One of the main drivers for afforestation is increased demand for timber products. Therefore, increasing use of responsibly sourced timber in construction can help to maintain and increase the forest resource.
Timber certification schemes, requiring responsible forest management, play an essential part in this approach. Certification schemes, such as the ‘Forest Stewardship Council’ (FSC) and the ‘Programme for the Endorsement of Forest Certification’ (PEFC) have been in existence since the 1990s. They were the early drivers for the responsible sourcing standards.
The embodied carbon of a material is the net carbon dioxide (CO2) emissions produced during the processes involved in its production and disposal. Carbon dioxide in this context includes all the greenhouse gases rolled up into a single quantity referred to as carbon dioxide equivalent (CO2e).
During the production of timber, trees absorb, or sequester CO2 from the atmosphere during photosynthesis as they grow. This is stored as carbon in wood, bark and leaf material. The carbon is then partly released back into the atmosphere when trees die and decay, with a proportion being retained in the soil and forest biological systems. If the area of forest remains constant, there is a net uptake of CO2. Additional uptake occurs if the area of forest increases, but if the area decreases there is likely to be a net outflow of CO2.
If wood is used in timber products there is a further increase in net uptake as its sequestered carbon continues to be stored for as long as the products remain in use. 1kg of (oven-dried) wood contains approximately 0.5kg of carbon, which represents approximately 1.8kg of absorbed carbon dioxide for each kg of construction timber used.
Some of this sequestered carbon is offset by the various processes involved in forestry work (eg planting, maintenance and harvesting) and in the wood processing activities (eg debarking, sawing, drying, planing). The larger number of processes involved in more highly engineered products (eg plywood, CLT and Glulam) increases the amount of processing CO2 produced. For most timber construction products process emissions are less than the amount sequestered, giving a net negative embodied carbon for the finished product.
According to WRAP4, 81% of UK construction timber is recycled or reused, with most of the remainder going to landfill. This compares with 99% for metals and 59% for concrete. The rate of timber recycling and reuse is expected to improve due to the UK Landfill tax escalator.
It is not generally possible to recycle timber in the true meaning of the word, by reprocessing into into a new product with identical properties, as with steel or aluminium. But it can be reused, recycled into lower grade products (eg wood particleboard, animal bedding, soil mulches) and used as biomass for energy production where it can substitute fossil fuels.
Landfilled timber slowly decomposes anaerobically giving off methane, a potent greenhouse gas. A proportion of this can be collected and used for energy production but the remainder is released into the atmosphere. However, according to Doka5, only around 3% of landfilled wood carbon is converted to methane. This can be viewed negatively, as only a very small amount of the potential energy in the waste timber is utilised, or positively because more than 96% of the carbon in the landfilled timbers remains locked up in the ground.
BS EN 15804 and related standards consider additional sustainability indicators for construction works:
- Ozone depletion
- Acidification of land and water
- Eutrophication
- Resource, energy and water use
Data from environmental product declarations (EPD) does not indicate any clearly consistent sustainable benefit for timber in terms of these indicators, but water usage is generally high for timber in comparison with other materials. This is partly due to inclusion of rain water usage during tree growth.
Forests are major global carbon sinks, absorbing atmospheric carbon. Afforestation and increased use of timber in long-use forest products, in a proven carbon capture and storage system, is one of the available strategies to mitigate carbon emissions and the climate emergency.
References
[1] British Standards Institution (2012) BS EN 15804:2012, Sustainability of construction works — Environmental product declarations — Core rules for the product category of construction products, London: BSI
[2] Timber Trade Federation (2013), Statistical Review 2013. Industry Facts and Figures for the Year 2012. London
[3] United Nations Statistics Division (2010) Change of Forest Area from 1990 to 2010. (adapted)
[4] WRAP (2009) Management of non-aggregate waste
[5] Doka, G. 2007. Building material disposal. Ecoinvent Report 13 - Life cycle inventories of waste treatment services