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Measuring embedded carbon - the next indicator of sustainabilityOctober 2009

BSRIA has published the Inventory of Carbon and Energy to help choose the ideal materials for low embodied carbon buildings.

Following on from BSRIA's article Embodied energy calculations design tool , David Churcher looks at applying the theory.

BSRIA is one of 26 partners on this major project sponsored by the EU
In recent years the main emphasis in reducing the carbon impact of buildings has been in reducing the energy used during occupation. This has been tackled in a number of different ways - improving the insulation in buildings to reduce heat loss or the need for cooling in hot weather, installing plant and equipment that uses energy more efficiently, rethinking the design of HVAC systems to use passive measures instead of mechanical systems.

Operational energy use for office buildings on a university campus have been benchmarked by Imperial College at 110-350 kWh/m2 Treated Floor Area/year . This covers heating, cooling, lighting, small power and ancillary building systems. Approximate figures for embedded energy in office buildings has been estimated at 1100-3300kWh/m2 (based on figures of 4-12 GJ/m2 quoted at an industry conference in 2004).

Hence to a first approximation, embedded energy is equivalent to 10 years of operating energy. As pressure continues to reduce operating energy then this multiple is likely to increase, and it will become more important to understand and then to reduce the embedded energy and CO2 emissions associated with manufacture and installation.

The difficulty with calculating specific figures for embedded energy and CO2 emissions is sourcing appropriate data. Ideally each component manufacturer would be able to quote the embedded energy and the CO2 emissions associated with each of their products. Contractors could be then combine these figures with other those for building materials and the construction processes to give an overall figure for embedded energy or CO2 emissions. In the benchmarking exercise in I3CON Work Package 4, we have used an elemental approach to calculate the carbon emissions in two different external cladding products. We have specifically not considered the or CO2 emissions associated with transportation or installation since these are so site specific.

The methodology we applied is as follows:

  1. Identify the principal materials used in the cladding system, for example aluminium, glass, concrete
  2. Identify the quantities of each material used in a sample area of cladding (in our benchmarking this was an area of wall 7.2m long and 7.6m high). These quantities start from drawings or specifications for the system to calculate volumes of each raw material. From this the mass of each material is calculated.
  3. Use the Inventory of Carbon and Energy developed by Bath University in the UK to establish an appropriate figure for embedded carbon per kg for each material.
  4. Multiply the weight of each material by the CO2 emission factors to give CO2 emissions for each material
  5. Sum the separate CO2 emission figures to give an overall total, and divide this by the elevation area of the cladding to give a unit figure of CO2 emission/m2 wall area.

Embedded CO2 in sample wall area (click image to zoom)
Although in this case we are primarily interested in the overall total for each cladding system, this approach does enable us to see the relative CO2 emissions for the different materials in the two cladding systems. This is shown in the chart.

The conclusion of this analysis is that at a product level, the precast concrete cladding had a much lower level of CO2 emissions over a typical façade detail, and that this is primarily because the aluminium in the unitised curtain walling has such a high level of CO2 emissions.

This article was written as part of the outputs for the EU funded I3CON research project.

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