lime figures range from strongly negative to clearly positive?
Because different sources include or exclude the biogenic uptake and, for lime binders, carbonation. A manufacturer EPD that nets uptake and carbonation into A1–A3 can show a negative total; a study that reports fossil emissions only shows a positive one. Both can be right for what they measure. The fossil column is the comparable one.
Can Elementa certify a product’s biogenic carbon?
No. Elementa reads the declared A1–A3 values from the ÖKOBAUDAT dataset pinned to a layer and presents them as indicative values and impact classes for early comparison. Certification and compliance reporting need a qualified assessor and the EPD itself.
The takeaway
EN 15804+A2 did the useful thing: it made stored plant carbon visible as its own line and forced EPDs to show it leaving as well as entering. A negative A1–A3 total is a storage figure. The saving is the fossil column, and the storage is worth exactly as much as the pathway that keeps the material in use. Compare on fossil, report biogenic separately, and let the pathway decide.
If you want bio-based and mineral options compared on that basis, element by element, before formal assessment, start a project.