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It may be a given that wood is a sustainable material, but what does that actually mean? For years, it traded on the assumption that it is less harmful to the environment than the use of other major construction materials.

The industry relied on the understanding that timber is a naturally growing, replaceable material, needing few inputs and causing little environmental damage. At the end of use, it can be burnt or allowed simply to rot away. Growing forests actually act as ‘carbon sinks’, absorbing carbon dioxide.

None of this is wrong, but it is not enough in a world that has an increasingly sophisticated understanding of sustainability, that knows that nothing is simple and that there are always trade-offs to be made, and which, quite rightly,  values measurable outcomes over mere assumptions, however well-intentioned.

While the timber industry was, frankly, a little complacent, other ‘rival’ materials got their acts together, benefitting from the fact that they were better financed and in the hands of fewer owners to commission studies that demonstrated their credentials. The timber industry was frustrated but it was not until AHEC (the American Hardwood Export Council) decided to take action a few years ago that there were any hard facts that wood lovers could use in arguments.

The approach that AHEC adopted (and that had been used in other industries) was the environmental impact assessment (EIA). This measures a number of impacts: primary energy demand (from renewable resources); primary energy demand (from non-renewable resources); global warming potential; acidification potential; eutrophication potential; and, photochemical ozone creation potential.

The first two of these are relatively self-explanatory. They are measured in GJ and take into account the conversion efficiency from the original energy source to electricity, where appropriate.

Global warming potential, which is measured in kg of CO2 equivalent, is calculated from the volume of greenhouse gases, such as CO2 and methane, produced during processing. It is a good marker for general environmental impact but, if one wants a full understanding of the sustainability of a process, should be seen alongside the other measures.

Acidification potential, measured in kg of sulphur dioxide equivalent, measures the impacts that can have acidifying effects, including the well-known and potentially devastating acid rain.

Eutrophication is the process by which water receives excessive amounts of nutrients, normally nitrates and phosphates, typically as run-off from fertiliser over-use. The potential is measured in kg of nitrate equivalent and phosphate equivalent.

Photochemical ozone depletion potential is a measure of the emissions that contribute to low-level smog. It is measured in kg of ethane equivalent.

The EIA measures a material from the start of production (typically extraction but in the case of timber, tree felling) to the ‘factory gate’ where products are made. In the case of a construction material this gate may actually be at the construction site. The results, produced by an independent, accredited organisation called PE International, were impressive and had some surprises – the impact of transport from American forests to Europe were relatively minor, whereas the impacts of kiln drying were relatively important. These results varied from one species to another – AHEC commissioned EIAs for each of its major timbers.

While this data is helpful to clients and specifiers, it is not enough. They need to know the details for their finished products, in this case of the CLT compared to alternative structural techniques.   This requires an understanding of what happens after the ‘factory gate’. Which other materials were used? What were the energy inputs in production? What type of transport was used and over what distance? What will the lifetime of the building be (or in the case of The Smile, of the components, since there is an intention to re-use them after the lifetime of the project)? How will they be disposed of at end of life?

Answering these questions requires a second EIA that is specific to the project and that draws on a wealth of outside information. Just as there is an EIA available for each of the main American hardwoods, so information can be found on other components such as glues, finishes and fixings. While many of these are relatively minor in volume, experience has shown that their impact, due to high embodied energy, can be disproportionately significant.

AHEC has commissioned EIAs of this kind for three previous projects: the Endless Stair, built outside Tate Modern in 2013 and the first use of tulipwood CLT, and two projects based around furniture and objects, Out of the Woods and The Wishlist. All proved enlightening. A similarly exercise is being carried out for The Smile which will provide valuable insight relevant not just to this project but also for the future application of tulipwood CLT in ‘live’ building projects.