The uk government has ambitious plans to transform the energy efficiency of construction, acknowledging the oft-quoted words of Norway’s Harlem Brundtland, that “sustainable development is that which meets the needs of the present without compromising the ability of future generations to meet their own needs”.

Houses alone account for nearly a third of the UK’s carbon emissions, so the construction sector is right at the heart of government strategy. Sustainability is the single most important future trend in construction.

The 2008 Climate Change Act is an important tool. It creates a new approach to managing and responding to climate change in the UK, by setting legally binding targets to reduce greenhouse gas (GHG) emissions (compared with 1990 levels) by 34% by 2020 and by 80% by 2050.

The Low Carbon Transition Plan was rolled out in 2009 as a route map to achieving the 2020 target. The wide-ranging plans have significant implications for construction:

Residential buildings
•    2020 – heating emissions to be down 29%
compared with 2008
•    2016 – all homes to be zero carbon
•    a major retrofit to increase the efficiency of
existing stock
Non-residential buildings
•    2020 – emissions to fall by 13% compared with 2008
•    2019 – private sector buildings to be zero carbon
•    2018 – public sector buildings to be zero carbon

An innovation and growth team drawn from the construction sector and government, operating under the Department for Business Innovation & Skills, estimates that for UK construction by far the greatest part of GHG emissions associated with a building’s life cycle occurs as a result of occupation and use (83.5%), followed by the manufacture of the building components (13%) (Figure 1).

The message is clear: energy emissions associated with building use must reduce, but so too must the emissions associated with producing construction components.

Timber has unique properties compared with other materials. Through photosynthesis, trees trap large amounts of carbon dioxide, store it as wood and release oxygen. Every cubic metre of sustainably-produced timber used instead of other typical building materials reduces CO₂ emissions. Timber can therefore contribute hugely to the reduction of GHG emissions, both from the construction phase of a building and during its service life.

As timber is reluctant to transmit heat it is an efficient thermal barrier, so timber window frames and studwork ensure that a building remains well insulated. In addition, wood fibres make efficient insulation products in their own right, both as insulation boards and insulation batts.

Figure 2: the comparative thermal conductivity of selected building materials

Figure 2 deliberately excludes steel and aluminium, which have thermal conductivity values some 400 times and 1,200 times greater than softwood respectively. Timber’s insulation qualities mean that less energy is required for space heating, and so timber can assist in reducing GHG emissions from fossil fuels.

If grown sustainably, woody biomass can be a source of low carbon heating, either in the form of traditional logs, or increasingly as pellets in more sophisticated boilers. When combustion takes place the CO₂ in timber is released. However, if new growth is planted to replace it, CO₂ becomes reabsorbed. The only emissions to accumulate in the atmosphere are those associated with processing and transport. This is in contrast to fossil fuels, where all of the CO₂ released from combustion accumulates in the atmosphere.

During photosynthesis, CO₂ from the atmosphere is combined with water which is absorbed via the roots. The carbon from the CO₂ is sequestered into carbohydrates which form the tree fabric. Wood is made from approximately 50% carbon and every tonne of wood that grows absorbs the carbon from approximately 1.8 tonnes of atmospheric CO₂. The carbon remains sequestered until the wood is burned or decays, when it re-combines with oxygen to release the CO₂.

Sequestration is not a permanent solution to the absorption of CO₂ from the atmosphere because, in time, the CO₂ will be re-released. However, use of wood in construction provides a reservoir of stored carbon, so the longer the wood is in service, the longer the atmospheric CO₂ remains sequestered. It is estimated that in 2009, timber in the UK housing stock held 19 million tonnes of absorbed carbon (representing nearly 70 million tonnes of CO₂) and has potential to increase to 29 million tonnes of carbon by 2019.

To bring building materials to site requires an investment in energy for extraction, processing, manufacture and distribution. Much of this energy will have been derived from fossil fuels, so all construction and other products will be responsible for the release of GHG before being incorporated into a building. In the interest of these emissions, consideration should be given to the materials used. The University of Bath has collected data on embodied carbon and energy for a full range of construction materials, publishing its findings as the Inventory of Carbon and Energy (ICE). Leading structural engineer Ramboll has applied the ICE figures to similar sized buildings constructed from different materials (Figure 4). The results indicate that a timber-based construction contains considerably less embodied carbon than one built from competing materials. If carbon sequestration is counted, the embodied carbon of the building falls below zero.

At the end of its service life there are various options that can avoid disposal and the immediate release of the sequestered carbon. It is often possible to reuse or recycle wood. For example, each year in the UK over a million tonnes of recycled wood is used to manufacture chipboard, offering the potential for the carbon to remain sequestered in a further construction project.

There is a rapidly growing market in the UK for biomass fuel. Electricity generated using recycled wood is eligible to contribute towards the UK’s renewable energy targets. Wood that is being combusted will inevitably release its sequestered carbon as CO2 back to the atmosphere. However, much of this near carbon neutral energy can replace highly carbon intensive fossil fuels.

Timber is a sustainable, multifaceted material that at key stages throughout a building’s life cycle can significantly reduce the carbon impact of construction. Choosing to build thermally-efficient homes and buildings in timber will surely aid the fight against global warming.

Figure 4: the Ramboll calculator showing timber as a construction material with low embodied carbon

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