Structural engineers have a significant influence on the initial embodied carbon due to the construction (or refurbishment) of buildings. The embodied carbon is often expressed as being equivalent to a number of years of operating energy and some benchmarks for different building types are provided.
While structural engineers have limited direct influence on building energy consumption (although facade design, thermal bridging and creating thermal mass can all have a small impact) it is useful to understand typical operating energy consumption of buildings so that the embodied carbon can be placed in context.
EPC and Part L
It is important to note that Part L and EPC calculated energy consumption has no correlation with actual metered energy consumption. This is because the modelling provides a measure of energy efficiency of the building fabric and some, but not all, of the building services compared to a compliant base case building.
It indicates the potential for the building to be low carbon, but does not predict likely energy consumption. Actual energy can be up to five times greater than the number stated on the EPC and so consequently EPC / Part L figures should not be used when comparing operating and embodied carbon impacts.
Annual energy consumption
Real energy consumption is hugely variable and depends on:
- Building type
- Thermal performance of the fabric
- Type and efficiency of heating, cooling, ventilation, lighting, domestic hot water and other systems
- Equipment plugged in
- Hours of operation
- Average climate that year (warmer or colder)
- Control strategy
- The expectations and behaviour of the building occupants
In Appendix C of What Colour Is Your Building various benchmarks for different building types have been collated from different sources. A summary of some of the benchmarks (including best practice and typical) can be found in this table.
These are based on: Electricity = 0.6 kgCO2e/kWh, Gas = 0.2kgCO2e/kWh, GIA = Gross Internal Area
Embodied versus operating carbon
If embodied carbon assessments are intended to estimate the actual CO2e emissions due to construction, then they should really be compared with the CO2e emissions due to actual operation.
The embodied carbon for new construction of office buildings in the UK is typically between 500 and 900 kgCO2e/m2 of GIA. This is equivalent to five to ten years of the CO2e emissions due to the energy consumption.
Embodied versus renewables
The contribution that onsite renewables can make to reducing the whole carbon footprint of a building varies significantly depending on the building type and configuration, including the area of roof available for solar panels.
For example, a ten-storey 10,000m2 office building in London with a roof full of photovoltaics could reduce annual carbon emissions by around 2 kgCO2e/m2.
The embodied carbon of the construction of the building (excluding fit out), spread over a 60 year design life, is between 6 to 15 kgCO2e/m2.
Taking a typical value of 5 kgCO2e/m2 and assuming the structure (primarily steel and concrete) represents 50% of this, then the structure accounts for 2.5 kgCO2e/m2.
If the structural engineer can, through clever design and material specification, reduce this by 30% then the saving is 0.8 kgCO2e/m2. This is comparable to the renewable energy savings (which will diminish over time as the electricity grid is decarbonised).
Another key point to note is the CO2 savings are made now and not in the future.
The future
Buildings need to have much lower energy consumption and legislation related to energy efficiency (building regulations) and transparent reporting of actual energy consumption (eg ESOS and DECs in UK).
The role of the structural engineer, as part of a multi-disciplinary design team, is to develop structural solutions that contribute to reducing energy consumption, and to deliver buildings with a long life and a low embodied carbon footprint.
Further information
Conservation of fuel and power: Approved Document L