What Is a Net Zero Carbon Building?

Hareth Pochee

Principal Engineer

Max Fordham LLP

In general, the carbon emissions associated with a building fall into two categories; operational carbon and embodied carbon.

The General Aims and Requirements of Net Zero Carbon Buildings

Operational carbon emissions are those associated with the energy used to run the building once complete for aspects such as heating, cooling, hot water generation, lighting, lifts, computers, mobile phone charging, cooking and more. In most existing buildings these energy demands are met by burning gas in boilers or by using electricity.

Embodied carbon emissions are those associated with the building materials and products including the production, construction, replacement, demolition and disposal stages. In most cases the production stage dominates. Examples include the emissions that occur at a foundry that produced steel beams used in a building structure or at a factory that produced cement used in the concrete of a building’s foundations.

In general, net zero carbon standards are achieved by limiting the operational energy demands and embodied carbon emissions to levels that are considered compatible with a net zero carbon future, where all demand can be met by a mixture of on-site renewable energy generation and/or zero carbon energy (such as electricity, and possibly gas) supplied via the national grid. In most cases it won’t be possible to meet all of a building’s energy demand using on-site renewables and so imports from the national grid will be required.

In the short and medium term (say 15 years), the national grid will not be fully decarbonised and so buildings will need to make use of carbon offsetting to achieve a net zero carbon balance. The UK Green Building Council (UKGBC) Net Zero Carbon Building Framework definition describes the steps required to produce and verify a net zero carbon building.

Net Zero Operational Carbon

  • As a priority the building should be designed, built and operated to minimise operational energy demands
  • The building should include as much on site renewable energy generation as possible
  • In-use energy consumption and associated carbon emissions should be calculated and publicly disclosed on an annual basis
  • Residual carbon emissions should be offset on an annual basis using recognised offsetting schemes with a preference for off-site renewable energy schemes

Net Zero Embodied Carbon

  • The building should be designed and constructed to minimise embodied carbon emissions
  • The embodied carbon impacts from the production and construction stages should be calculated and offset at practical completion using recognised offsetting schemes
  • The details of the offsetting should be publicly disclosed at or soon after practical completion

Some Characteristics and Other Requirements of Net Zero Carbon Buildings

A net zero carbon building will have very low operational energy demands. For new buildings it is proposed that they should achieve annual energy demands that are around 70% lower than typical existing buildings. For example, a typical existing school building consumes around 200 kWh/m²/yr whereas the target for a net zero carbon compatible school is proposed by LETI and has been adopted by school providers as 65 kWh/m²/yr.

In order to achieve such low energy demands, net zero carbon buildings will have highly efficient passive design features and highly efficient building services systems. They will be highly insulated and draught proof, potentially triple glazed and will include technologies such as mechanical ventilation with heat recovery and heat pumps for heating and hot water. Buildings will also require to be managed and operated efficiency (not leaving things on when not required) and will need control systems to facilitate this.

It’s vital that buildings stop using fossil fuels as soon as possible. Whilst low or zero carbon gas might be available in decades time, it is unlikely to be widely available soon. Therefore, electric heat pumps will likely be the best option for most buildings.

Limiting peak electrical loads is an important part of a net zero carbon society. This is because the national peak electrical load determines the number and size of renewable energy power stations required. Being able to shift loads in time is also beneficial to be able to make the most of intermittent renewable energy sources. Therefore, net zero carbon buildings will include passive and active measures to limit and /or shift peak loads. The energy efficiency measures already mentioned contribute. Other examples include thermal mass to reduce peak heating and cooling loads, thermal water stores within heating systems, as well as batteries and smart appliances that reduce their demand at times of high grid stress.

A net zero carbon building needs to be designed and constructed with limited embodied emissions. Current targets are to aim for 40% less embodied emissions than current benchmarks of “business as usual” practice. For example, a typical new build office incurs embodied emissions of around 1000kgCO2e/m² for the production and construction stages of its life cycle a net zero carbon office should aim for 600 kgCO2e/m².

In many cases the best way to minimise embodied carbon emissions is to re-use, remodel and/or retrofit an existing building rather than building a new one. New Buildings will need to make use of low embodied carbon materials. Examples include sustainably sourced timber, materials with high recycled contents and low carbon cement replacements.

Irrespective of the choice of structural material (e.g. steel, concrete or timber) the main structure should be designed to last many decades and be adaptable for future changes of use.

Designers will need to consider how to minimise the quantity of material, in particular materials with high embodied carbon content such as metals and concrete and to facilitate future re-use or recycling of their buildings. Net zero carbon buildings should be designed and constructed so that they can be disassembled into high grade components, ideally of single material composition.

Net zero carbon buildings are a vital part of our response to the Climate Emergency and the impending thread of Climate Change.