Timber is widely regarded as the obvious solution in the quest for a sustainable construction material, but it is not as simple as that. Simon Wyatt considers the evidence – and draws a cautious conclusion
There is lots of conflicting talk around timber in the construction industry, with many declaring it a panacea, an all-encompassing solution to the net zero carbon challenges we face. But is it really?
In an ideal world, where we have abundant supplies of sustainable resourced timber from well-managed forests, potentially it is. But, in the real world, it is much more complicated and sadly there is no silver bullet when it comes to the climate emergency.
Over the course of this Countdown to COP26 series, I have explored many of the challenges and possible solutions we have when it comes to decarbonising the built environment. As COP26 draws ever closer, the debate around the use of timber as a net zero solution is heating up.
The main benefits of using timber are well known; trees are a renewable, natural resource and they absorb carbon when they grow. This is referred to as “carbon sequestration”, which is the process by which carbon dioxide is removed from the atmosphere and incorporated as “biogenic carbon” in “biomass” through photosynthesis and other processes associated with the carbon cycle. However, accounting for sequestered carbon is often a source of debate, confusion and inconsistency.
In simple terms, growing trees and locking away carbon in timber buildings can provide significant carbon sinks. But would it be better to leave forests to grow naturally? Do we have time to plant new forests?
Steel and timber are framed as direct alternatives but it is not a straight choice. It also depends on the relative quantities required
The recent report from the Intergovernmental Panel on Climate Change (IPCC) indicates that the climate emergency is happening now and that we are at “code red”, meaning we need low carbon solutions that have an immediate impact. When we look at timber sequestration, we can see that this lag can take years or even decades to be realised. So, should we really be cutting down any trees in a climate emergency? Should we not be planting as many as we can?
This, however, is more complicated when we consider land scarcity and the variation in sequestration rates based on a tree’s maturity. Carbon uptake in newly planted saplings is initially slow but then accelerates as these become established. In an unmanaged forest, sequestration continues until the total carbon eventually tends towards a steady state, meaning that the greatest sequestration occurs when a tree is in its young growing stage.
A managed forest can therefore achieve carbon storage similar to or even greater than that of an unmanaged forest between harvesting cycles. But the greatest benefits come from the carbon storage in the products produced from it. If these are amassed sufficiently over time, then the total carbon sequestered accumulates and could eventually be greater than that of an unmanaged forest, assuming that the timber is used for a significant period to allow for its replacements to sequester enough carbon to offset its production.
The question is what happens to it at the end of its life. Will it be sent to landfill or combusted? If so, then the carbon released in the process will undo most of the gains. The answer is that all timber must be grown in sustainably managed forests that replace felled trees with an equal or greater number of new trees than are cut down.
Updated guidance from the London Energy Transformation Initiative, the Whole Life Carbon network, the Institution of Structural Engineers and the RIBA advises that sequestration should only be aggregated with emissions when end-of-life values are also included, and where the stored carbon is typically cancelled out by re-emission at the end of life.
Carbon accounting for timber should therefore not start at negative. This means that credit should not be taken for a tree planted 50 years ago, even if this eventually ends up being used to construct a building. The benefit should be taken during its use. Where trees are harvested and not replaced (deforestation), no sequestration should be accounted for, which is in line with current European standards.
Without sequestration, timber buildings still typically have lower carbon footprints than traditional steel or concrete structures. However, where high levels of cement replacements or recycled steel are used, then the numbers can be very close. This is due to the production emissions for timber products, from harvesting, drying and sawing, which can be significant.
Steel and timber are framed as direct alternatives but it is not a straight choice. It also depends on the relative quantities required – the aim is the lowest overall impact for the building, not the lowest carbon material. But what is clear is that, if we are simply looking at upfront embodied carbon which affects carbon emissions now and not in the future, then retrofit projects that reuse significant parts of an existing structure are almost always the best option, despite some projects claiming the sequestration benefits to justify demolition and new-build. Additionally, as repurposed steel and material passports become the norm, then steel has the potential to be significantly lower for the upfront embodied carbon as well as for the end of life, because it can form part of a circular economy.
If all things are equal, or carbon neutral, then the question is: what is more recyclable and reusable, steel or timber? Steel can be more easily reused, repurposed and even melted down and reshaped, whereas timber is a lot more difficult to reuse. So, if we are looking at it from a circular economy point of view, steel makes more sense.
There is also the fire concern around timber safety, which can be addressed through the right approaches, but the embodied carbon of the whole solution (fire-proofing materials) must be considered in addition.
So timber is not a silver bullet. Don’t get me wrong, in most new-build cases it is a great solution, assuming it is used efficiently and is sustainably sourced. But, as sustainably sourced timber is currently limited, we must use it wisely: on buildings where it can be locked away for long periods of 60 years plus or even over 100 years.
Low embodied carbond design hierarchy
1/ Rationalise and reuse existing buildings
2/ Retrofit and repurpose existing buildings
3/ Repurpose and recycle materials using circular economy and whole-life carbon principles
4/ Prioritise natural low carbon materials, while considering material efficiency
5/ Design for flexibility and durability to ensure carbon sinks are long-life
6/ Consider end of life, prioritise circular economy principles
It should not be used in short-life structures such as in retail, or in short-life products, for instance as a fuel source in power stations, as a way of playing accounting games with grid emission rates. Timber currently is a finite resource and so it should be treated as one – over‑consumption should not be encouraged.
The timber solution of sustainable forestry is potentially a panacea, but we are talking about a supply chain system that will take decades. Is that too late? Given the urgency of the climate emergency, the answer is yes. We need to look at a raft of measures and these will evolve over time, but they should always start with the idea of “retrofit first”.
Simon Wyatt is sustainability partner at Cundall