Few claims in sustainable construction generate as much interest, or as much scepticism, as the idea that a building material can remove carbon from the atmosphere rather than simply avoid emitting it. The evidence says hempcrete can do exactly this. Understanding why means looking at three things: what happens in the field before construction starts, what happens inside the wall for years afterwards, and what the finished mix actually is, because that last part turns out to matter more than most summaries let on.
What "carbon negative" actually means
"Carbon negative" doesn't mean a material emits no CO₂. Every material carries some embodied energy from production, processing and transport. It means the CO₂ locked away in the material over its lifetime exceeds the CO₂ emitted in making it.
For most conventional building materials, that balance runs the wrong way. Portland cement production is one of the more carbon-intensive processes in construction, and once concrete is cast it stores no meaningful carbon. Hempcrete works differently on both counts, though as the next section covers, not automatically.
How hemp sequesters carbon during growth
Hemp absorbs CO₂ through photosynthesis like any plant, but it does so fast: the plant reaches 2 to 4 metres in four to five months, producing a dense, woody stem in a single season. Timber is also credited as a carbon-sequestering material, but even fast-growing tree species take years to reach comparable maturity. Hemp's growth cycle gives it a real speed advantage as an agricultural carbon sink.
The peer-reviewed figures for cultivation are more conservative than some of what circulates in industry material, and it's worth separating gross uptake from the net figure once fertiliser and processing emissions are subtracted:
- Gross CO₂ uptake during growth: 1.29 to 1.6 kg CO₂e per kg of hemp (Scrucca et al., 2020; Portland State review, 2022)
- Net cultivation GWP, after accounting for farming inputs: -0.315 kg CO₂e per kg of hemp in a French context (Scrucca et al., 2020), or a more optimistic -1.734 kg CO₂e per kg in an earlier Italian study (Zampori et al., 2013)
The Hempcrete Book cites 1.5 to 2.1 kg CO₂ per kg of hemp during growth, which sits at the high end of the gross uptake range above. Per-hectare figures for hemp cultivation vary widely across the literature, from a conservative 3 to 4 tonnes CO₂ per hectare in institutional reviews to considerably higher numbers in trade and advocacy material. The per-kilogram figures are the more defensible ones to build an argument on.
Once hemp shiv is mixed into hempcrete and cast into a wall, the carbon fixed during that growing season stays locked into the building's fabric for the structure's lifetime, which life cycle studies suggest exceeds 100 years.
The double sequestration effect: hemp and lime
Hempcrete's carbon story doesn't end with the plant. The lime binder adds a second sequestration mechanism through carbonation.
When lime (calcium hydroxide) is exposed to air, it reacts slowly with atmospheric CO₂, converting back to calcium carbonate, the mineral it originally came from. This hardens and strengthens the hempcrete while absorbing CO₂ at the same time, partially reversing the emissions produced during the lime's manufacture.
That reversal is measurable rather than a vague benefit: Arehart et al. (2020) found that carbonation recovers roughly 18.5% to 38.4% of the CO₂ emitted during binder manufacture, depending on the binder and mix. Together, the hemp aggregate and the carbonating lime binder make hempcrete unusual among building materials in that it keeps sequestering carbon after it's built, not just during production.
Not every hempcrete mix is carbon negative
This is the nuance most summaries skip, and it's worth being upfront about it: carbon negativity is a property of the mix design, not something hempcrete gets automatically.
Arehart et al. (2020) modelled 36 different hempcrete formulations and found that high-density mixes, above roughly 300 kg/m³, especially those using Portland cement rather than a lime binder, can end up net carbon positive. Denser mixes need more binder relative to hemp shiv, and Portland cement's manufacturing emissions are high enough to outweigh what the hemp and lime carbonation store. The same study found the best-performing formulation they tested, a lower-density lime mix, achieved a theoretical storage of around -16.0 kg CO₂e per m² of wall (functional unit basis).
This reinforces something builders already need to know for other reasons: binder choice isn't just about set time and building control approval, it's the single biggest lever on whether a given wall is carbon negative at all.
What the numbers look like in practice
Here it matters to be precise about which quantity is being reported, since the literature reports at least two distinct things and they get blurred together often.
