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In July 2007 a government policy document called Building a Greener Future declared that all new build homes would progressively work towards becoming net zero carbon by the year 2016 as depicted in Fig 14.
Fig 14: Timeline route to compliance. Source: Zero Carbon Hub 2009
In order to meet government targets the construction industry must identify and reduce where possible the causes of harmful emissions being released into the atmosphere. According to Crane Environmental (1999) in the report entitled Sustainable Homes: A Guide for Registered Social Landlords, buildings are the greatest source of energy demand in the UK of which houses account for approximately 30% of national energy consumption. Figures from HMSO (1998) show that 580 million tonnes of CO2 emissions were released into the atmosphere in 1990 due to energy consumption from homes and offices. However it not only takes energy to run a building but it also takes energy to produce the materials used to build it.
The Green Consumer Guide (2007) emphasise the significance of construction materials in reducing CO2 emissions from new build construction projects and recommend increasing the use of sustainable materials. The significance of the environmental impact of materials is examined by Kruse (2004) who concludes that the extraction and production of building materials have a huge impact on the environment with the production of cement alone contributing in excess of 5% of global CO2 emissions. This is backed by Roaf (2007) who indicates that the production of cement and steel account for 10% of global CO2 emissions.
4.1 CO2 Emissions
As the overriding emphasis of the research is to determine whether or not the performance of timber framed housing is an improvement over traditional brick and block construction then clean renewable energy solutions such as solar panels, photovoltaic's, heat pumps and wind turbines will not form part of the research. Therefore the research into CO2 emissions will be related to the materials themselves.
There is no debate that if CO2 emissions from new build housing are to be reduced then materials must play a significant role. This is emphasised by the Green Consumer Guide (2007) and backed by Bed Zed who claim to have reduced emissions by 25% in the development entirely through material selection and specification.
Considering the transportation of construction materials alone then Lazarus (2007) concluded for the Bioregional Development Group that 30% of all road freight in the UK is the haulage of construction materials with every 100 tonnes of materials being moved a distance of 10 miles a total of 91kg of CO2 emissions are produced. However before materials can be transported to construction sites they first must be manufactured, extracted or refined all of which consume energy and therefore contribute towards CO2 emissions. This use of energy to produce materials is known as the embodied energy. This embodied energy is considered by National Green Specification (2010) as being;
"the total primary energy consumed which would normally include extraction, manufacturing and transportation. Ideally the boundaries would be set from the extraction of raw materials until the end of the products lifetime, known as 'Cradle-to-Grave".
In its simplest terms embodied energy is described by Crane Environmental (1999) as the energy needed to transform a product from raw materials in the ground into the completed final material. Thus the embodied energy of a house can be deemed to be the total energy needed to build the house which cannot be recovered during the lifetime of the building, no matter how efficiently it operates.
Traditionally in the past building materials were indigenous to the local area and evidence of this exists all over the UK with examples such as granite being predominately used in Aberdeen whilst sandstone was extensively used in Glasgow. The use of local materials ensured embodied energy was relatively low as the materials were sourced and processed locally minimising transport and other associated energy costs. This has changed dramatically in that materials are now sourced from all over the world with a prime example being that slate is imported from China for use in roofs as opposed to traditional Scotch or Welsh slate. Shipping materials from the other side of the world increases the energy used to have the material available for use on a building site. This is done for reasons of cost alone with no consideration to the environment.
If embodied energy is considered as the energy required for extracting, manufacturing and transporting a material then this energy can be calculated and used as a tool to specify materials which are low in embodied energy. Normally the embodied energy is expressed as CO2 per unit mass (kgCO2/tonne) or CO2 per unit area for a completed building (kgCO2/m3).
To this end BRE (2008) have created the "Green Guide" which is a list of generic building materials with calculated data relative to a materials environmental credentials allowing architects and designers to be able to make objective choices regarding materials knowing that the material is suitable to the needs of the development and the environment. This proved to be a challenging concept as different materials can have similar structural capacities but different masses. However the Green Guide is now gaining momentum and is referred to by codes such as the Code for Sustainable Homes. In order to make the guide user friendly the materials are rated on a grading scheme which lists a material as A+, A, B, C, D and E with A+ obviously being the best rated in terms of the environment.
As the research has shown it takes energy to produce building materials with some materials having more embodied energy than others. Conversely to embodied energy some materials posses internal energy stored in the material which can be released through combustion or chemical processing, a good example being that timber burns providing energy in the form of heat. This type of energy is described in a Guide for Registered Social Landlords (1999) as inherent energy and must be factored into the environmental equation when specifying materials to give a more rounded view to materials overall energy performance.
