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What is net zero carbon?

UK Construction Intelligence Report

In June 2019 the UK Government legislated for a target to reach “net zero carbon” emissions by 2050. This means all sectors of the economy will need to reduce their emissions of carbon dioxide and other greenhouse gases by moving away from fossil-fuels for heat and power production as well as in transport, industry, agriculture and buildings. These changes are needed as part of coordinated global response to prevent dangerous levels of climate change occurring this century.

Approach

Focusing on the built environment, the UK Green Building Council has developed a framework definition for what net-zero means for building construction, operation and demolition in what is known as "whole life carbon".2 This framework is illustrated below.

Trends

The framework is useful because it helps us to target practical steps to reduce emissions at each stage of the building lifecycle. For example:

  • At the construction stage, materials with low or zero “embodied” carbon should be selected. For example, timber products have the benefit of storing carbon during their growth phase.
  • Reducing operational carbon should start with energy efficiency (e.g. high standards of building insulation), but also include designing out the use of fossil fuels (e.g. gas boilers) and using renewable energy (e.g. solar PV).
  • Designing buildings to be flexible for adaptation to changes in use and for disassembly at end of life further supports whole life carbon reductions.

Examples

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The building construction industry has been taking steps towards carbon reduction for some time and there are many leading examples of low carbon buildings. However, progress needs to be accelerated for both new construction and refurbishment across the sector. There are a range of existing design standards and guidance available support low carbon and sustainable buildings, such as BREEAM, Passivhaus and Ska. Taking the example of PassivHaus, its approach to low carbon buildings is based on the following design principles:

  • Super-insulation, including triple-glazing
  • Optimisation of the orientation and shading of a building to harvest heat from the sun in winter but keep it out in the summer
  • Careful design of the building envelope to design out thermal bridges that increase heat loss
  • Very low air leakage
  • MVHR (Mechanical Ventilation with Heat Recovery)
  • A study completed by the Passivhaus Trust indicated a capital cost uplift of circa 9% is typical of current best practice for Passivhaus buildings.i

Summary

A net zero building is:


  • At the construction stage, materials with low or zero “embodied” carbon should be selected. For example, timber products have the benefit of storing carbon during their growth phase.
  • Assembled on site using materials and processes that have low or zero embodied carbon impact.
  • Highly energy efficient and powered by clean, renewable energy.
  • Maintained, repaired and refurbished with low or zero embodied carbon and can be reused or recycled at the end of its life.
  • At any one of these stages, residual emissions can be offset.