The (timber) Stadthaus in London

By Lynne Blundell

21 January 2011 – The forestry industry and union have been stirring up controversy on the way Green Star ratings account for timber sustainability certifications. Now we look at the underlying sustainability of timber and how it stacks up against other materials.

The debate over which construction material is the most sustainable – timber, concrete or steel – is hotting up with each industry enthusiastically pushing its particular barrow. As we reported recently, a ratings tool is being developed for building products in Australia; in the meantime a raft of studies have been coming up with some strong results in favour of timber.

In New Zealand the Ministry of Agriculture and Forestry recently conducted a research project to compare the sustainability of timber with steel and concrete. Timber won by a sizeable margin.

The research project, Environmental Impacts of Multi-Storey Buildings Using Different Construction Materials, found that using predominantly timber in large-scale commercial buildings could significantly reduce their environmental impact.

The project looked at both the life cycle energy use and carbon dioxide (CO2) emissions of four similar office building designs that used different materials as their main structural element: concrete, steel, timber and timber-plus. The timber-plus option also used wood-based non-structural elements such as windows.

Results showed that the timber-plus building had the lowest net environmental impact, producing 4571 tonnes CO2 equivalent, while the steel building had the highest net impact producing 6789 tonnes. The timber building was the second lowest (5454 tonnes), followed by concrete (6627).

The NZ Ministry of Agriculture and Forestry’s Sector Performance Director, Iain Cossar, says the study fills an information gap on how much wood should be used in commercial buildings.

“It shows that increasing the use of timber can decrease the total energy consumption and environmental impact of the building over its 60 year lifetime.

“In addition the study found that from a technical point of view, a commercial building up of to six storeys could feasibly be constructed on a timber structure, something that does not typically happen in New Zealand,” said Mr Cossar.

The study considered the full life cycle of the buildings, including the initial embodied energy of the materials used, maintenance, transport, operational energy and end-of-life scenarios. All four buildings were designed for a 60-year lifespan and were based on the design of an actual six-storey concrete building.

The research used Life Cycle Assessments, which measure the environmental impacts associated with a product, process, or activity by identifying energy and materials used and wastes released through the course of a product’s lifespan.

The study was led by researchers from the University of Canterbury and included work by Crown research Institute Scion and Victoria University of Wellington.

Timber could help reduce carbon emissions

In Australia a recent study by University of Melbourne researchers, co-funded by Forest & Wood Products Australia and the Department of Climate Change, found timber’s carbon storage capacity could help in reducing greenhouse gas emissions. The study set out to improve existing estimates of carbon stocks in Australian housing, where more than 70 per cent of harvested sawn wood products are used.

Professor Rod Keenan, Director of the Victorian Centre of Climate Change Adaptation Research at the University of Melbourne, who was involved in the study and has extensive experience in analysis of carbon sequestration and greenhouse accounting, said: “Provided the timber is coming from forests where carbon stocks are being maintained, carbon in timber used in housing is a potentially important contribution to Australia’s effort to reduce greenhouse gas emissions.

“Our study indicates there is significant capacity for increasing the rate of addition to carbon stocks in housing. This would require a reversal of the trend for reduced wood use in housing,” Professor Keenan said.

For example doubling the volume of wood currently used in Australian homes would result in 2.5 times as much annual carbon storage in Australian houses in 2050 –  equivalent to the carbon dioxide emissions of more than half a million motor cars or about 0.7 per cent of Australia’s greenhouse gas emissions in 2006.

A key factor in determining timber’s sustainability however, is forest management, something that is being hotly debated right now in Australia (see our story on this).

According to Professor Keenan, in Australia forest management is particularly relevant because of bushfire risk.

He says that neither of the two international schemes currently used to assess the sustainability of forest production give rigorous consideration to the effect of forest management on greenhouse gas emissions.

In Australia this is a particular challenge because bushfires are the major cause of carbon emissions. Greater use of controlled burning may reduce this risk, says Professor Keenan.

“Using timber from sustainably managed forests, a properly-designed timber house will generally produce less CO2 emissions over the full life cycle than a concrete and steel structure. Ideally, accreditation processes such as green star building ratings and forest sustainability assessments would incorporate full life cycle assessment of different types of building products.”

But the use of timber has declined in Australia over the past few decades. The researchers found that while the average dwelling size has more than doubled across Australia in the past 60 years, the amount of wood being used in residential construction has decreased.

The study examined the use of wood products, the generation and fate of construction and demolition wood waste, the lifespan of residential dwellings and the factors influencing patterns and longevity of residential housing, as well as opportunities to encourage greater use of timber and to increase longevity of timber in housing.

It looked at which policies governments (State and Federal) and industry could introduce to promote increased use of wood products in residential construction.

The results: 86 percent of building practitioners suggested that reducing the price of wood products compared to competing products would have the greatest impact; 76 per cent indicated better training and education for building practitioners on wood products’ carbon storage benefits; 62 percent thought changes to government building regulations would result in the greatest impact on the choice of wood as a building material.

Even in intense bushfires, timber is resilient

And what of the risk of timber use in bushfire prone areas?

According to Justin Leonard, CSRO research scientist and expert on urban design for bushfire areas, while timber is a combustible material, when used in a dense form such as logs, timber can provide considerable protection in bushfires.

“The idea of a log cabin is quite robust. There is no timber that won’t eventually ignite but the thickness of the timber in a log cabin means it will take some time to ignite and will char. The downfall of such as system is that if the outside of the wall is burning, there will also be flames on the doors, windows and eaves. It is important to make sure these other features are protected and able to withstand the heat,” said Leonard.

“And if the outside of the cabin walls were burning for an hour you have to think how you’d safely leave the building. The argument is that a log cabin will eventually self-extinguish. This is true for some types of timbers but not all, so it is important to pay attention to timber types.”

It seems the push for greater timber use is a global trend. In the UK the world’s tallest timber building –a residential development – has received the Judges’ Special Award at the British Construction Industry Awards 2009 for pushing design boundaries in the use of structural timbers.

Called Stadthaus, the eight storey residential building in Hackney, London was designed by London-based architecture firm, Waugh Thistleton, for developer Telford Homes. It is built from pre-fabricated, cross laminated timber panels which form not only load bearing walls and floor slabs but also stairs and lift cores.

According to the developer, the all timber cross laminated panels were fabricated off site from a sustainable resource.

It could be a sign of things to come.

lblundell@thefifthestate.com.au

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