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NCM Call for Evidence - Joint Submission by CIBSE and LETI


In order to collate the most information possible in this short space of time, and in a single location making it more straightforward for the BRE and DLUHC to analyse, CIBSE and LETI have worked together to provide this joint submission. A large number of companies have provided support to this submission, through their expertise and modelling, including Buro Happold, Elementa, Etude, Hawkins Brown, Inkling, and Max Fordham.

This CIBSE-LETI response focuses on essential points, including:

  • Evidence that Space Heating is consistently and systematically under-estimated by the NCM (both in SBEM and DSM calculations). This devalues the effect of fabric and services efficiency measures. We have submitted evidence of this as comparisons between NCM calculations and other types of calculations, best practice (i.e. Passivhaus space heating demand), and in-use space heating data. All support the fact that space heating demand is under-estimated by NCM
  • The need to review the NCM database: The submission identifies the relatively short amount of time given to respond, highlighting that to comment meaningfully on all of the assumptions within the NCM, including occupancy density and profiles; assumed heating gains; and set points, for all building uses, would require significantly more time and should receive dedicated attention through a systemic study. 
  • The need to review hot water demand assumptions. In hotels, demand seems over-estimated.
  • Hourly vs monthly calculations (for SBEM) A hybrid or hourly approach in SBEM may be useful but it could introduce complexities with yet unclear benefits. Smaller time steps should not be confused with increased accuracy. On balance, this is probably not a priority for development, as long as DSM remains an option. As alternative, an option may be to expand the occasions where DSM should be used.

We stress that more comments and more evidence could be gathered given proper time.

This is just a snapshot of issues and evidence. CIBSE and LETI strongly recommend a comprehensive review of the NCM, starting with a scoping study in a similar fashion as was carried out for the Standard Assessment Procedure 11 (SAP11), to ensure NCM is suitable for the Future Buildings Standard and for future versions of Part L applied to work on existing buildings.

Download the full response

NCM in context: Part L

While we understand that NCM is the calculation methodology, and is to some extent independent from its context, in order to effective energy and carbon savings it needs to operate within a supportive ecosystem of regulations. Both CIBSE and LETI have expanded on this previously, and we would be very happy to have the opportunity to discuss this in more detail. In summary:

  • The notional building approach needs to be reviewed, as it hinders comparisons
    between building design options and over time, and does not drive absolute
    improvements. It is very difficult for policy-makers, clients and designers to actually compare design options.
  • Regulations on commissioning should be enforced. Ways to incentivise this through the Part L assessment should be investigated e.g. penalty on building systems efficiency used in as-built Part L calculations unless evidence is provided that commissioning has been carried out.
  • The ADL2A requirement for energy forecasting should be meaningful i.e. through
    performance modelling such as TM54, NABERS or PHPP. Current options such as
    “benchmarks” and “design calculations” can mean all sorts of things. They may be of very little use, or even misleading, to building occupiers.
  • A significant measure to reduce the performance gap would be to require monitoring and disclosure of in-use energy performance. There was a previous government working group on proposals to introduce such a measure beyond public buildings. This workstream needs to be re-invigorated, and the policy should be finalised and implemented.


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