Jersey RdSAP Convention 3.14
Within RdSAP, it is not necessary to change the U-value of a building element from the default 'as built' values based on the dwelling age band.
If insulation is added, the retro-installed thickness can be entered, and the calculation engine will assume an appropriate U-value for the element.
Where these defaults have been overwritten, you must be able to provide specific documentary evidence that the U-value has been:
1. Calculated for the whole element, not just insulation;
2. Produced by a suitably qualified person;
3. Produced in with BRE "Conventions for U-Value Calculations".