A Case Study to Investigate the Life Cycle Carbon Emissions and Carbon Storage Capacity of a Cross Laminated Timber, Multi-Storey Residential Building
About this book
A Case Study to Investigate the Life Cycle Carbon Emissions and Carbon Storage Capacity of a Residential Development Built Using Cross-Laminated Timber, authored by H.J. Darby, A.A. Elmualim, and F.
Kelly and presented at the Sustainable Building Conference (SB13) in Munich in April 2013, is a rigorously quantified contribution to the body of evidence supporting mass timber construction as a credible low-carbon alternative to conventional structural systems in multi-storey residential development. The study addresses a specific and practically significant research question: when a residential building is constructed using a cross-laminated timber structural frame, what is the net carbon position across the full life cycle — including material production, construction, operational energy use, and end-of-life treatment — compared with an equivalent reinforced concrete frame solution? Cross-laminated timber, the structural system at the centre of the analysis, is an engineered wood product manufactured by bonding successive layers of timber boards at right angles under pressure, producing large-format panels of considerable dimensional stability and structural capacity.
Its relevance to whole-life carbon accounting is twofold: first, CLT has a substantially lower embodied carbon footprint than concrete and steel structural systems because timber growth sequesters atmospheric carbon dioxide and manufacturing the panels requires far less energy than producing cement or primary steel; second, the timber in a CLT structure continues to store the carbon that was sequestered during the growth of the source trees for as long as the material remains in use in the building, meaning the structure functions as a carbon reservoir as well as a structural system. The research team constructed a detailed whole-life carbon model for a multi-storey residential development, tracing emissions and sequestration through the complete building lifecycle. Embodied carbon — covering raw material extraction, processing, manufacturing of products and components, and the construction process itself — was calculated for both the CLT and the reference reinforced concrete scenario.
Operational carbon — arising from energy consumption in heating, cooling, hot water, lighting, and appliances over the building's anticipated service life — was assessed on equivalent terms for both structural systems, allowing the relative contribution of the structural frame choice to the total whole-life carbon budget to be isolated. The findings on biogenic carbon storage are among the most striking quantitative outcomes of the study. The total carbon stored within the CLT structural frame was calculated at 1,215 tonnes of CO2 equivalent across the development — equivalent to approximately 30 tonnes of CO2 per individual housing unit.
This stored carbon represents a genuine reduction in atmospheric carbon load relative to a scenario where the timber had not been harvested and converted into a long-lived building product, assuming the source forests are sustainably managed and regenerating. The end-of-life treatment scenarios reveal the high sensitivity of the whole-life carbon outcome to decisions made at the building's end of service. Under a reuse scenario — where the CLT structural panels are recovered and deployed in another building application — the whole-life embodied carbon of the structural frame reaches -1,017 tonnes of CO2 equivalent, meaning the CLT structure represents a net carbon benefit over its life rather than a net liability.
At the opposite extreme, incineration without energy recovery results in a positive figure of +153 tonnes of CO2 equivalent as the stored biogenic carbon is released back to the atmosphere without offsetting benefit. Intermediate scenarios including landfill, incineration with energy recovery, and structural reuse within the same building sit between these poles, but all scenarios examined resulted in a lower total emissions outcome than the equivalent reinforced concrete frame. This comparison with conventional construction is the study's most directly applicable finding for practitioners.
The reinforced concrete alternative consistently showed higher net carbon emissions across all end-of-life scenarios modeled, confirming that the carbon advantage of the CLT system is robust rather than contingent on a single optimistic assumption. The authors discuss how this advantage accumulates from multiple sources: lower production emissions, the biogenic storage effect, and the potential for material recovery at end of life. The study's methodological contribution lies in its integrated treatment of biogenic carbon, embodied carbon, and operational carbon within a single quantified model, at a time when many practitioners still focused on operational performance alone.
It remains a foundational reference in the literature on CLT whole-life carbon assessment and continues to inform the design decisions of developers, structural engineers, and sustainability consultants evaluating mass timber against conventional alternatives.