National Calculation Method
Introduction to NCM
The National Calculation Method for the EPBD (Energy Performance of Buildings Directive) is defined by the Department for Communities and Local Government (DCLG). The procedure for demonstrating compliance with the Building Regulations for buildings other than dwellings is by calculating the annual energy use for a proposed building and comparing it with the energy use of a comparable 'notional' building. Both calculations make use of standard sets of data for different activity areas and call on common databases of construction and service elements. A similar process is used to produce a 'asset rating' in accordance with the EPBD. The NCM therefore comprises the underlying method plus the standard data sets.
The NCM allows the actual calculation to be carried out either by an approved simulation software (click here for details), or by a new simplified tool based on a set of CEN standards. That tool has been developed for DCLG by BRE and is called SBEM - Simplified Building Energy Model. It is accompanied by a basic user interface - iSBEM.
Introduction to SBEM
SBEM is a computer program that provides an analysis of a building's energy consumption. SBEM calculates monthly energy use and carbon dioxide emissions of a building given a description of the building geometry, construction, use and HVAC and lighting equipment. It was originally based on the Dutch methodology NEN 2916:1998 (Energy Performance of Non-Residential Buildings) and has since been modified to comply with the recent CEN Standards. Details of the calculation method, the algorithms used and the assumptions made are provided in the SBEM Technical Manual, now available from the Download page.
SBEM makes use of standard data contained on associated databases and available with other software.
The purpose of SBEM and its interface iSBEM is to produce consistent and reliable evaluations of energy use in non-domestic buildings for Building Regulations Compliance and for Building Energy Performance Certification purposes. Although it may assist the design process, it is not primarily a design tool. It does not calculate internal temperatures, for example.
As iSBEM is a compliance procedure and not a design tool, if the performance of a particular feature is critical to the design, even if it can be represented in SBEM, it is prudent to use the most appropriate modelling tool for design purposes. In any case, SBEM should not be used for system sizing.
In line with the revision to Approved Document ADL2A, SBEM and iSBEM have now been revised and updated to include additional functionality. This version - v4.1.e - is now available for demonstrating compliance with Part L 2010 and generating Energy Performance Certificates. The tool, along with a user guide, can be downloaded free of charge from the Download page.
SAP for dwellings
The calculation method for dwellings under Part L 2010 is a new version of SAP - the Standard Assessment Procedure for the Energy Rating of Dwellings. This is specified in documents available at http://www.bre.co.uk/sap2009/ and will be implemented through a number of commercially available software tools.