Wood in architecture
London || United Kingdom
The Endless Stair demonstrates the potential of new environmental life cycle assessment (LCA) and modelling tools designed to introduce genuine sustainability into the building design process. One outcome of the project should be to encourage a decisive shift away from the “greenwash” which has done so much to devalue the concept of sustainability in recent years.
Claims of green probity are very often based on single issues - “recyclable”, “low carbon”, “organic”, or “locally sourced”. Such labels miss the bigger picture, ignoring the fact that efforts to improve environmental performance at one stage of a products life cycle may simply increase damage at other stages.
Use of “local” products may increase environmental impacts if local growers are more wasteful or energy-intensive than overseas competitors. Crops may be “organic” yet require twice as much land as conventional crops, increasing pressure to convert forests and other valuable ecosystems. “Recycling” more often involves “downcycling” to produce goods of lower quality and requires large inputs of energy and leads to high concentrations of toxic chemicals at the recycling plants. In each case, the public is given a false sense of security that environmental problems are being solved when in reality they are simply being shunted from one issue to another.
This emphasis on single-issue and simplistic labels may have been an inevitable consequence of the sheer complexity of the global environment and the need to package issues for public consumption. But it doesn’t deliver real sustainability or progress towards forms of resource use which satisfy human needs while still maintaining, or even enhancing, the world’s stock of environmental capital.
Developing technologies promise to deliver genuine sustainability
However, a combination of technologies is now coming together which promises to deliver a much more enlightened approach.
Increasingly we live in a world of “big data”, driven by ever cheaper hardware for data storage, access to a widening range of information sources via the internet, faster processors and sophisticated computer modelling and visualisation software to facilitate analysis and communication.
These developments are combining with the evolving science of LCA, which is already twenty years old, to offer a path towards integration of real environmental performance into key stages of the design process. Both governments and companies such as PE International, who were commissioned to undertake the LCA research and modelling work for this project, are now building comprehensive “Life Cycle Inventories” containing verifiable quantitative data on the use of the world’s resources and outputs of emissions and pollutants.
At the same time, there has been significant progress to standardise the process of LCA to ensure comprehensive coverage of impacts and consistent results. The process is now subject to the ISO14040 series of international standards. A global framework is emerging for issue of Environmental Product Declarations (EPDs) to communicate data on individual products to consumers.
At European level, the Construction Products Regulation (CPR) which became law in July 2013, requires use of EPDs to meet standards for environmental performance for construction products in all Member States. The EU has published EN15804 to provide core rules for construction product EPDs. In the UK there is a strong emphasis on Building Information Modelling (BIM) Systems. BIM is essentially a sophisticated form of computer aided design which integrates traditional software tools for architects and designers with a whole range of other data, including environmental impact of materials. The UK government has already announced its intention of requiring BIM on its projects by 2016.
So there’s every reason to believe that construction professionals will soon be given the tools needed to integrate genuine concern for sustainability into their designs. They need to start grappling with the full range of environmental impacts and the trade-offs between them. A key aim of the Endless Stair project is to demonstrate the power of the new LCA tools and to communicate the meaning and implications of the various impact categories to designers. Endless Stair builds on a two-year study, commissioned by AHEC and undertaken by PE, to assess environmental impacts associated with delivery of U.S. hardwood material into world markets. This involved a wide-ranging independent assessment of hardwood forestry practices and a survey of the hundreds of U.S. companies engaged in the processing and export of hardwood products. This information was then combined with data gathered from the production process of Endless Stair –the delivery of timber to Europe, the CLT production at Imola Legno in Italy, the fabrication of the stairs at Nüssli in Switzerland and the installation at Tate Modern in London. It was also combined with PE’s existing life cycle inventory database which covers an expanding range of non-wood materials and product groups.
Using PE’s Gabi software for LCA, the Life Cycle Inventory data was analysed to quantify environmental impacts. PE's Envision modelling tool was used to visualise impacts at different process stages. Envision also provides a mechanism to assess how alterations in certain parameters (e.g. types of timber, kilning methods, transport scenarios) will influence overall impacts.
