BRE has unveiled a prototype version of a software tool that can assess the energy performance of buildings. The tool, commissioned by the ODPM, has been designed to demonstrate a building's compliance with the new Part L of the Building Regulations as well as its rating under the European Union's Energy Performance of Buildings Directive.

Known as the simplified building energy model, the tool is based on the national calculation methodology. This is the process laid down in the consultation document on Part L that forms the basis for demonstrating buildings' compliance with minimum energy performance standards. And because the new Part L has been written to ensure compliance with the EU's EPBD, the national calculation methodology is also the process that will be used to work out a building's EPBD asset rating. This reveals how much energy the building uses under normal design conditions and will have to be displayed in larger public buildings.

To determine the integrated energy performance of buildings, the national calculation methodology has to take into account a large number of factors. These include the building's fabric, its heating, ventilation and air-conditioning systems, its position and its orientation. The building's occupancy and usage must also be considered and the methodology rationalises the infinite possible variations by breaking down the activity going on in distinct spaces within the building � in terms of the number of people, their activity level, the hours of occupation, the light levels required to carry out work and the amount of heat generating equipment within the space � into standardised "activity area" categories.

Despite this kind of standardisation, however, the methodology is necessarily complex � which is why the ODPM commissioned BRE to develop a simplified calculation method, to be used for the majority of ordinary buildings. The simplified building energy method is based on a Dutch public-domain methodology, which has been used in the Netherlands to set performance standards and employs monthly average calculations.

However, some buildings are so sophisticated in their design, that their energy performance can only be calculated by means of sophisticated energy simulation models that use hourly calculations. In this case, as simulation is expensive, time-consuming and requires significant expertise, the ODPM's proposal is that simulation using certain accredited tools will be allowed as a means of assessing a building's energy performance.

The national calculation methodology relies on the use of a notional building. This is a building with the same geometry and activity data as the actual building, but built with fabric and service systems in accordance with the 2002 Part L2 standards. The methodology then requires that two calculations are performed: one on the actual building with the actual fabric and actual heating, ventilation, air-conditioning and lighting systems; and one on the notional equivalent building. For Part L compliance purposes, the actual building has to perform better than the notional building by a specified percentage improvement.

This approach has to be used whether using simulation or BRE's SBEM software to do the calculation. It requires access to a database of activity area data, which is kept the same in both buildings. At the design stage, the resulting relationship between the two buildings yields a design rating; once the building is completed, the final EPBD asset rating is determined, based on the building "as built".

In order to carry out the calculation, BRE has also developed a basic user interface, which captures all the required information about the building in terms of its geometry, its construction, the activities within it and the service systems. This interface, know as iSBEM, works with SBEM as illustrated below right.

The interface has been developed using Microsoft Access in the short term, though ideally it should be independent of any specific software application. It asks the user a series of questions through Access forms, in order to produce the required input for the calculation core itself. Part of the process involves interacting with the activity database to input the relevant "activity area" categories in the building. The tool can also interrogate a database of fabric details, which contains descriptions of many different wall, floor, roof and glazing systems and can be used in two ways. If the user knows the detail of the wall construction, for example, they can select it from the library of details; however, if details are not available �for example in an existing building � there is a crude but effective inference procedure that will provide a default construction system based on the age of the building. A similar process provides details on heating, ventilation, air-conditioning and lighting systems.

The first version of the whole package is now available for what is optimistically called "beta test" from a special BRE website (
). It contains the calculation tool SBEM, plus the interface and fledgling databases. The data contained in the databases is subject to review and refinement in consultation with a range of industry representatives, but the basic framework should remain fixed. The interface requires the user to define their building in advance, and guidance on using the tool is also available.

It should be emphasised that the tool is not yet complete, and that the answers it produces are not yet suitable for use either in Building Regulation submissions or in calculating EPBD asset ratings. BRE and ODPM welcomes feedback of the functionality of the tool and on suggestions for improvement, using the feedback form on the website. BRE will be launching a revised version of the tool in July, which will take into account the feedback received, and the final version will be released in conjunction with the revised Part L of the Building Regulations. It might not be perfect, but it does seem to offer the promise of a flexible and relatively simple calculation method.

To see the national calculation method in action, a demonstration is being held at BRE's Resource 05, the low-carbon technology showcase, on 14 September.

For more information visit the website at
[external__blank::http://www.resource05.co.uk]