Biogenic Carbon
Carbon that plants absorb from the air and store in timber, straw, hemp and cork, and how life-cycle assessment counts its uptake and release.
Biogenic carbon is the carbon that plants pull from the air as carbon dioxide while they grow and lock into their tissue. When that plant material becomes a building product, such as timber, straw, hemp or cork, the carbon stays in the material for as long as the material lasts. For a designer, the term links the carbon story of a timber wall to the forest it came from, and it raises a question that concrete and steel rarely force: how should a life-cycle assessment count carbon that is taken up now and may be released decades later?
What is biogenic carbon?
Biogenic carbon is carbon that originates in biological processes, chiefly photosynthesis. A tree takes CO2 from the atmosphere, builds wood from the carbon, and releases it again when the wood rots, burns or is consumed by fungi and insects. The word biogenic separates this short cycle from fossil carbon, which has been held in geological deposits for far longer and re-enters the atmosphere as a new addition when fossil fuels are burned. That distinction is why two houses that use the same energy in operation can still carry different carbon accounts.
How do building materials store biogenic carbon?
Timber, straw, hemp and cork store biogenic carbon in four different forms, and each stays in service for a different span of time. The table summarises the logic. The actual carbon content depends on species, moisture, density and processing, so it should come from a product declaration rather than from a rule of thumb.
| Material | Source of the carbon | Typical role in a house | What keeps the carbon stored |
|---|---|---|---|
| Timber, solid or cross-laminated | Tree trunks and branches | Structure, floors, cladding | Protection from rot and the service life of the building |
| Straw bale | Cereal stalks, a crop residue | Infill walls and insulation | Keeping the bales dry for the life of the wall |
| Hemp (hempcrete) | Hemp stalks held in a mineral binder | Infill walls and insulation | Binder choice and the moisture regime of the wall |
| Cork | Bark of the cork oak, harvested without felling the tree | Insulation, flooring and wall tiles | Lifetime of the finish and whether it can be recovered |
In Slovak residential work the question arises most often with drevostavba (timber-frame construction) and with krížom lepené drevo (KLD), the panel product known in English as cross-laminated timber. Both place a large mass of wood in the primary structure. Straw and hemp usually sit in infill and insulation layers, so their carbon is held in walls rather than in the frame.
How does life-cycle assessment count biogenic carbon?
A life-cycle assessment divides the life of a building into stages and assigns each flow of greenhouse gases to one of them. The European standards behind the method are EN 15804, which governs environmental product declarations, and EN 15978, which governs the building as a whole. Both are adopted in Slovakia as STN EN versions. In the usual layout, the uptake of carbon during growth appears as a negative flow in the product stage, module A1 (raw material supply). The release at end of life appears as a positive flow in the end-of-life modules, which EN 15804 calls C3 for waste processing and C4 for disposal. Benefits beyond the system boundary, such as reuse or energy recovery, are reported separately in module D.
What is the -1/+1 approach in EN 15804?
The -1/+1 approach is the named convention in EN 15804 for booking biogenic carbon in a product declaration. Carbon taken up from the air is booked as minus one unit of carbon dioxide equivalent per unit of carbon, and the same carbon released at end of life is booked as plus one. A product whose carbon is fully released within the study period therefore nets to zero. The conversion from carbon to carbon dioxide follows the molar-mass ratio of 44 to 12, which is why a given weight of carbon corresponds to a larger weight of CO2.
The biogenic carbon content is also declared as a separate item, so a net zero result never hides how much carbon a product holds.
A common misconception
A net zero result does not make a product climate neutral. It means the uptake and the release cancel within the study period. If the material is replaced early, burned at end of life, or harvested from a stand that is not regrown, the cancellation does not hold in practice, and the net figure overstates the benefit.
How long does stored biogenic carbon stay out of the atmosphere?
Storage is temporary, and the honest answer is that it lasts only as long as the material does. A sound timber floor keeps its carbon out of the air for as long as it stays in use. Once the wood is burned, or decays in conditions that release its carbon, the storage ends and the carbon returns. The benefit therefore counts only when three conditions hold: the material outlives the product it replaces, it is not burned at end of life, and the forest it came from is managed so that harvested stands are regrown.
| End-of-life route | What happens to the carbon | Effect on the account |
|---|---|---|
| Reuse or continued service in a new building | Storage continues past the study period | Stays stored, not credited in the core modules |
| Energy recovery by burning | Carbon released quickly as CO2 | Release in C3 cancels the earlier uptake; recovered energy is reported in D |
| Landfill | Slow decay, partly released as methane and CO2, partly retained | Depends on site conditions and the method used |
How is biogenic carbon different from embodied carbon?
Biogenic carbon is one component inside the wider metric of embodied carbon, not a separate category. Embodied carbon totals the greenhouse gases from manufacturing, transport, construction and end of life. The biogenic share is the part that comes from carbon taken up by plants, while the rest comes from fossil energy and process emissions. Timber can have modest processing emissions alongside a large biogenic component, so a timber envelope may look better than a concrete one for reasons that have nothing to do with storage. Reporting the biogenic content as its own line keeps a manufacturing advantage from being mistaken for a storage benefit.
Where does biogenic carbon matter in a Slovak timber project?
In practice the question reaches an architect at three points. First, the product declaration for each timber element should state its biogenic carbon content, and the specification should ask for that figure rather than accept a generic one. Second, the wood should come from a supply chain with forest certification and traceable origin, so that the uptake can be linked to a managed forest. Third, the detailing should keep the material recoverable for reuse, which is where storage is most likely to continue. A whole-life carbon assessment brings all three points into one account, adding operational energy and the remaining embodied emissions to the biogenic figure.
Frequently asked questions
- Is timber carbon neutral?
- Not on its own. Timber stores carbon taken from the air, but that storage is only as durable as the building it sits in, and the forest must be regrown after harvest. Timber is best described as a store of carbon with a conditional benefit, not as a neutral material.
- What does the -1/+1 convention mean in plain terms?
- It books the carbon taken up by the plant as minus one and the same carbon released later as plus one. If all of the carbon is released within the study period, the two cancel and the product nets to zero. The biogenic carbon content is still declared separately so the stored amount stays visible.
- Does burning timber at the end of its life cancel the storage benefit?
- Yes, in the accounting. Burning releases the carbon in the end-of-life modules, which cancels the earlier uptake. Any energy recovered is reported in module D, separately from the core figure.
- Are straw and hemp counted the same way as timber?
- The accounting logic is the same, because their carbon is also biogenic. The difference lies in service life and form, since straw and hemp usually sit in infill and insulation layers whose durability depends on moisture control.
- Why should I ask for the biogenic carbon content in a product declaration?
- The net figure can hide how much carbon a product holds. A declaration that reports the content separately lets you compare products on storage, not only on the net result, and check whether the claimed benefit fits the expected life of the building.