screed ufh insulation

Wondering if anyone could shine some light om my flooring? Ive got amtico spacia, ontop of an electrical ufh system which has been set into a screed. Its been done by a professional but after 2 weeks the floor started bubbling, and the mat that the cables sit in has buckled in several areas of...

I'm planning to install UFH on the ground floor at my house which currently has a suspended wooden floor. I have around 300mm void under the floorboards with a concrete base. Assuming I go with a 75mm screed to fill, my question is what should I fill it with as I would like to do this myself if...