Gross storage is the total CO₂ physically locked in the material. Net lifecycle benefit is what's left after subtracting the emissions from producing, transporting and building with that material. Ip & Miller (2012), whose life cycle analysis covered the full wall system including timber frame and coatings in French and UK contexts, give both figures for a 300mm hempcrete wall:
- Gross storage: approximately 82.7 kg CO₂ per m² of wall area
- Net GHG reduction: approximately 36.08 kg CO₂e per m²
The gap between those two numbers is the embodied carbon of everything else in the wall assembly. Other studies land in a wide net-lifecycle range depending on wall type and system boundary, from roughly -1.6 to -79 kg CO₂e per m² (Pretot et al., 2014, via Arehart et al., 2020), and a six-study review modelling a 100-year lifespan found net figures ranging from 15.9 down to -36.08 kg CO₂ per m², a range that crosses zero depending on assumptions.
At the material level, a 2003 study by Pervaiz & Sain (published in Resources, Conservation and Recycling) put stored CO₂ at approximately 325 kg per tonne of dried hemp. Lime Technology's widely cited figures for their Tradical® system, 110 kg CO₂ per m³ for sprayed application and 165 kg CO₂ per m³ for hand-placed or shuttered work, describe gross storage rather than net benefit; HempcreteHub's own carbon tracker uses these same rates, and the caveat above applies there too.
The honest summary: hempcrete's directional claim, that it's carbon negative, is well supported. The exact magnitude depends heavily on mix, binder, wall type and what's included in the accounting, and treating any single headline number as universal would be a mistake.
Why local sourcing makes or breaks the case
The most current full-building study available, a 2025 life cycle assessment of the Narara Ecovillage in Melbourne (presented at the 58th Architectural Science Association conference), found something that should reshape how the sequestration argument gets framed.
The hempcrete wall's total embodied carbon came to 49.57 t CO₂e, a 19.9% reduction versus an equivalent brick veneer wall (61.81 t) and a 92.5% reduction versus lightweight steel (927.34 t). Looking only at the construction-and-use stage, the hempcrete wall was net -7.51 t CO₂e, meaning sequestration outweighed emissions at that stage.
But the single largest source of emissions in the whole study wasn't the binder or the hemp shiv. It was transport: 51 t CO₂e from hauling shiv and binder 176 km to site. That's more than the entire construction-and-use sequestration benefit.
The implication is direct: how far your materials travel can determine whether a hempcrete build ends up carbon negative in practice, regardless of how good the mix design is on paper. This is exactly why sourcing locally matters beyond cost and lead time. HempcreteHub's supplier directory is built around helping you find hemp shiv and lime binder closer to your build site.
What happens at end of life
Carbon stored in a hempcrete wall doesn't vanish when a building is eventually demolished. The material either biodegrades slowly, releasing its stored carbon back over time, or gets reused in new construction where possible. Either way, a structure standing for a century or more represents a sustained reduction in atmospheric CO₂ during its service life, regardless of what happens afterwards.
Hempcrete's carbon case holds up under scrutiny, but it's a case that depends on choices: a lime binder over cement, a reasonable density, and shiv and binder sourced as close to site as you can manage. Get those right and the numbers in this article are yours to claim. Get them wrong and you can end up with a wall that's carbon negative in name only. If you want to see what real projects are storing, HempcreteHub's community carbon tracker totals up submitted builds using the rates above, and you're welcome to add your own.
Sources
- Stanwix, W. & Sparrow, A., The Hempcrete Book — newsociety.com
- Ip, K. & Miller, A. (2012), "Life cycle assessment of hemp-lime composite walls," Resources, Conservation and Recycling — sciencedirect.com
- Arehart, J. H. et al. (2020), "On the theoretical carbon storage and carbon sequestration potential of hempcrete," Journal of Cleaner Production — sciencedirect.com (open PDF)
- Pervaiz, M. & Sain, M. (2003), "Carbon storage potential in natural fibre composites," Resources, Conservation and Recycling 39(4), 325–340 — doi.org/10.1016/S0921-3449(02)00173-8
- Narara Ecovillage LCA (2025), 58th Architectural Science Association conference — minerva-access.unimelb.edu.au
- Scrucca, F. et al. (2020) and Portland State University sustainability review (2022) — cultivation GWP figures