The role of Hemp in Construction Stored Carbon
Construction Stored Carbon
The UK has committed to ambitious ‘Net Zero’ targets to reduce CO2 emissions by 68% by 2030, and achieve carbon neutrality by 2050, to combat climate change. Biomass materials and products in new construction could make a significant contribution to this goal.
When biomass materials are used in construction, CO2 absorbed from the atmosphere during plant growth is stored as biogenic carbon for as long as they are incorporated in a building. This storage of biogenic carbon results in a long-term negative greenhouse gas (GHG) emission. Though the benefit of negative CO2 emissions in buildings can be estimated by quantifying the effect of delayed emissions on global warming potential (GWP), the value of biogenic carbon storage is not included in most mainstream Life Cycle Assessment (LCA) standards.
The publication of the ‘Biobased Construction Certification Protocol’ this year seeks to address this omission. Responding to the demand for separate targets for emissions, avoided emissions, removal and storage, the Protocol provides a calculation tool to quantify the value of carbon removal and storage in built assets. The proposed metric can be used in calculating sequestered carbon within the value chains of real estate based financial products, allowing policy makers to design incentives.
Together with policy measures such as the French RE2020 and initiatives such as ‘Bio-based building products in the Dutch Environmental Database (NMD)’ there is a growing consensus that mainstream ‘static’ LCA provides an incomplete picture of carbon impacts in the built environment.
“Show me the incentives and I’ll show you the outcome.”
Incentive Reward Structures
Inasmuch as the built environment is a physical outcome of incentives, we can see the results all around us. Sometimes described as ‘oil age architecture’ [1] the design of buildings in the 20th and early 21st century has favoured extractive materials, manufactured at scale, using high energy processes.
More recently, incentives have been introduced to reduce fossil fuel inputs in both operation and during construction, with point-based sustainability assessments, such as the British Research Establishment Environmental Assessment Methodology (BREEAM). Developers are rewarded with an aggregated score against sustainability criteria, including a LCA to calculate operational and embodied carbon impacts. A high BREEAM rating gives developers a commercial advantage by meeting the environmental, sustainability and governance (ESG) criteria for end users, increasing marketability and asset value. Other whole life carbon (WLC) targets have been set in the RIBA 2030 Climate Challenge, and by regional bodies such as the Greater London Authority, which can have a direct bearing on development value and finance costs.
Therefore, the way we measure carbon has a critical role in shaping outcomes in the built environment. LCA aims to account for the likely GWP consequences of design and material choices in built assets, throughout their service life and ultimate end of life. Various approaches and standards have been developed, with a range of calculation methodologies.
Biogenic Carbon
The question of how to account for biogenic carbon is contentious, because the central premise of LCA is to quantify both actual and hypothetical carbon impacts over time, looking into the future. Biogenic carbon accounting is problematic because CO2 emissions absorbed in biomass and stored in buildings are both a temporary GWP benefit, and a future impact. In the context of buildings, ‘temporary’ is measured in decades, according to a notional time horizon.
It might be assumed that utilising biomass materials in construction would always result in lower embodied carbon emissions, but this is not necessarily the case when considering only the fossil carbon impacts of production and distribution, particularly in relation to engineered timber and manufactured products.
In accordance with international standards, most LCA calculation methodologies adopt a static -1/+1 accounting of biogenic carbon, so that ‘negative’ emissions sequestered in biomass are assumed to be re-emitted at end of life. Using this approach there is zero GWP benefit attributed to temporary carbon storage over the whole life cycle of a built asset. Depending on end-of life assumptions, fossil carbon embodied in biomass products are therefore often only marginally lower than extractive materials, and sometimes higher. For example, a comparative LCA of structural steel versus glue-laminated timber (glulam) reveals that in some scenarios steel has lower embodied carbon than timber [2].
Static LCA methodologies have the merit of relative simplicity but are criticised because they ignore the effect of timing on carbon emissions and the influence of rotation periods relating to biomass growth.
Box A
Radiative forcing is a measure of the imbalance caused in the Earth's energy system by factors such as greenhouse gases, aerosols, and changes in solar radiation. It quantifies the change in net radiation at the top of the atmosphere. A positive radiative forcing leads to warming, while a negative forcing leads to cooling.
During growth plants sequester CO2 from the atmosphere, and then later emit it back to the atmosphere through combustion or decomposition. Carbon stored in durable biomass products has a ‘buffering’ effect in relation to climate change, by not exerting additional radiative forcing for the duration of storage (see box A). The argument in favour of buffering carbon release is that it addresses the urgency of climate change, by temporarily storing carbon that would otherwise contribute to global warming, and ‘buying time’ while mitigation strategies are developed[3]. If adopted at sufficient scale, buffering could help avoid global temperatures reaching critical tipping points[4]. Others argue that biogenic carbon should be considered as permanently stored in built assets, since future circular economy legislation will necessarily prohibit the incineration of biomass[5] in favour of re-cycling and re-use, to meet treaty obligations prior to 2050.
