The value of carbon storage | Heyne Tillett Steel

Could the cost to offset carbon emissions improve the cost-competitiveness of timber structures?

Natural building materials such as timber are more expensive than established manufactured materials such as steel and concrete but timber structures have lower embodied carbon^[1] and can also sequester carbon. Despite the design of timber structures being well understood in the industry, this higher cost can be prohibitive, leading clients and developers to stick with traditional steel and concrete structures and their associated higher carbon emissions. The emission of one metric ton of carbon dioxide (or equivalent) has an estimated cost to society of $258/tCO2e ^[2]. If timber avoids emissions, can the carbon benefits of timber construction be utilised to improve its cost-competitiveness?

In London, Boroughs such as Westminster request payment for overshooting emissions targets, with one tonne of additional carbon dioxide equivalent costing £2850 (over an assumed 30-year period), paid at the start of the development. This can result in developments paying hundreds of thousands of pounds to offset the carbon of “cheaper” concrete or steel frame designs. These funds pay for trees in the form of carbon offsets from forests.

This system, therefore, encourages emitting with concrete to offset with trees rather than building in timber in the first instance.

A key consideration in valuing timber fairly is how to quantify the sequestered carbon stored in it. In the fullness of time carbon storage is temporary: sequestered carbon is reported separately under RICS Whole Life Carbon Assessments, and (rightly) cannot be used to cancel out the initial upfront embodied carbon. In addition, methods of considering this sequestered and delayed emission carbon benefit need to be encouraged, due to the urgent need to decarbonise today rather than tomorrow. Arup recently published a report on the time-benefits of carbon storage, listing three benefits of delaying carbon emissions: 1. delaying buys time to avert emissions later, 2. delaying reduces the impact at a static time horizon (e.g. 2050), and 3. we should prioritise the wellbeing of society today as the existence of society in the future is not guaranteed^[3].

Methods of considering this sequestered and delayed emission benefit need to be further developed but current methods include:

        
                        1                    
                    
                        The Climate Cleanup Foundation has developed a certification protocol that allows biobased construction projects to record the carbon stored and gain financial value from it through government subsidies, selling it to companies or for inclusion in the project’s carbon accounting.  Their approach is overly favourable as it doesn’t consider the future re-emission of sequestered carbon.                     
                
                        2                    
                    
                        In France the environmental regulation RE2020 allows for a “dynamic lifecycle assessment” (DLCA) where emissions are weighted depending on when they are emitted, similar to the second approach mentioned by Arup, with future emissions being reduced compared to upfront emissions.  A hypothetical CLT structure’s embodied carbon was reduced by 21% by using DLCA with a 50-year design life rather than a typical LCA approach. DLCAs can reasonably account for the benefits of sequestration of bio-based building materials, and while being common in France, are not commonly used in the UK.

The benefits of long-term carbon storage should be considered in conjunction with a wider view that includes the reductions in offset payments, particularly at the early stages of projects when the structural frame material is being decided. Balancing timber’s higher upfront costs with offset savings in this way could help broaden timber’s use in industry which would be good for all: saving clients’ money, making healthier natural spaces for building users, and reducing the carbon and biodiversity impacts of construction for the environment in the future.

[1] Buro Happold Engineering. Structural Sensitivity Study. (2020).
[2] Bressler, R. D. The mortality cost of carbon. Nat Commun 12, (2021).
[3] Allwood, M., Wild, W. & Lau, J. The Time-Value of Carbon: An Introductory Exploration to Support Better Decision Making. (2024).

Written by

Ben Brown

Research and Innovation Engineer