Perhaps the most common of the insulation boards and certainly the cheapest is the Expanded Polystyrene boards that can be used on new build or retro fit as can the other types of board.
The main characteristics of the EPS board are :
Can be rasped and shaped to fit round curves etc
Accepts most renders and brick slips
Suitable below ground level and below DPC line
Ranges from 20mm to 300mm in depth
Special cavity drain system for NHBC warranties
60mm depth = 0.43 U value on a solid 9 inch wall