By considering softwood as an example then the inherent energy will be high as the wood can be burned for energy. This level of inherent energy is fixed as it does not depend on location, transportatation or processing as timber is timber the world over, however the embodied energy will change significantly from a home grown timber to an imported timber therefore the net energy will change as the energy to transport the timber is not recoverable. Therefore there is an important relationship between inherent and embodied energies which only add to the confusion for specifiers as to what materials should be used to minimise emissions.
At this point in the research it should be pointed out that the embodied energy of a building is far less than the energy used to run the building for heating, lighting, hot water etc and therefore the emissions from general use will outweigh those of materials. This is emphasised by BRE who conclude that the environmental impacts of the materials in a house are less significant than the actual performance of the house over its lifetime with domestic household energy consumption accounting for 29% of the UK's CO2 emissions whereas the materials used in a house's construction account for just 2-3%. However as previously stressed the aims of this research are underpinned by materials in the method of build and not the subsequent operation of the structure.
Brown and Buranakarn compared the total life-cycle energy required to make major building materials with the results shown in Fig 15. As can be seen Aluminium required the greatest amount of energy with timber being the lowest. However this points to the difficulty as discussed by BRE in defining the energy used from material to material as although steel fairs quite poorly in the table using more than fours times the energy of timber the strength of steel is far superior to timber therefore a smaller cross section of steel will support the same load as a greater section of timber therefore the actual application of the material must be considered and on some occasions although limited in number steel could out perform timber (Boyle 2005).
Emergy: X 109 solar energy J/g
Fig 15: Material Extraction & Energy Intensity of Building Materials
With regards to timber framed housing then UKTFA concludes that the lowest embodied energy of new build housing is achieved by using timber frame provided that the timber is locally sourced and that it can be built off shallow, excavated pile foundations, which are the least energy-intensive foundation type.
This is backed by the Forestry Commission (2006) who emphasise that by using locally produced sustainable timber for the structural frame and cladding of a house as opposed to brick and block the CO2 emissions could be reduced by as much as 86% by reducing the embodied energy attributed to construction materials. Further evidence comes from Wood for Good (2007) who are of the opinion that timber frame construction could be vital in the challenge of reducing emissions. The potential for reduction of CO2 emissions possible by building timber framed houses in comparison to traditional building methods and materials can be seen in Fig 16.
Fig 16: CO2 Emissions of Wall Compositions Source: BRE 2008
B & DB Brick and Dense Block
B & TF Brick and Timber Frame
RDB & TF Rendered Dense Block and Timber Frame
B & AB Brick and Aerated Block
RAB & TF Rendered Aerated Block and Timber Frame
SC & TF Softwood Cladding and Timber Frame
Additional benefits of timber as a building material are detailed by Trada (2009) who are of the opinion that timber
is the most versatile building material currently available as it can be used in a variety of applications such as; structural framework of timber kits, insulation in the form of cellulose fibres, internal and external finishing of houses for flooring or cladding and can even be used for furniture. They add that it is also the ultimate in 'green' materials, being:
Renewable - as long as replanting is carried out in line with sustainably managed forests then timber will always be available as a renewable material. In Europe, where over 90% of UK timber originates it is current practice in FSC forests for two trees to be planted for every one that is felled.
Natural. Only limited energy and processing is required to convert felled timber into a usable construction material with the energy often coming from bio energy from waste wood and off-cut material.
Reusable and recyclable. Timber can be burnt for fuel and it will also bio-degrade. One of the big growth areas for recycled wood is biomass.
The UK Timber Frame Association (2009) view timber frame as the most environmentally friendly commercial building material available citing timber as being; renewable, organic and non-toxic, whilst only requiring low levels of embodied energy to fell process and transport. They have also calculated that in a typical timber framed house there will be approximately twelve to twenty cubic metres of timber, equivalent to the absorption of about fourteen tonnes of CO2. Therefore as little as a 10% rise in the market share of timber framed housing could potentially reduce CO2 emissions by hundreds of millions of tonnes.
This feel good factor concerning timber frame is not limited to the UK as the European Commissions DG Enterprise (2003) are also of the opinion that timber will play a vital role in the challenge against climate change stating that:
"Greater use of wood products will stimulate the expansion of European forests and reduce greenhouse gas emissions. The commission is examining ways to encourage these trends".
As can be seen from Fig 16 materials traditionally used in the construction of new housing such as brick and block fare the worst in relation to CO2 emissions. One of the major drawbacks of these materials is the use of cement to bond them together. According to Pritchett (2003) 10% of all global CO2 emissions are attributable to the manufacture of cement, therefore it cannot be considered as an environmentally friendly construction material. Further estimates from Pritchett (2003) indicate that in the region of 3000 million bricks are being produced in the UK alone for construction use and as long as houses are built by these methods then the energy associated with the manufacture of the materials will remain high.
