9 January 2018
By Priyanka Arora
Sustainable Timber in Construction
If sustainably sourced, timber is undoubtedly one of the most environmentally friendly materials currently available, being a natural carbon sink and truly renewable. This has made timber a very popular material among sustainability champions and protagonists of green construction. Although its use in construction dates back many centuries, the advent of Cross Laminated Timber (CLT) has now made it possible to construct complete buildings in timber.
What is Sustainable Timber?
Sustainable timber refers to timber that has been harvested responsibly from well managed forests that are continuously replenished; and ensure that there is no damage to the surrounding environment, or to native flora and fauna.
In the UK two main certification schemes, FSC and PEFC, assure that all wood and wood-based products originate from sustainable sources. At KLH, we help our clients and partners ensure timber is procured from certified, sustainable sources.
Timber and climate change
Timber has a significantly lower embodied carbon footprint compared to other mainstream construction materials. This is due to the minimal processing required, even when accounting for the process of laminating (glulam or cross-laminated) which is generally required to create structural timber products.
If sustainably sourced, using timber can have an additional positive environmental impact because trees absorb carbon dioxide through photosynthesis and lock it away as carbon, thus removing it from the atmosphere. This phenomenon is called sequestration and can essentially offset the processing and transportation energy associated with timber products. Therefore, timber can be considered a carbon negative material. It is however important to remember that the sequestered carbon will be released at the end of life of the timber product (unless it is reused or recycled). The global warming implications of disposal options vary and are detailed in the table below.
It is interesting to note that in response to the Paris Agreement (COP21), scientists proposed a range of “negative emissions technologies (NETs) in order to limit climate change to “well below 2C”, three of which relate to timber and its capacity to absorb and store carbon from the atmosphere: afforestation and reforestation, building with biomass and biomass with carbon capture and storage.
The recently published UK’s 25 year environmental plan also recognises the significance of using sustainable resources, and specifically recommends increasing supplies of timber.
Timber and life-cycle impacts
Recent studies have found that the life cycle emissions from a CLT framed building (without including sequestration) can be about 30-50% lower than a typical concrete framed building. When sequestration is included, the benefit can be much more significant. However, it is important to note that these results assume that 100% of the timber is diverted from landfill at the end of its useful life. If timber is landfilled at the end of its useful life, analysis has suggested that the net emissions from a CLT framed building could exceed the life cycle emissions from a typical concrete framed building.
When timber is landfilled it rots and releases up to 60% of the sequestered carbon back to the atmosphere as methane, which is 25 times worse than carbon dioxide in terms of global warming impact.
The table below shows the net life cycle emissions for different end of life scenarios for timber. It illustrates the importance of considering the end of life of timber (and other construction products) used in construction.
At KLH Sustainability, we always explore the wider implications of sustainable solutions with our clients to ensure that we are not creating problems for future generations!
CLT life cycle emissions in kgCO2e
Source: Darby, Elmualim and Kelly (2013)
 The case study has been assessed in accordance with EN 15978 and does not include carbon sequestration for timber within the frame of the study