Is it time to consider the carbon potential of timber?
Dr Morwenna Spear FIMMM discusses a project investigating embodied carbon and carbon storage in the construction sector.
2019 saw a global shift towards addressing climate change. In part, this was driven by people, think of Greta Thunberg and the distinctive pink ark of Extinction Rebellion. But it was also marked in governments – for instance, the UK moved from a commitment to reducing emissions by 80% by 2050, to the intention of reaching net-zero by 2050 instead.
Several reports produced by the Committee on Climate Change (CCC) reached the headlines. Possibly the most controversial policies gained the greatest time in the spotlight, such as should we be going vegan?, and will we all be driving electric cars in future? Many of the projections and suggested routes to reducing carbon emissions require lower effort and are less controversial, so can be easily overlooked. But if these are enacted together, they could achieve large emission reductions.
Quantifying timber’s contribution
One such question posed was, should we be using more timber? This was considered in a 2019 study, Wood in construction, commissioned by the CCC, and also fed into its 2018 review, Biomass in a low-carbon economy.
The main focus of the work was to weigh up whether it was of greater benefit to burn wood chip for bio-energy – displacing coal or other sources of electricity generation – or to use the timber in structures such as houses, offices, schools and shops.
The assessment of potential greenhouse gas abatement through using timber rests on two distinct, but complementary, effects. Firstly, timber has a low embodied carbon value, which is assessed as the global warming potential (GWP) in lifecycle assessment studies. When timber is used in place of other materials, which may have higher embodied carbon values, there can be a reduction in the embodied carbon of the structure.
This can be assessed for a building in order that the design of the building can be fine-tuned to reduce its carbon footprint, while ensuring that all other design criteria are met, e.g. cost, energy-efficiency and safety. Secondly, timber acts as a store for the carbon sequestered by the growth of that tree in the forest. When the timber is used in a long-life application, such as a trussed rafter or in a timber-framed house, the duration of this storage may be very long.
Houses in the UK are typically designed on the basis of a 60-year service life, but in practice, people continue to occupy them well beyond this date. Therefore, carbon stored in the structural elements of the building may be considered to be stored in a pool of sequestered carbon, preventing its release back to the atmosphere until eventually the house is re-modelled or demolished 60-100 years into the future.
It is possible to quantify both of these effects. Within the study for the CCC, this was done for different types of dwelling – detached and terraced houses, bungalows and flats – and for different construction systems. The main comparison was between a masonry house – brick and block with cavity insulation – and an open panel timber-framed house. Both of these systems are widely used across the UK. The Structural Timber Association reported in 2017 that the current level of timber frame usage is 28.1% of all housing starts.
An additional comparison was carried out for flats, where concrete-framed and cross-laminated timber (CLT) structures were considered. CLT is a relatively new technology in the UK, but uptake is increasing rapidly (see Materials World, September 2018, page 56).
For each dwelling and each construction system the materials present in the structural elements were quantified, so that embodied carbon could be calculated for the house or flat.
Using the same data on quantities of materials present, the timber and wood elements were used to quantify stored sequestered carbon present in that unit.
First let’s consider the effects for embodied carbon. A typical house contains concrete, bricks, concrete blocks, mortar, timber, various forms of insulation, and so on. The actual volumes of bricks, blocks and insulation used are altered by the design details of the project. In a timber-framed house, the walls are still faced with bricks, but the structural element is formed of timber panels filled with insulation, placed inside this exterior face. Thus, quantities of bricks may be broadly similar, but concrete blocks greatly reduced for this example. Insulation used may be of a different type, and different quantity, to achieve the required performance under Part L of the building regulations.
It is important to consider the complete dwelling unit, with equivalent thermal performance and service life in order to make a sensible comparison. By tallying the volumes of materials and the associated embodied carbon for each of these, it is possible to derive a GWP value for the dwelling, and to compare this with a similar one formed using a different structural system.
In our study, the change of materials within the dwelling led to a reduction in embodied carbon for timber-framed houses, compared with masonry houses. Approximately three tonnes (t) of carbon dioxide equivalents (tCO₂e) were saved for a detached house, and approximately 2tCO2e for a mid-terraced house, relating to the different exterior and interior wall relationship.
Many details within any house can be optimised if embodied carbon becomes part of the decision-making process during design. For example, the type of brick used can have different embodied carbon (kiln temperatures, ingredient mix etc.) just as the type of concrete (type of aggregate, recycled content etc.) or insulation (glass fibre, mineral wool, polyurethane foam, blown cellulose etc.) can also alter this parameter.
There is a set of guidance for chartered surveyors and for architects in this area, from both RIBA and RICS. So there is potential to make greater use of this criterion at the design stage, if seeking to reduce the carbon footprint of new buildings.
Secondly, when bio-based materials such as timber are used, there is a carbon storage benefit as the carbon in the timber has already been sequestered during growth of the tree. The duration this carbon remains stored in the material can be altered by the application in which it is used. If more timber is used in longer term applications, this carbon storage benefit lasts longer, keeping this carbon dioxide out of the atmosphere until the end of the product life. In the case of construction, this is substantial in both volume and time.
For shorter duration applications, such as paper, recycling offers a method to extend the storage for multiple lives. Many other timber applications have lifespans between months and decades.
For medium-term items in buildings, e.g. window frames, fitted kitchens, standard replacement intervals can be used, in BS EN 15978, or when looking at national carbon inventories a stocks and flows approach can be taken.
For the individual dwellings, we demonstrated a significant increase in stored sequestered carbon for the timber-framed system, compared with the masonry system. The increase was between 33% and 50%, compared to the masonry values. Remember, timber is the predominant material in roofing trusses and floor joists, as well as the widespread use of wood-based panels in the flooring surface, so can already be substantial within masonry systems. For the cross-laminated timber flats, a five-fold increase in stored sequestered carbon was seen, compared with concrete framed flats.
The big picture
As before, there is considerable scope to fine-tune this parameter when designing new structures and increase the timber content to provide carbon storage. The study excluded joinery timbers – skirtings, door architraves etc. – or fitted kitchens. Although these elements are replaced more often than the structural parts, they still offer medium to long-term storage. Using timber cladding for exterior walls or a proportion of exterior wall area, would offer a different approach to increasing carbon storage if this becomes adopted as a carbon mitigation strategy.
The big picture remains important. For each new dwelling there is scope to reduce carbon footprint, as well as store sequestered carbon. If we think at national level, there were 206,680 new housing starts in 2018, and if each house saved 2-3tCO₂e through materials choice, this would be substantial. The study indicated this was possible, and the saving can be even higher for substituting concrete by CLT.
The use of timber and low GWP materials offers real opportunity to reduce carbon emissions, if designs are optimised in this area. The construction sector has a target to reduce emissions to 50% of 1990 levels by 2025. Materials choices must become part of the method for delivering this change.
*Dr Morwenna Spear FIMMM, is a research scientist at the BioComposites Centre, Bangor University, UK. She is also Vice-Chair of the IOM3 Wood Technology Society.