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Timber structures: sustainability Are timber structures good for the planet?
Are timber structures good for the planet?
Will Hawkins investigates how a sustainably built environment offers carbon benefits.
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The prospect of long-lived timber products, such as building structures, creating an anthropogenic carbon sink and thereby acting in opposition to climate change is a tantalising and exciting one for structural engineers. As well as being described in scientific literature,1 this idea is gaining attention in mainstream media2 as the public appetite for positive climate solutions understandably grows.
The equivalent mass of carbon dioxide stored within timber is greater than that of the timber itself, at around 1.64 kgCO2/ kg. Timber has been considered a carbon-negative material on projects, with designers claiming that biomaterials, used in sufficient quantities, can ‘more than compensate’3 for emissions used in concrete foundations and steel connections, creating structures with negative embodied carbon. Is it true that increased material consumption can lower emissions? Is this a sustainable design approach?
Understanding carbon
In discussing embodied carbon, a distinction is made between fossil carbon, emitted through combustion of fossil fuels, and biogenic carbon, which is sequestered via photosynthesis, stored in biological matter such as timber, and typically re-released through combustion or decomposition at end of life as part of a cycle.
Fossil carbon
For most timber produced today, significant quantities of fossil carbon are released through growing, planting and protection of seedlings, construction and maintenance of access roads, thinning, harvesting, debarking, limbing, sawing and kiln-drying (if gas-fired), as well as the production of surface treatments, adhesives and any post-processing into engineered products. These emissions occur at the beginning of a building’s life cycle, creating an immediate and longlasting warming effect.
Transport distances to factories and then to site can also be large, as can the quantity of timber wasted through offcuts. Despite this, studies show that timber building structures have a lower embodied fossil carbon than concrete and steel equivalents.4 This gap is likely to increase in future, since most timber production processes can be electrified relatively simply, leading to reduced emissions with grid decarbonisation, whereas the production of cement and steel typically features hard-to-avoid emissions from chemical reactions and the high temperatures that drive them.
Biogenic carbon
Biogenic carbon is removed from the atmosphere as trees grow and can be stored within timber structures. The longer this remains locked away, the greater the potential climate benefits. If a building gets demolished, the fate of its biogenic carbon depends on the waste disposal method and cannot be predicted with certainty.
In the UK, the majority of waste timber is currently burnt as biofuel to generate electricity,5 offsetting grid production. The remainder is largely downcycled into chipboard, animal bedding, MDF or compost, most of which are short-lived products likely to re-enter the waste stream or decompose quickly. Less than 1% of the UK’s waste timber goes to landfill. Based on these statistics, it is currently reasonable to assume that most of a timber structure’s biogenic carbon re-enters the atmosphere shortly after demolition. We might reasonably hope and expect that this may not be the case in future, perhaps due to a reduction in demolition rates, new expertise in component re-use or even new technologies for bioenergy with carbon capture and storage. However, today’s codes of practice for life cycle carbon assessment stipulate that end-oflife assumptions must be based on today’s norms.
Climate benefits
Even when temporary, the storage of biogenic carbon in longlived structures has climate benefits. Although harvesting a tree stops its growth, it enables the re-planting of new saplings which, once they reach their vigorous growth phase, sequester carbon more rapidly than mature trees. If this is combined with the steady accumulation of biogenic carbon in timber products, then the total quantity can, in theory, exceed that of a mature forest (Figure 1). This highlights the benefit of locking timber away in structures compared to alternative use as biofuel or in short-lived products. The longer the structure remains in use, the better. >>
Figure 1: Biogenic carbon storage example for a UK stand of sitka spruce with 50-year rotation period, plus accumulation in forest products, compared to a similar unmanaged forest6
Despite the potential benefits of biogenic carbon storage, the most recent 2019 version of the European Standard EN 15804,7 which covers Environmental Product Declarations (EPDs), does not enable these to be fully captured.8 It takes a product system approach, where any biogenic carbon entering must also leave at end of life, specifically forbidding permanent biogenic carbon storage. However, there are several options available to designers wishing to communicate the benefits of biogenic carbon storage:
1.Report biogenic carbon separately, acknowledging its temporary nature.
2.Show the life cycle embodied carbon of a project graphically, highlighting the timing of fossil and biogenic carbon fluxes.
3.For a more detailed analysis, any carbon emission history can be converted directly to climate impacts including absolute temperature change using dynamic life cycle assessment (DLCA).9
The latter two options require an assumption to be made about the timing of biogenic carbon sequestration. As well as the ‘instantaneous’ approach adopted by EN 15804, which models biogenic uptake with production, alternative ‘backwardslooking’ and ‘forwards-looking’ approaches are sometimes considered, particularly when using DLCA. The former accounts for past sequestration from harvested trees, whereas the latter starts at zero and synchronises sequestration with those replanted, thereby capturing the importance of replanting and the benefits of using faster-growing species.9
Other factors
Much of the discussion and analysis of timber structures is focused, understandably, around carbon and climate. However, biodiversity loss driven by pollution and habitat destruction is an equally urgent crisis. Across the EU, forest areas have increased by 9% since 1990,10 although one-third of the total are single species. Compared to multi-species forests, these monocultures are worse for biodiversity, soil health, recreational value and resistance to disturbance (and carbon loss) from pests, fire and wind.11 While biodiversity is considered a key element of sustainable forestry, evidence suggests that there is an inverse relationship between biomass production and biodiversity conservation,12 although of course steel and concrete also require land to produce and thus negatively impact biodiversity.
Another common question surrounding increased uptake of timber is supply. Across EU forests, 73% of the annual increment is currently harvested,9 total stored carbon is increasing and there is some scope for additional removals. However, demand for forest products is set to increase dramatically in the coming decades, driven by rising demand for low-carbon materials and biofuel for transport, heating and electricity.
