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For the TRADA study, sequestration is considered to be the most important factor

Carbon profit and loss
Published:  25 March, 2010

TRADA membership & marketing manager Rupert Scott discusses a new study designed to quantify the carbon footprint of different timber species

The timber industry has long been confident that, in terms of its carbon footprint, timber’s environmental credentials are sound. However, engineers and designers want to see the numbers that back up this premise.

Measuring CO2 emissions isn’t like using a ruler or a set of scales, though. It’s more akin to profit and loss accounting, weighing up what is paid in against what is taken out. Assessing the carbon footprint of any material is therefore complex. Nor can a comparison be made per unit measure of one material against another – the quantity of timber required for a given application will differ from the volume of concrete, steel or plastic needed for the same purpose.

One of the problems with carbon footprinting and similar studies is that, regardless of how well controlled the processing of the data, the outcome can vary greatly depending on where the boundaries are drawn – ie factors included/excluded – and assumptions made about, for example, end of life treatments and the level of atmospheric CO2 sequestered within the wood. For timber products, the level of sequestration of CO2 has a big influence on the final result.

TRADA commissioned Davis Langdon to undertake some calculations using their in-house carbon footprinting model. This uses protocol similar to the recently published Publicly Available Specification (PAS) 2050, which aims to develop product carbon footprinting by building on established BS EN Life Cycle Assessment (LCA) methods. Following PAS 2050 guidelines, a product carbon footprint assesses the environmental impact of a selected product in terms of the CO2 released throughout its life cycle, either entire or partial, depending on the study being undertaken. Where greenhouse gases (GHG) other than CO2 result as part of the life cycle, these are added to CO2 emissions and the combined value expressed as carbon dioxide equivalent (CO2e). 

The work commissioned by TRADA compares different species grown in different parts of the world. It also considered different assumptions, for example, about how much CO2 is absorbed from the atmosphere and sequestered within the timber. For each timber type, the study assumed that primary processing had been undertaken at point of harvesting and that the timber had been transported to central England for manufacture, then taken a further 200km for delivery to site, a nominal figure to represent distribution in the UK.

The scenarios investigated are:

For housing, it is appropriate to assume more than 100 years of use

  • Sitka spruce, Scotland, preservative treated – cladding
  • Redwood, Sweden, hydro power – timber frame
  • English oak, central England – green oak timber frame
  • Iroko, Cameroon ­– decking

For the TRADA study, sequestration is considered to be the most important factor. Thus, if timber is credited with the CO2 that it absorbs from the atmosphere during its service life, it can be shown to have a negative carbon footprint. However, as the level of  sequestration falls, the performance in carbon footprint deteriorates.

Where sequestration is included at 100% or 70%, the carbon footprint of timber can be negative, regardless of the end of life option. PAS 2050 takes the stance that products formed from plant-based carbon (not fossilised carbon) are acting as a carbon store by extracting CO2 from the atmosphere and so are generating negative CO2 emission and provides a method of determining the appropriate level of sequestration to include. The longer the timber is to be kept intact, the greater the level of sequestration that should be included. PAS 2050 states that for 100 years of life or more, 70% should be taken – hence the inclusion of this proportion in our tables.

This stance is somewhat at odds with the Inventory of Carbon and Energy (ICE), a database produced by the University of Bath, which provides an inventory of embodied carbon and carbon co-efficients for building materials. ICE considers the embodied energy (carbon) of a building material to be the total primary energy consumed (carbon released) during its life cycle, ie extraction, manufacturing, transport.

While the University of Bath does acknowledge that the carbon sequestration for timber is a real benefit, it leaves users of the ICE data to make their own amendments where they are justified. PAS 2050 is more explicit in suggesting that account should be made of carbon sequestration, however, and suggests a formula to calculate the effect it will have. For timber in, for example, housing, it is quite appropriate to assume 100 years-plus of use, so using PAS 2050 guidance one would take 70% of the sequestered values – which are shown in Tables 1 and 2.

TRADA intends to develop its study with further scenarios. It will be critical to ensure that the sequestration of carbon throughout the life cycle is taken into account in any calculations, by TRADA and others, if valid comparisons with other materials are to be made.

A TRADA Construction Briefing Timber carbon footprints – calculated values is available in the members’ only area of the website www.trada.co.uk.