RdSAP Rating


The RdSAP (Reduced Data Standard Assessment Procedure) has been devised for DEFRA (Department of Environment, Food and Rural Affairs) as the preferred and accepted method of measuring the overall energy efficiency of existing dwelling. The SAP is related to the energy cost factor (ECF), based on the calculated annual energy cost for space heating, water heating and lighting. In addition to producing a cost based rating it also produces an estimated carbon dioxide (CO2) emission.

The SAP rating of a property determines the annual cost of space heating, water heating and lighting per square metre. The SAP calculation assumes standard occupancy and heating pattern derived from the floor area of the property. The rating is normalised for the floor area so the size of the dwelling does not affect the result.

RdSAP is the Governments recommended method of assessment of existing dwellings.

Factors affecting RdSAP

The rating is affected by the heating system, fuel type and controls. In addition, the ventilation, thermal insulation and lighting of a property are factors which determine the SAP rating.

What is assumed and excluded from the calculation

Assumptions made for the SAP calculations include;

  • Standard occupancy
  • Standard heating pattern
  • Standard lighting pattern

Exclusions made for the SAP calculations include;

  • Cost of cooking
  • Household size and composition
  • Other electrical appliances (washing machines etc)
  • Individual heating patterns and temperatures

If these factors which are excluded were taken into account then it would produce a wide variation in the rating which would make it difficult to decide if one property was more energy efficient than another similar property. RdSAP replaces the variability that real occupants would introduce with a standard occupancy. For example, a two bedroom flat would have a standard occupancy of two, and a one bedroom flat would have a standard occupancy of one.

For instance, two completely identical houses should in theory have the same SAP rating. But, if one house had two more occupants than the other, it would be likely the house with more occupants would use more energy, and energy bills would be higher. For this example it would be unfair to say one house is more energy efficient than the other. RdSAP uses standard occupancy and living conditions to eliminate this problem of variation and allows prospective buyers to compare similar properties on a like for like basis.

The running costs are calculated using a standard (average) heating pattern of 9 hours heating a day during the week and 16 hours a day on the weekend, with the living area heated to 21°C and the rest of the dwelling to 18°C.

About the EPC inspection

The assessor will require access to various parts of the property to observe and collect data. On some occasions, the assessor may need to ask the homeowner questions about the build date of an extension and any insulation added to the property. In some cases a certificate may be requested i.e. when a cavity has been filled/insulated. If the assessor is unable to find suitable evidence of insulation levels, then an assumed value will be defaulted which would reflect the standards to which the property was originally constructed.





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