Environmental Impact Categories
For Endless Stair, data is available on seven environmental impacts: Global Warming Potential (or “carbon footprint”); Acidification Potential; Eutrophication Potential; Photochemical Ozone Creation Potential (POCP); Ozone Layer Depletion potential ; primary energy demand from all resources; and primary energy demand from non-renewable resources. These seven impact categories are those to be reported for construction products according to EN15804. Before looking at how Endless Stair performs against these impact categories, something should be said about the source of timber. The LCA of American hardwood undertaken by PE assessed the impact of U.S. hardwood extraction on land use and biodiversity drawing on regular U.S. government forest inventory data and other independent reports.
The study concludes that harvesting of hardwoods in the U.S. does not contribute to deforestation. It observes that between 1953 and 2007, the volume of U.S. hardwood growing stock more than doubled from 5,210 million m3 to 11,326 million m3. The study also notes that: "in the system under investigation the main material – wood – comes from naturally re-grown forests. The harvested areas had undergone several iterations of harvesting and re-growth. After harvesting, the land is returned to forest so there is no direct land use change to account for in the timeline of few hundred years."
On biodiversity impacts, the study concludes: "conversion of any other commercial land into the hardwood forest would most probably be a positive impact on the land quality including biodiversity and associated ecosystem services."
The CLT used in Endless Stair is composed of tulipwood, one of the most abundant hardwoods in the U.S. forest accounting for nearly 8% of wood volume. U.S. government forest inventory data shows that American tulipwood is growing at a rate of 35 million m3 every year while annual harvests average around 17 million m3 each year. This means that even after harvesting, an additional 18 million m3 of tulipwood accumulates in U.S. forests every year. Such is the productivity of the U.S. tulipwood resource that it takes less than 2 minutes for the 100 m3 of tulipwood logs harvested to produce the CLT for the Endless Stair to be replaced by new growth in the forest.
Cradle-to-cradle design concept
Endless Stair ties in with the “cradle to cradle” design concept which models industry on nature. Cradle-to-cradle aims to eliminate the whole concept of “waste” by encouraging us to view materials as constantly circulating nutrients. The output of one process should always be seen as the feedstuff of another. Designs should build in this concept from the very start and ensure materials can be readily reused.
Throughout the Endless Stair project, there has been a major focus on efficient use and reuse of materials. CLT is an inherently efficient use of wood. Each sheet is built up from a number of planks so any local weak areas such as knots have relatively little effect on the overall strength. This makes it possible to use lower grades of timber than are traditionally considered for construction, without any loss of quality in the finished product.
The design ensures that panel sizes match those that may be most readily engineered from the raw materials available, minimising the need for cutting during fabrication. During all stages of lumber production in the U.S., manufacturing of CLT at Imola Legno in Italy, and fabrication at Nüssli, those smaller pieces of wood which could not be used for this application were put to good use elsewhere. Endless Stair is composed of 23m3 of CLT panels manufactured from 54m3 of kiln dried tulipwood lumber. The conversion process generated 14.1 tonnes of excess wood material, of which 4.2 tonnes were reused for animal care and panel products and 9.9 tonnes was used for energy production.
This focus on efficient use of materials means that some processes effectively offset environmental impacts. For example, the production of 23 m3 of CLT at Imola Legno in Italy offsets the Global Warming Potential of Endless Stair to the tune of 9.9 tonnes of CO2 equivalent. This is primarily due to the use of wood offcuts as a fuel leading to avoided consumption of natural gas in the manufacture of other products.
Carbon emissions and storage
The carbon footprint of Endless Stair, including all processes to extract, transport and process materials, and to fabricate, deliver and install the structure in London, is 13.1 tonnes of CO2 equivalent. That’s roughly equivalent to 2 people flying from Heathrow to Australia and back, or driving 50,000 miles in a 40MPG petrol car.
Note that the 13.1 tonnes does not include the carbon stored in the wood – around 50% of the mass of any timber is composed of carbon. The carbon in Endless Stair will, sooner or later, be released at the end of its life cycle. Because we don’t know yet what will become of the structure after the London Design Festival, the potential carbon storage benefits cannot yet be accounted for in the carbon footprint.
