What does zero mean?

The industry needs a clear set of definitions for buildings, and CIBSE Members have two opportunities to have their say, as Julie Godefroy explains

Having a set of commonly agreed, clear and comprehensive definitions for ‘net zero’ buildings is necessary for investment, specifications, trusted and consistent reporting, certification and, ultimately, regulations.

There has been great progress on this in the past few years – first with the UK Green Building Council (UKGBC) framework, and then the London Energy Transformation Initiative (LETI) one-pager on operational carbon (energy use), which is supported by CIBSE. As a next step, LETI and the Whole Life Carbon Network (WLCN) recently produced a full set of definitions, including operational carbon, as well as embodied and whole-life carbon.  

CIBSE now has two important opportunities and decisions on which members can engage: 

CIBSE – LETI FAQ on definitions 

The LETI-WLCN definitions provide principles, but it is difficult to cover, in a succinct and clear way, the range of situations that a building and its energy supplies may face in practice. CIBSE will now be working with LETI to produce a set of FAQs on these definitions, to support clients and project teams, and bring consistency in how definitions are applied. Clarifications will be provided, along with illustrations of a range of situations. 

Some examples of what the FAQs will cover are provided below. They focus on the definition for a ‘ net zero carbon – operational energy building’, which currently reads ‘one where no fossil fuels are used, all energy use (Module B6) has been minimised, meets the local energy-use target (for example, kWh.m-2 per year) and all energy use is generated on or off site using renewables that demonstrate additionality. Any residual direct or indirect emissions from energy generation and distribution are “offset”.’

  • The incorporation of energy-use targets in the definition may be queried by some: why is it required if energy supplies become zero carbon? Energy efficiency and demand reduction are, of course, supported by CIBSE, and it is right that they are integral to the definition, as they are required to support decarbonisation of the energy system, as highlighted by National Grid ESO.  
  • The definition implies that either the building is supplied by its own onsite renewables, or it uses a Power Purchase Agreement for renewable energy or a ‘green tariff’ demonstrating additionality, with guidance on tariffs provided by the UKGBC. 
  • Once the electricity grid achieves carbon, this will include nuclear energy – so, not meeting the definition’s requirement for all energy to be from renewable sources. A clarification in the definition is, therefore, required to cover that point. In addition, and assuming that the end point is accepted as  carbon, this triggers the question of what happens in the few preceding years: is there a point at which grid electricity is not yet carbon, but is so low carbon that additional renewable capacity is no longer considered beneficial or required by the definition – and, if so, when? Under some National Grid scenarios, such ‘near zero’ content could be achieved in the first half of the 2030s – the not-distant future when planning a building – so, again, a clarification in the definition may be useful already. 
  • Biomass is provided as an example of ‘residual direct emissions’. ‘Residual indirect emissions’ intend to cover those from the transmission and distribution of electricity, and ‘well-to-tank’ emissions. This needs explaining, with guidance on how to calculate them.  
  • If hydrogen did become available for use in buildings in the future, the definition’s requirement that all energy should be generated by renewables implies that only ‘green’ hydrogen would be allowed, and all indirect emissions associated with generation, storage and distribution would have to be offset. This needs explaining and, at a future point, guidance given on calculations. Again, this may seem a long way off, but the government’s ambition is for trials at the neighbourhood and village scales in 2023 and 2025 respectively.1 
  • How does the definition apply if a building is supplied by ‘green gas’ produced on site from, say, anaerobic digestion? Should ‘green gas’ tariffs be allowed as they are for electricity – for example, using the UKGBC guidance on such tariffs? Or would this risk encouraging continued reliance on the gas grid while its decarbonisation is still uncertain? 
  • How are buildings supplied by heat networks to be addressed, including boundaries for calculations of energy use and carbon offsets, and with networks using varied energy sources? 

Some of these themes are complex and not necessarily clear-cut, so we will be looking for opinions on what is the right thing to do to truly support the UK’s decarbonisation, as well as technical advice on points of detail. 

Should ‘green gas’ tariffs be allowed, as they are for electricity, or would this risk encouraging continued reliance on the gas grid?

Should CIBSE adopt these definitions? 

CIBSE could decide to adopt the LETI-WLCN definitions. However, engagement with members is essential on such an important topic, to create the best possible set of definitions for us all to work from. So, CIBSE is keen to hear whether members think we should: 

  • Adopt the current set of definitions
  • Adopt them subject to modifications and, if so, which ones
  • Create a different set of definitions – probably undesirable when industry needs clarity rather than, say, an engineer and architect working to different goals 
  • Not adopt any particular definition – again, not an ideal situation if we want to give clear advice on the end goal for building performance.  

Tests when deciding whether to adopt the definitions should include: 

  • Do the definitions support real decarbonisation?  
  • Are they clear and flexible enough to apply in all possible situations (alongside an FAQ)? 
  • Do they work in different contexts than the UK? 
  • Should they be ‘absolute’ and continue to work until 2050, or should we accept that they may need future revision to reflect a changing energy system? 

Have your say

Please send your thoughts on the definitions, the questions above, what you think the FAQs should cover, and what position you think CIBSE should adopt, to the technical team at JGodefroy@cibse.org 

Also, look out for events and a survey in the next few weeks, when we will be collating views.  


  1. UK Government, UK Hydrogen Strategy, August 2021.

Dr Julie Godefroy is head of sustainability at CIBSE