The changes and updates to RdSAP methodology and software have increasingly made the EPC for existing dwellings more consistent and accurate. The energy rating of a property calculated using RdSAP 9.91, to be introduced in April 2012 will differ from an energy rating previously carried out on the same dwelling. The main reasons for this are as follows:

RdSAP 9.91 is more sophisiticated with data entry on issues previously ignored eg insultaed doors and floor insulation.

RdSAP 9.91 will provide an option to include a precise U-value of construction enements if known.

Additional items will be added to the Product Characteristics Data File (PCDF) in the new release of RdSAP 9.91. For example, flue gas heat recovery and waste water heat recovery systems are to be included.