However, we can estimate that about 14.5 tonnes of CO2 is stored in the structure – which is actually greater than the 13.1 tonnes of carbon emissions required to manufacture, deliver and assemble the whole structure for the Festival.
From an environmental perspective, a key advantage of CLT is its longevity. There’s no reason why it shouldn’t remain in a building without need for replacement for many years. If Endless Stair were to become a permanent installation and to store the carbon for several decades, it might then be appropriate to claim it is “zero carbon” – but not before!
Role of transport in global warming impacts
Transport often plays less of a role in the total carbon footprint of a product than the manufacturing process. This is true of the kiln dried hardwood lumber at point of delivery at Imola Legno in Italy. The “cradle to gate” carbon footprint of the tulipwood at that point in the supply chain is 13.5 tonnes CO2 equivalent (again excluding carbon storage), of which 44% is due to transport and 56% is due to manufacturing.
Transport is less important than manufacturing despite the tulipwood having to travel 650km by truck to port of export in the U.S. and 7750km by container ship across the Atlantic. Note also that tulipwood is a relatively quick-drying hardwood species, requiring no more than 6 to 10 days in the kiln. Transport’s relative contribution is even less for hardwoods like oak that need much longer (and therefore more energy) in the kiln.
Rather than just focusing on “local sourcing”, it is often worthwhile from a carbon perspective to import products from regions where manufacturing is most efficient or less energy-intensive. There is a particularly strong rationale for importing wood from countries like the U.S. which have a large sustainable supply well in excess of domestic demand and where the wood is destined for long-lived products like CLT. In effect the pool of carbon contained in the CLT supplements that contained in the U.S. forest.
When considering the whole structure as it stands now at the London Design Festival, transport accounts for 59% and manufacturing for 41% of the total carbon footprint. The relatively high contribution of transport at this stage of the life cycle is due to some unusual features of the supply chain within Europe. Particularly important is the carbon offset attributed to Imola Legno in Italy for reuse of offcuts. This greatly reduces the carbon footprint of the CLT manufacturing process. Another factor is the relatively limited number of manufacturers capable of producing hardwood CLT and delivering such an innovative one-off exhibition project. This resulted in products having to be moved first to Italy and then to Switzerland before delivery to the UK.
The impact of non-wood materials
Another notable feature of the carbon footprint of the Endless Stair is that a relatively high proportion is attributable to materials other than the CLT despite their low visibility. The carbon footprint (without stored carbon) of the 23m3 of tulipwood CLT at point of despatch from the Imola Legno facility in Italy was 5.8 tonnes CO2 equivalent (around 250kg CO2 equivalent per cubic meter).
For comparison, the 15.75m3 of concrete footings laid on-site in London, for which all materials are assumed to be locally sourced, had a carbon footprint of 4.1 tonnes CO2 equivalent (around 260 kg CO2 equivalent per cubic meter). The carbon footprint of all non-wood materials used by Nüssli in Switzerland to assemble the structure (such as glues, metals and coatings) was 2.2 tonnes CO2 equivalent.
Eutrophication potential measures the excessive nutrient enrichment of waters by release of phosphorous or nitrogen compounds (such as fertilisers) and organic matter (e.g. in effluents). This leads to build-up of algal blooms and depletion of oxygen levels of the water. The problem is widespread all over the world and the severity is increasing.
The total eutrophication potential of the Endless Stair is 24.4kg of phosphate equivalent - about the same as that caused each year by conventional farming of a single hectare of land for wheat in the UK. So while not negligible, the eutrophication potential is probably not as significant an issue for Endless Stair as the carbon footprint.
Perhaps surprisingly, hardly any of the eutrophication associated with the Endless Stair is linked to the growth of tulipwood. Fertilisers are very rarely used to encourage growth of American hardwoods which thrive under natural conditions. Instead, nearly all eutrophication potential of the structure is due to nitrate emissions during burning of fuels for transport and processing of materials.
