Passivhaus and Embodied Carbon

Rachel Mitchell, Sarah Lewis, Richard Pender, John Butler.

It is estimated that buildings are responsible for over one third of greenhouse gas emissions globally. There are six ways buildings can cut these emissions:

  • reduce operational energy use
  • decarbonise operational energy use
  • reduce construction material use
  • decarbonise the manufacture of construction materials
  • reduce activities on construction sites
  • decarbonise activities on construction sites.

As buildings become more energy efficient, and with a rapidly decarbonising National Grid, the balance between the carbon emitted when running a building (operational carbon) and the carbon emitted to build it (embodied carbon) is changing. The whole life carbon footprint is a way to assess both operational and embodied carbon. It runs from “cradle to grave”, that is, from product material extraction to ultimate building demolition (or better still deconstruction). The whole life carbon footprint thus gives a good indication of the carbon impact of a building though its lifetime.

In new buildings, embodied carbon can represent as much as 40-70% of a building’s whole life carbon footprint. Both the RIBA 2030 Climate Challenge and the LETI Climate Emergency Design Guide have set minimum standards for embodied carbon, and the lack of embodied carbon calculations in UK Building Regulations is being challenged through the proposed introduction of a Part Z in England.

The government’s approach to achieving zero carbon homes in the UK is based on the decarbonisation of the grid and the switch to heat pumps (which have their own embodied carbon implication as shown later). This will require a huge increase in grid capacity to meet the electrical needs of housing alone, unless we continue to drive down the operational energy used in our buildings. Passivhaus buildings are optimised for a decarbonised grid, by slashing the peak energy demand, thus facilitating a smooth transition to renewable energy for all sectors, not just buildings. In addition, well insulated buildings also allow for ‘load shifting’ which means you can be more flexible with the timing of heating while still retaining comfortable internal temperatures. This means you can heat your home outside of peak times, when energy prices will be cheaper.
Consequently, we need to rethink how we design, construct and supply energy to our buildings. Once the grid has completely decarbonised, the only emissions associated with a building will be the whole life carbon of its construction materials, component replacement and the infrastructure needed to generate and supply its energy demand.

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