Accounting for Time
Alternative LCA approaches have been developed to more accurately describe the effect of time. In 2010 Levasseur et al published a dynamic LCA (DLCA) methodology[6] to consistently measure time-dependant impacts of emissions on radiative forcing. While this approach is regarded as the most accurate calculation method, it is considered laborious and impractical for widespread adoption. Other methodologies allow for the value of temporary biogenic carbon storage over time with linear approximations of DLCA, to account for delayed emissions. These include the French RE2020 standard, the British Publicly Available Specification (PAS) 2050 (2011) and the European Commission’s International Reference Life Cycle Data System Handbook (ILCD 2010), all of which account for the positive GWP effect by calculation of a credit for temporary carbon storage.
Box B
Hemp Fibre Insulation (λ= 0.039W/m.K)
A1-A3 (fossil) : 10.1kg CO2e/m3
A1-A3 (biogenic) : -78.8kg CO2e/m3
RE2020 : -35.4kg CO2e/m3
PAS2050 : -39.3kg CO2e/m3
Glass wool Insulation (λ=0.037w/m.K)
A1-A3 (fossil) : 12.9kg CO2e/m3
A1-A3 (biogenic) : -2.3kg CO2e/m3
RE2020 : 11.36kg CO2e/m3
PAS2050 : 11.8kg CO2e/m3
The application of different LCA methodologies gives rise to significant variations in results. For example, comparing a functionally equivalent cubic metre of hemp fibre insulation, to a cubic metre of glass wool insulation, the difference in A1-A3 fossil carbon is only 2.8kgCO2e/m3. Accounting for biogenic carbon using PAS2050 gives a difference of over 51kgCO2e/m3 (see box B).
Ignoring Time
Notwithstanding advantages of DLCA or linear approximations of DCLA, static -1/+1 methodologies prevail, as seen in the recently published Royal Institute for Chartered Surveyors (RICS) Whole Life Carbon Assessment (WLCA) Standard 2nd Edition. Here we find that biogenic carbon storage is of neutral effect in calculating carbon impacts, based on a notional time horizon of 60 years and end-of-life assumptions where biomass is generally incinerated, or oxidised through decomposition. Carbon emissions sequestered from the atmosphere during plant growth, and temporarily stored in materials, are assumed to be fully re-emitted back into the atmosphere when a building is demolished. Thus, the GWP benefit of removing carbon from the atmosphere and storing it even for a relatively short period is overlooked.
Indeed, the RICS WLCA guidance strikes a decidedly agnostic tone in respect of materials, explicitly referring to the necessity of avoiding ‘perverse incentives’ in relation to timber and biomass. Appendix N3 states that LCA practitioners may wish to ‘explore the benefits of biogenic carbon storage in their assessments’ (using recognised approaches such as PAS 2050:2011), with results reported separately. However, this optionality means the outcome of such a calculation carries little weight in comparison with the mandatory quantification of fossil carbon, or in relation to BREEAM and other sustainability assessments.
Leaving aside the question of whether cost focused developers really would add functionally unnecessary biomass to buildings to ‘game’ LCA results, we should question whether static -1/+1 methodologies provide a realistic picture of carbon impacts. Comparative studies of DLCA against static LCA show very different results[7]. And by negating the value of temporary biogenic carbon storage over time, calculations using static LCA bring forward hypothetical future emissions to be compared on equal terms with actual fossil CO2 already emitted and immediately contributing to global warming.
Moreover, powerful market incentives already exist for the use of extractive materials in construction, because environmental externalities are not fully priced into product costs, giving materials like concrete and steel a price advantage over biomass products. Economies of scale in established industries that have benefitted historically from cheap fossil fuel inputs, mean that relatively small-scale biomass product manufacture is at a further cost disadvantage. Nor are the carbon impacts of extractive materials properly understood or assessed.
“Contemporary construction across the world has two additional poorly researched yet relevant impacts on the carbon cycle: first, the production of cement, concrete, asphalt, glass and so on requires vast amounts of sand extracted from beaches, rivers and seafloors; second, mining can lead to extensive local deforestation. The sand mining not only exerts substantial pressure on available deposits, which have become an increasingly scarce global resource, but also compromises the carbon uptake capacity of the aquatic ecosystems disturbed during extraction.
Together, the mining infrastructures and the development of mineral commodity supply chains are responsible for a disproportionate loss of forests surrounding mines and resulting loss of stored carbon. Mining-induced deforestation in Brazil alone was responsible for 9% of all Amazon forest loss in 2005–2015: twelve-times more than the area deforested within per- mitted mining leases.”