To their credit the British Cement Association realised that some form of action was required to not only reduce the embodied energy of cement but to ensure its longevity as a mainstream building material for future generations (Collins 2003). To this end they have invested in new technology leading to efficiency improvements of 25% from 1990 levels until 2010 as shown in Fig 17.
Fig 17 Reduction in Energy Consumption to Manufacture Cement
However this does not detract from the overwhelming conclusions derived by the UKTFA in their 2009 report entitled Comfort and Cost where they are categorical that overall the CO2 emissions from timber frame construction is up to six times lower than the emissions from masonry construction methods.
As previously noted (National Green specification) the energy associated with materials should be considered on a cradle to grave basis. The research has extensively discussed the cradle element through embodied energy in the manufacture of materials however it has not focused on what happens to materials at the end of the natural lifespan of a building.
Construction and demolition waste can be described as all wastes that arise from construction, renovation and demolition activities (C.I.F. 2003) with approximately 17% of all UK waste production being attributable to the construction industry (Cameron 2003) much of which is recyclable.
The design process for new buildings must be to ensure that the materials used and the method of build is such that the building lends itself more readily to recycling than is currently the norm at the end of its life cycle. However buildings currently being demolished were not constructed in this manner as the recycling of materials was not a major consideration when they were built therefore it can be more onerous to attempt to recycle than to simply demolish and discard the rubble ((Khalaf & DeVenny 2004).
Considering bricks as a typical example then according to Sherwood 1995 whether or not bricks are recycled depends to an extent on the type of mortar used when laying the bricks. As lime mortar can be removed from bricks fairly easily then the brick becomes more commercial in terms of recycling however if a cement based mortar is used then it becomes difficult to clean the bricks for reuse normally resulting in them being crushed for aggregate (Khalaf & DeVenny 2004).
The recycling process for some construction materials can be considerable (Trada 2009) however the timber industry has turned this negative into an environmental positive by using wood in biomass generators making the processing of timber a net contributor to the national grid as shown in Fig 18.
Fig 18: Net CO2 Emissions of Construction Materials
This clearly shows the reduction in energy consumption that can be achieved by recycling. Taking steel as an example then net emissions of 17500 Kg CO2/m3 of raw steel is reduced to 4000 Kg CO2/m3 for recycled steel. The graph also clearly shows from the major construction materials considered timber is the only material that can be considered as a net energy contributor rather than a net consumer of energy.
This is expanded upon by a Swedish study undertaken in 2001 to compare the embodied energy of timber houses and that of steel/concrete houses. A timber house used 2300 MJ/m2 less energy in the materials and construction of the house which would be enough energy to heat one of the houses for six years furthermore the timber house also outperformed the steel/concrete house by producing 370 kg/m2 less in CO2 emissions (Trada 2010).
In excess of nine million tonnes of post consumer waste wood is generated in the UK every year. Less than 2% of this waste was recycled in 1996 but ten years later in 2006 the rate of recycling had risen to over 16% according to the UK Wood Recyclers Association with one of the emerging markets for the waste being the biomass sector. Woodwise (2008) describe the major benefits of recycling off cuts of wood from the construction industry into a major source of renewable fuel as the following:
Wood fuel is both carbon-neutral and renewable reducing CO2 emissions.
Reducing timber off cuts going to landfill reduces the methane gas produced by rotting wood. This is particularly relevant as methane gas is more than twenty times as damaging as CO2 emissions.
Wood is a renewable source of energy available throughout the year without being dependent on factors such as the sun or the wind.
The evidence from the research is quite clear that using timber as a construction material is more beneficial to the environment than using masonry or steel materials. The major benefits are that the embodied energy associated with timber is considerably lower than of the other materials therefore CO2 emissions are considerably lower. Additionally timber performs better in terms of inherent energy as it can be burned to release energy at the end of its lifespan.
The construction industry has made considerable efforts to recycle more materials when buildings reach the end of their natural lifespan thus reducing the energy required to extract, manufacture or refine new materials. This is evident from the reuse of bricks, the crushing of masonry rubble into aggregates and the use of timber off cuts in biomass, however there can be no debate that in terms of environmental performance timber easily outperforms masonry construction in relation to the reduction of harmful CO2 emissions.
Every source researched came to the same conclusion that using timber as a construction material provides greater environmental benefits compared to the use of steel, brick or masonry types of construction.