Through the careful specification of timber it is possible to reduce the whole-life carbon footprint of a school by over 15%
01 / INTRODUCTION
Our last article, on prestige offices (18 May 2012), previewed the new CEN/TC 350 European standards for whole-life carbon assessment. This time we examine new-build schools, contrasting a conventionally constructed school’s carbon performance with a low-carbon alternative. We then focus specifically on schools’ structural systems and the potential of timber construction to improve a school’s carbon performance.
The whole-life carbon issues this article raises include:
- How significant are the “end of life” issues associated with timber construction? What can we do to help mitigate future problems?
- What does it cost to reduce the embodied carbon performance of a school building?
- What role does the durability of timber have on whole-life carbon performance?
- Is sourcing timber products locally always the best option?
- Should the lifespan of a building affect the choice of structural timber system used?
- How do different types of timber act as a carbon sink, and how important is the manner in which trees are grown and forests managed?
02 / WHOLE-LIFE CARBON ASSESSMENT: A BRIEF RECAP
CEN/TC 350 identifies four stages in the life of a building - product manufacture, construction, in-use and end-of-life - with more detailed subcategories used to pinpoint specific sources of emissions.
The first stage encompasses the extraction of raw materials, their transport to a point of manufacture and the process of transforming them into construction products.
The second involves transporting construction products to site and the on-site processes involved in assembling them into a building.
The third, most complex, stage, covers the maintenance, repair, replacement and refurbishment cycles of the building, as well as the use of energy and water during its occupation.
In the final stage, the building is deconstructed and its redundant components transported off-site, processed and disposed of.
At each stage, whole-life carbon analysis can identify solutions with lower carbon impacts, delivering considerable savings overall. The first part of the standards, EN 15804, was published early in 2012 and establishes product category rules for environmental product declarations.
Please note that by the use of the word “carbon” in this article, what is being referred to is the whole basket of greenhouse gases which have the potential to cause damage in the atmosphere. These are measured in carbon dioxide equivalents or KgCO2e for short.
The table above provides a summary of the carbon and financial costs of delivering a 15% reduction in the whole-life carbon cost of a school, concentrating solely on embodied carbon solutions. None of the embodied carbon interventions identified require any change to the design or appearance of the building, relying only on procurement and construction detailing changes.
As a consequence of the overall whole-life carbon figures decreasing, the relative percentage of operational emissions goes up, although the overall amount stays the same, as shown in the graph’s opposite.
The analysis here does not cover reductions in operational energy and BER achievable through interventions such as renewable energy technologies. To provide some comparison, however, achieving the emissions reductions illustrated here using renewables would cost £150-160 per m2 more.
03 / LOW EMBODIED CARBON INTERVENTIONS
To optimise the building’s whole-life carbon performance, the 50 most carbon-intensive components are first identified. Each component is then examined to see whether materials of a lower carbon content could be substituted, or if products can be sourced closer to the site or assembled by a more carbon-efficient process.
For some products, such as insulation, the relationship with operational emissions needs to be considered in order to avoid over- or under-specification. To enable these specification choices to be made requires dynamic simulation, in which the impact of improving operational performance is considered with respect to the embodied carbon required to achieve it. This analysis often leads to less material specified than if each product was considered in isolation.
The following table and diagrams show a direct cost comparison between 15% whole-life carbon reductions through operational or embodied means.
BREAKDOWN OF CARBON EMISSIONS OF MAJOR BUILDING COMPONENTS
COMPARISON OF WHOLE-LIFE CARBON EMISSIONS BY BUILDING COMPONENT
COST AND CARBON BREAKDOWN OF LOW EMBODIED CARBON SCHOOL
04 / WHAT IS SEQUESTERISATION?
Sequesterisation is the carbon benefit attached to the use of mainly organic materials that have the capacity to remove carbon from the atmosphere through their growth or manufacture.
At the same time, these materials will often cause carbon emissions to be generated through the use of the machinery and transport to process them.
In this regard, timber is unique as the carbon it sequesters during its growth often outweighs the carbon generated during its processing, so by specifying timber it may be possible to help reduce the amount of carbon in the atmosphere.
Claiming these benefits is contingent on many processing and end-of-life assumptions, which is why some practitioners choose not to include sequesterisation in their calculations. We believe it is important to consider it as an additional benefit to be clear what has been put into the atmosphere through the building’s lifecycle and then to understand the potential benefits that may arise as a consequence of good forestry and waste processing methods.
05 / FACTORS INFLUENCING THE CARBON IMPACT OF TIMBER
Timber is one of the few construction materials the use of which does not entail the depletion of finite resources, as the forests from which it is sourced are sustainably managed. The growth of timber also absorbs carbon dioxide from the atmosphere, locking it away as cellulose, hexacellulose and lignates.
In nature, this locked-away carbon usually gets returned to the atmosphere when the tree dies and decays. In managed forests however, this process is delayed by the removal of the trees as timber. So long as this timber is not left to rot, the net effect is that carbon is removed from the atmosphere, which allows timber in some situations to be claimed as a carbon sink.
