Timber structures Sustainability
Are timber structures good for the planet?
Will Hawkins investigates how a sustainably built environment offers carbon benefits.
T
he 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 www.bmtrada.com
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. >> Timber 2022
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