The method of calculating energy performance in homes – SAP – has been updated for 2005. Meanwhile a new version for non-dwellings is being developed – and it’s a much more complicated affair, says Paul Davidson of BRE

One of the most challenging aspects of the EU’s Energy Performance of Buildings Directive is the requirement for a calculation methodology that will measure energy performance in a very accurate and detailed way.

For dwellings, coming up with a new calculation has been relatively straightforward – the Standard Assessment Procedure has been tweaked to calculate the energy performance of dwellings (see “SAP 2005”, below). However, the development of the National Calculation Method for non-dwellings has been a more complex and time-consuming affair, so much so that with five months to go before the EPBD comes into force a final version has yet to be published.

The National Calculation Method meets two of the most important requirements of EPBD – it will demonstrate compliance with the minimum performance standards set by Part L2 in accordance with EPBD, and it will derive an asset rating in accordance with the energy performance certificate.

The EPBD requires that when calculating energy performance designers take into account the building’s fabric, HVAC systems, position and orientation. But the tool must also consider the specified conditions of occupancy and usage – the activity in the building. These requirements go well beyond what’s contained in Part L of the Building Regulations.

The predicted energy demand of any building is determined in part by the thermal performance of its fabric and its resistance to unwanted air infiltration. It will be influenced by the form, orientation and thermal mass of the building and by the extent to which daylight is able to penetrate. The energy needed to meet that demand will depend on the type and efficiency of the heating, ventilation, cooling and lighting systems.

Energy demand is also driven by the activity going on in the building. Factors include the number of people in the space, activity levels, how often the space is occupied, temperature of the space, levels of fresh air and light required and the amount of equipment giving off heat.

This leads to the concept of different activity areas within buildings. In order to calculate the performance of a building, it is necessary to define sets of standard values for activity types.

The approach adopted by the ODPM for demonstrating compliance with Building Regulations, and for calculating the asset rating, 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 regulations standards. The NCM requires that two calculations are performed, one on the actual building with the actual fabric and actual HVAC and lighting systems, and one on the notional equivalent building. To comply with Part L, the actual building has to perform better than the notional building by a specified percentage improvement.

This approach requires access to a database of activity area data. At the design stage, the resulting relationship between the actual and notional building yields a design rating. Once the building is completed, the final asset rating is determined, based on the building “as built”.

The proposed NCM recognises that some buildings are so sophisticated in their design that their energy performance can only be calculated by means of sophisticated energy simulation models. But simulation is expensive and time-consuming and for existing buildings and for more straightforward new-builds the BRE has developed the Simplified Building Energy Method, which calculates the energy performance of non-dwellings.

To help designers carry out the calculation, BRE has developed a basic software user interface SBEM, which captures all the required information about the building in terms of its geometry, construction, activities within and the service systems.

If the designer does not have details of the building available (for example in an existing building), SBEM will provide a default construction system based on the age of the building. A similar process provides details on HVAC and lighting systems.

The first version of the whole package is now available for “Beta testing” from BRE. Guidance on using the tool is also available.

SAP 2005: Assessing the energy performance of homes

In the new version of Part L, the energy performance of dwellings has to be measured using the Standard Assessment Procedure 2005. Elemental and target U-value methods will no longer be permitted.

SAP 2005 assesses the energy performance of dwellings by considering the energy costs associated with space and water heating, ventilation and lighting, and meets the requirements laid out in the Energy Performance of Buildings Directive. The latest draft of SAP 2005 differs from the current SAP 2001 in a number of ways:

  • SAP scale is revised to 1-100 where 100 represents zero energy cost. Dwellings that are net exporters of energy can receive a score of more than 100.
  • The dwelling CO2 emission rate replaces the carbon index.
  • Energy for lighting is included.
  • Solar water heating is revised.
  • Manufacturer’s data for heat loss is now the preferred source of cylinder loss.
  • The effect of thermal bridging is taken into account.
  • It incorporates additional renewable and energy-saving technologies.
    Existing dwellings will be assessed using a reduced version of SAP. SAP 2005 is applicable to dwellings with a total floor area not exceeding 450 m2. The latest draft of SAP2005 can be found at