When is timber the right choice?
Dr Qian LiView bio
Originally published in Architecture Magazine in April 2022.
As an architectural designer and embodied carbon specialist, timber is one of my all-time favourite materials. It is aesthetically pleasing, it gives you a sense of nature, it promotes biophilic feelings and is also a truly renewable material. You can’t grow steel or concrete just yet!
As a sustainable building material, the benefits and drawbacks of timber are a hotly debated topic. Some argue that it is a panacea, an all-encompassing solution to the net zero carbon challenges we face, while others argue that we need low carbon solutions now and that timber sequestration, the process of capturing and storing atmospheric carbon dioxide through sustainably managed forests, can take years or even decades to be realised.
While timber’s effectiveness as a renewable material depends on our ability to plant and grow sustainable forests, for the most part, it is still the most effective construction material to minimise the carbon footprint of a building and improve quality of internal spaces.
The new music school at King's College School Wimbledon, designed by Hopkins Architects, is a beautiful example of when timber was the right choice for both aesthetic and sustainable reasons. When designing the award-winning structure, the project team, which included my colleagues at Cundall, went through extensive deliberations before arriving at the choice of timber as the dominant structural material. Concrete and steel were also evaluated during the concept design stage, but in the end, timber was chosen for its sustainability credentials as well as its ability to achieve the architect’s aesthetic vision.
In this case, timber was the best structural system for the job because it was a low embodied carbon material and all elements could be fabricated offsite to precise tolerances, meaning onsite waste was negligible. It also addressed other issues including durability, insulation, lightness, thermal performance, and of course, acoustics (very important for a music school).
Naturally, we would like to create buildings like this that use as much sustainable timber material as possible. However, is it realistic to advocate for the mass use of timber in the built environment? When considering how I advise my clients on how to achieve the lowest embodied carbon on their projects, I sometimes struggle to convince myself that timber is always the right choice.
This is particularly true for complicated structural designs that might require huge amounts of timber in places where other materials like steel or masonry could be used in far smaller volumes. It is at times like these, when despite its properties as a renewable building material, the amount of timber used means it has far greater embodied carbon, that timber is not necessarily the most sustainable choice.
One example of this is Phase One of the Salford Crescent Redevelopment, a residential scheme that makes up part of the £2.5 billion Salford Crescent Masterplan, a major regeneration programme with ambitious goals of leading the way towards carbon neutrality. For this we looked at an extensive range of facade and structural materials and decided that a full timber frame was not suitable. Instead, analysis indicated that masonry construction (recycled and reclaimed bricks) combined with timber joists was the most sustainable choice because it offered the lowest upfront embodied carbon.
It’s an example of how we need to use scientific analysis and embodied carbon databases to consider the effectiveness of timber on a case-by-case basis, considering the design at every stage and renewing our embodied carbon calculations to reflect every change. Simply deciding to use timber at the outset, without considering the embodied carbon calculations that other materials might produce, is not the most sustainable way to design a building.
Another issue with timber’s reputation as a sustainable wonder material, is the opportunity for it to be exploited. One of its most attractive properties is its ability to offset the carbon emissions from other building components (foundation, building services etc) through sequestration. We have worked on projects where people want to use more timber than necessary by inserting timber backing everywhere, just to get the negative CO2e values of the timber sheet.
In an effort to discourage this practice, the London Energy Transformation Initiative (LETI) rating systems, as well as GLA and RIBA, did not include sequestration in its guidance for the calculation of upfront embodied carbon to avoid the distortion of timber sequestration. According to LETI's Embodied Carbon Primer, sequestration can only be included in calculations when you consider the end-of-life impact of timber elements (for instance, the carbon emission if we incinerate it for energy or the methane emissions should it be landfilled).
Ultimately, when we consider the embodied carbon of materials, less is always more. While timber does have a lot of qualities that make it seem like the perfect solution to the challenges of achieving net zero carbon (in new builds at least), the reality is that the design and intended use of a building may mean it isn’t suitable. As designers, it is our job to ensure that we consider the embodied carbon of the materials used at every stage, from concept design to construction. If we do not, we are failing in our responsibility to help our industry decarbonise and achieve net zero carbon.