Insulation boards, which to use?

[Accord200]

I need to fit some insulate boards to a room before self leveling over the top of the boards to level the floor to the same height as adjoining floor.
Both floors will then have a piped hot water underfloor heating system fitted which will be screeded again before laying either tiles or laminate.
Question is what type of insulation board can I use? Your thoughts welcome.

 

[Lou]

Hi what sort did you end up using?