I have a concrete base that is slightly inground from a previous pool. The base is currently buried under rubble and topped with gravel but I am having all this removed next week to expose the base. The top of the base was quite bumpy where the builder used the wrong concrete mix so it was topped with sand but the bumps never totally disappeared.

The base is going to have a top screed to make it level and smooth. Just wondering what could be used for insulation. Is it possible to put some insulation board down before the screed. I will of course ask the builder but I would be interested to know if anyone else has done this and what they used.