So, far from removing perverse incentives, static -1/+1 LCA supports existing incentives for business as usual, using the same extractive materials, albeit with relatively minor future efficiency adjustments at the margins (an estimated 24% for steel and 13% for cement by 2050[8]). Indeed, mainstream static LCA standards beg the question; is it not more perverse to carefully ignore the GWP benefit of long-term biogenic carbon storage? This question resonates when considering short rotation construction crops such as industrial hemp.
Short Cycle Carbon Capture Crops
The need to avoid over-use of timber is understandable, because forestry has long rotation periods and supply constraints. Slow growth periods means that timber cannot be considered carbon neutral in a short time horizon. On the other hand, fast growing construction materials such as industrial hemp and straw have short rotation periods, providing an effective mitigation effect on emissions by rapidly removing carbon from the atmosphere[9].
The amount of biogenic carbon currently stored in buildings annually can be extrapolated from global wood consumption in the construction sector, which is about 473 Mm3/a of sawn wood and 367 Mm3/a wood-based panels. An estimated 247 Mtonnes C/a or 0.91 Gtonnes CO2e/a is stored in these materials, which is about 2.5% of annual fossil CO2 emissions[10]. That’s equivalent to capturing and storing all of the annual emissions from global aviation every year.
Graph from Göswein et al, Buildings and Cities, 2022
Increased use of carbon capture crops such as industrial hemp in construction would multiply this benefit. Globally, if only 20% of conventional masonry, steel and concrete buildings were instead to be built primarily with timber and hemp, this would store an additional 222mtCO2e annually and displace 912mtCO2e fossil carbon emissions (a net saving of 1,134mtCO2e).
In contrast to forestry, hemp is grown in rotation with food crops, enhancing soil health, breaking pest cycles and improving subsequent food crop yields. By not competing with food production, the cultivation of industrial hemp has far greater capacity than forestry as a natural carbon capture mechanism.
“‘…most assessment methods and standards have not been allowing accounting for carbon storage by biological materials. Some decades ago this made sense, because accounting for stored carbon might overstate positive climate impact. At this moment in time however, this conservative approach results in the opposite: an understatement of the advantages of biobased construction.’ ”
New Incentives
There is no neutral space where mathematical abstractions align perfectly with real world outcomes; conceptual models relating to LCA create incentives, perverse or otherwise. Accounting for the GWP benefit of biogenic carbon removal and storage is necessary to accurately measure carbon impacts, but also, to encourage a shift away from extractive, high embodied carbon materials. Measures such as that proposed by The Biobased Construction Certification Protocol are an important step in this direction. Given the urgency of climate change, incentives are needed to store carbon and keep it out of the atmosphere for as long as possible. We can then begin to imagine a future built environment where most construction materials are grown, locking away biogenic carbon indefinitely.
Incorporating hemp and other short cycle biomass into durable construction products offers transformational potential for the future of construction, so that buildings can become part of a solution to climate change, rather than part of the problem.
Natural Building Systems continues to work alongside agricultural stakeholders on projects such as the Centre for High Carbon Capture Cropping (CHCx3) to develop new UK grown construction products and enhance the evidence base for the carbon capture potential of locally grown biogenic construction materials. You can read more about the CHCx3 project and our research in their latest newsletter.
[1] ‘Time to move beyond the architecture of the oil age’ – 2017 - Marc Ó Riain - Passive House Plus magazine
[2] ‘On the embodied carbon of structural timber versus steel, and the influence of LCA Methodology’ 2021 - Freya Morris, Stephen Allen, Will Hawkins
[3] ‘Temporary storage of carbon in the biosphere does have value for climate change mitigation: A response to the paper by Miko Kirschbaum’ – 2008 - Dornburg and Marland.
[4] ‘Assessing Temporary Carbon Storage in Life Cycle Assessment and Carbon Footprinting’ 2010 - Miguel Brandão and Annie Levasseur ref Jørgensen
[5] ‘Construction Stored Carbon’ 2021 – ASN Bank & Climate Cleanup
[6] ‘Dynamic LCA and Its Application to Global Warming Impact Assessments’ – 2010 - Annie Levasseur, Pascal Lesage, Manuele Margni, LouiseDeschênes, Réjean Samson
[7] ‘On the embodied carbon of structural timber versus steel, and the influence of LCA Methodology’ 2021 - Freya Morris, Stephen Allen, Will Hawkins
[8] ‘Buildings as a global carbon sink’ 2020 -Churkina et al.
[9] ‘Biogenic carbon in buildings: a critical overview of LCA methods’ 2020 - Endrit Hoxha et al.
[10] ‘Bio-based building products in the Dutch Environmental Database (NMD)’ – 2024 - Martien van den Oever