Acidification Potential is a measure of the emissions that cause acidifying effects to the environment and lead to serious damage to eco-systems. The acidification potential of Endless Stair is 172kg of SO2 (sulphur dioxide) equivalent. Acidification is mainly caused by the burning of fossil fuels and the scale of impact is directly related to their sulphur content. For example, Acidification Potential tends to be very high for products manufactured in regions where the national grid is heavily dependent on coal which has high sulphur content.
Over half of the acidification potential of Endless Stair is due to emissions during shipping of tulipwood lumber from the United States to Europe and reflects the relatively high sulphur content of marine fuels. The extent to which this impact can be reduced for products shipped from overseas is very dependent on international progress to reduce the sulphur content of these fuels. Efforts are being made to progressively tighten requirements under the International Convention for the Prevention of Pollution from Ships (MARPOL). A target has been set that the sulphur content of any fuel oil used on board ships shall not exceed 0.5% from 2020 onwards. This target is a long way from being met internationally and the Endless Stair study uses a figure of 2.7% which is the assumed current global average.
One factor partly mitigating this impact for Endless Stair is that the main location of emissions contributing to Acidification Potential is over the ocean rather than in populated areas.
Photochemical ozone creation potential (POCP)
POCP measures the contribution of the structure to the creation of so-called "photochemical smog". Such smog regularly afflicts modern cities and impacts human health and vegetation. It is caused by increased levels of ozone at ground level resulting from the reaction of volatile organic compounds (VOCs such as ethene) with oxygen compounds in the air under the influence of sunlight. Endless Stair has a POCP of 47.4kg of Ethene equivalent. Most is due to emissions of terpenes, which are VOCs released from wood resins. The terpenes are naturally released as trees grow, but processes in which wood is heated (such as a kiln drying) result in more significant emissions. In practice there is substantial variation in the level of VOC emissions between species and depending on drying times and other factors such as the mix of heartwood and sapwood. This finding highlights the need for more work to understand the specific impacts of terpene emissions within the context of the U.S. hardwood kilning facilities and the actions required to mitigate these impacts.
Endless Stair demonstrates both the environmental potential of hardwood CLT and the power of the innovative LCA tools now becoming available. It is extraordinary that the finished structure has the potential to store more carbon than is actually released during all the processes of material extraction, processing, transportation, fabrication, and installation. For some commentators that would be enough to label the structure “zero carbon”. However, the LCA tools also teach us to be highly sceptical of such single-issue claims. They highlight that a product can only be “zero carbon” if there is reliable accounting of carbon pools at every stage from extraction to disposal and for every component of a product. It’s unlikely that any industry is yet at the stage where it can make legitimate claims against that standard. Endless Stair also highlights the critical importance of measures to minimise waste and encourage greater re-use of materials in order to mitigate a wide range of impacts. The efficiency measures already undertaken by the U.S. hardwood mills, CLT manufacturer and the fabricator indicate that at least this section of the wood industry is well advanced down the road towards genuine “cradle-to-cradle” resource management. There are lessons here for the wider industry. The Endless Stair project demonstrates how it is now possible for designers and their material suppliers to identify the type, scale and location of environmental impacts of their products. These impacts can be identified early on in the design process. Prototypes can be assessed and designs altered to reduce environmental impacts before going into mass production. Material suppliers can visualise and understand where their environmental strengths and, more importantly, weaknesses lie and develop action programs accordingly. In short, Endless Stair, as well as being a beautiful structure, offers a path towards genuine sustainability in design and construction.
Rupert Oliver, Sustainability Consultant
Client: London Design Festival
Architects: dRMM Architects – timber studio, ARUP Engineering Structures, Material and Fire Specialists
CLT Production: Imola Legno
Lighting Designer: Seam
American Tulipwood: Allegheny Wood , Blue Ridge Lumber, J&J, Northland Corporation, Northland Forest Products, Pike, Lumber Company, Shenandoah Hardwood Lumber Co, Verde Wood International
Photography: 1/3-8 © Judith Stichtenoth, 4/7-9/11-13 © Jonas Lencer, 12 © Thomas-Etchells