A recent Forestry Commission report predicts that if additional 4% of UK land was devoted to forestry by 2050, this would allow trees to absorb up to 10% of anticipated UK carbon emissions.
After a tree is cut and removed, many factors can still affect the carbon storage potential associated with its timber.
If trees are not subsequently replanted and the forest left fallow, carbon stored in the organic matter in the ground will soon also rot and leach away. Replanting is essential in stemming this process and also sustaining the supply of timber as a renewable resource.
Foresters can also change the carbon storage potential of forest by increasing growth rates by moving from natural forests to plantations.
Selective pruning and thinning will also improve the quality of the timber yield, helping to ensure timber produced is put to longer term uses such as furniture and construction. Species selection is also a factor in determining growth rates and durability of the final product.
Cutting down trees, transporting them to a saw mill and drying them all adds carbon to the atmosphere.
Timber processing is, however, incredibly waste efficient as generally all by-products are put to use, such as wood offcuts being used to make lower grade timber products such as chipboard or as biomass. In total the amount of carbon involved with these processes is usually only 25-40% of the initial carbon sequestered by the tree during its original growth.
The role of transportation
Transport mode is generally more important than transport distance when it comes to selecting suppliers. For instance an American oak transported over 8,000km by boat from Minnesota to Southampton will give rise to less carbon emission than a European oak driven by HGVs from Poland for less than 2,000km. Furthermore, the significance of transportation is also quite small in the overall footprint and usually only accounts for 10-13%.
Different timber construction products have different lifespans, which effectively determines how long the carbon in the timber is stored away. Maximising this period should therefore be the goal of any building designer when using timber for carbon storage.
Designers should avoid using timber where other products may be more appropriate - for example, temporary works where the majority of timber used has a very short lifespan, cannot be reused afterwards and enters straight into the site’s waste stream.
Avoiding timber being sent to landfill is the key to ensuring it does not release its stored carbon back into the atmosphere or, at worst, in anaerobic conditions release methane instead.
The release of methane is of particular concern as it has up to 20 times the greenhouse warming potential of a single molecule of carbon dioxide.
At the moment in the UK the majority of timber entering the waste stream ends up as being reprocessed as panel board, which prolongs the life of the carbon stored in the timber.
Construction preservatives however such as CCA (chromium, copper, arsenic) or CCB (chromium, copper, boron) can prevent such recycling, as panel board has to conform to European safety standards in order to be used in certain types of manufacturing, such as children’s toys.
06 / WHOLE-LIFE CARBON ANALYSIS OF DIFFERENT STRUCTURAL SYSTEMS AND TIMBER SOLUTIONS
The table below shows the impact of different timber types, sources and end of life treatments. Key recommendations include:
- Specifying FSC timber, even if it ends up being sent to landfill at the end of its life, will result in a better whole-life carbon performance than using brick and block construction.
- If timber is from a non-sustainable source this results in a chain of events that give rise to five times more carbon being emitted than building out of brick and block.
- Timber generates the greatest carbon benefits when it is used for long-term uses such as structural frames. Ideally, the slower a timber grows the longer term its should be.
- If possible, avoid putting timber to short-term uses such as temporary works, which result in its stored carbon being put into the atmosphere sooner. If necessary robust waste management plans can help mitigate these impacts by diverting wood waste from landfill.
- The processing emissions of different species types are small in comparison to the carbon they can potentially sequester. Hardwoods generally store more but this need to be balanced against the fact they are slower to grow.
- Avoid using timber preservatives wherever possible as these can prevent timber from being recycled into new products in the future.
- Bio-gas capture, incineration, animal bedding and biomass wood chip formation all have a role to play in diverting wood waste from landfill. However, re-use or recycling of timber into new products/materials will always generate the greatest carbon savings in the long run, even when fuel displacement benefits are factored in.
CASE STUDY: WAINGELS COLLEGE
Waingels College is a exemplarily low carbon timber framed comprehensive school near Reading. It demonstrates many of the potential embodied carbon and operational carbon savings described in this article. Through the use of the timber framing the main contractor Willmott Dixon managed to save time off the construction programme and also reduced the amount of carbon from the build process by over 6,000 tonnes. This time saving was essential as the main building works needed to take place during a restricted build program designed to fit in the school calendar.
07 / ACKNOWLEDGEMENTS
Whole-life carbon cost models have been produced by Sturgis Carbon Profiling, and construction costs provided by Gardiner & Theobald. Special thanks to Willmott Dixon, Roger Forsdyke, Alastair Wolstenholme and Richard Francis at Gardiner & Theobald, Pierluigi Chinellato at Sheppard Robson, Liam Dewar at Eurban, Ben Hunt at Skelly and Couch and Richard Whidborne at e-Griffin.
More information on compiling a Whole Life Carbon Assessment is available in Whole Life Carbon and Offices (British Council For Offices, 2012).