One study13 estimates that timber demand will outstrip supply by 40% to 100% in the EU by 2050, based on current decarbonisation plans. At a global scale, the World Bank predicts a quadrupling of demand for roundwood by 2050. Ideally, long-lived products should be prioritised, re-used and then recycled progressively with energy only as a final option; however coming decades will likely see increased competition for timber across all users. The UK is already seeing record timber prices, due in part to supply and import constraints, but also to demand being at an ‘all-time high’.14 Meanwhile, concrete prices have remained steady (Figure 2); lean design will be increasingly important if timber structures are to remain cost competitive. >>
Figure 2: Most structural timber used in the UK is imported. The price of timber imports to the UK have more than doubled since 2015, while that of concrete has remained flat.15
Conclusion
Although many of the issues surrounding timber, the climate and biodiversity are nuanced and hotly debated, several key principles for sustainable design and specification can be concluded:
• Design with timber in mind – Timber frequently has a lower embodied fossil carbon than alternatives, regardless of biogenic carbon storage.
• Know your material – Sustainable certification (e.g. FSC® ,
PEFCTM) is a minimum requirement, but practices can still vary considerably. It’s best to trace and investigate supply chains, forestry practices, rotation periods and transportation distances wherever possible.
• Delay re-release of biogenic carbon – This can be achieved by designing for durability and component re-use, diverting low-value fuelwood into structures, or reusing timber destined for recycling, incineration or landfill.
• Design lean – Efficient material use remains the most sustainable approach to timber structures, minimising fossil emissions, costs and pressure on land and biodiversity while ensuring that more projects can make use of timber: a valuable, sustainable, yet finite resource. n
About the author
Will Hawkins Lecturer in Structural Engineering Design University of Bath
Further reading
To find out more about timber and sustainability, visit www.trada.co.uk/sustainability
References
1. Churkina, G., et al., ‘Buildings as a global carbon sink’, Vol 3,
Nature Sustainability, 2020, pp269–276 (www.nature.com/articles/ s41893-019-0462-4)
2. Smedley, T., ‘Could wooden buildings be a solution to climate change?’, BBC, 2019 (www.bbc.com/future/article/20190717climate-change-wooden-architecture-concrete-global-warming)
3. Fairs, M., ‘Serpentine Pavilion’s use of biomaterials “more than compensates” for concrete emissions, says Aecom’, dezeen, 2021 (www.dezeen.com/2021/06/16/carbon-emissions-serpentinepavilion-biomaterials-concrete-aecom) 4. Hart, J., D’Amico, B. and Pomponi, F., ‘Whole-life embodied carbon in multistory buildings: Steel, concrete and timber structures’, Vol 25, Issue 2, Journal of Industrial Ecology, 2021, pp403–418 (https://onlinelibrary.wiley.com/doi/10.1111/jiec.13139?af=R);
Skullestad, J. L., Bohne, R. A., Lohne, J., ‘High-rise Timber
Buildings as a Climate Change Mitigation Measure – A Comparative LCA of Structural System Alternatives’, Vol 96, Energy
Procedia, September 2016, pp112–113 (www.sciencedirect.com/ science/article/pii/S1876610216307512?via%3Dihub); Himes, A., Busby, G., ‘Wood buildings as a climate solution’, Vol 4,
Developments in the Built Environment, November 2020 (www.sciencedirect.com/science/article/pii/S2666165920300260#)
5. For more information, see WIS 2/3-59 Recovering and minimising wood waste, BM TRADA, 2020
6. Adapted from Morison, J., Matthews, R., Miller, G., Perks, M., Randle, T., Vanguelova, E., White, M., and Yamulki, S., Understanding the carbon and greenhouse gas balance of forests in Britain. Research
Report, Forestry Commission, UK (No.018), 2012
7. BS EN 15804:2012+A2:2019 Sustainability of construction works.
Environmental product declarations. Core rules for the product category of construction products, BSI, 2021
8. Anderson, J., Assessing the carbon-related impacts and benefits of timber in construction products and buildings, Timber
Development UK, Technical Paper, 2021
9. Hawkins, W., et al., ‘Embodied carbon assessment using a dynamic climate model: Case study comparison of a concrete, steel and timber building structure’, Vol 33, Structures, October 2021, pp90–98 (www.sciencedirect.com/science/article/pii/
S2352012420307323)
10. State of Europe’s Forests 2020, Forest Europe, 2020 (https:// foresteurope.org/wp-content/uploads/2016/08/SoEF_2020.pdf)
11. Felton, A., Lindbladh, M., Brunet, J. and Fritz, Ö., Forest ecology and management, 260(6), 2010, pp939–947
12. Naumov, V., et al., ‘How to reconcile wood production and biodiversity conversation? The Pan-European boreal forest history gradient as an “experiment”’, Vol 218, Journal of
Environmental Management, July 2018, pp1–13 (www. sciencedirect.com/science/article/pii/S0301479718303281?casa_ token=dOXam2xFyVQAAAAA:M_dQAz5n00yiE3tEb_3qF4NVv4P 3zKDWge8VOYWj-fBzrJdm1FfBGUTt7NTg6eXucKED4Itj)
13. EU Biomass Use in a Net-Zero Economy: A course correction for EU biomass, Material Economics, 2021 (https:// materialeconomics.com/publications/eu-biomass-use)
14. Trading Post-Brexit, Timber Trade Federation, 2021 (https:// ttf.co.uk/download/trading-post-brexit-report-timber-tradefederation-member-survey-feb-2021)
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Picture courtesy of Stronghold Preservation Ltd
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Picture courtesy of Constructional Timber Ltd
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