Underpinning foundation wall and installing thermal break

[Clennls]

Hi all,

 

I have about 20 m where i need to underpin my existing wall. Between the new foundation and the existing wall i would like to install a thermal break (the height would be perfect for our underslab insulation). The problem is i don't see a way to use Celcon blocks or xps-concrete blocks because i don't think it is possible to place them and protect them from water and damp.

Currently we plan to use Perinsul because it is water and damp proof without the need for extra foils or membranes. But this is very expensive. Are there any other solutions? Liquid waterproofing?