I have a property with a 150mm (6") thick concrete slab throughout. The slab has a step down of approx 300mm / 1 ft, running the width of the house. This means the front rooms are at a higher elevation than those at the rear.
I want to connect a 3m^2 room adjacent to a front room, without a step between them. This requires raising the level of the connected room by 300mm.
The finished buildup over both slabs will be: 150mm PIR rigid insulation, 50-60mm screed (with wet underfloor heating) then tiles.
An added complication is there is a timber stud wall parallel to the step, with the base plate on the lower part, thermally decoupled from the slab it sits on using thermoblock. It is decoupled from the higher slab by 50mm PIR sandwiched between the stud wall and the higher slab to a height level with the upper slab.
What is a recommended way to raise the lower rooms floor level?
A couple of options I have thought of:
On top of the PIR, add 300mm of concrete. This would mean the upper layer of concrete was inside the thermal envelope of the building. Seems like a lot of concrete? And will cause lagging effects on the heating / cooling of the space - this might be a benefit though?
bound the borders/flanks of the connected room by 50mm PIR to a height of 300mm, then fill with compacted aggregates / hard core / rubble, possibly cap off with some concrete blinding and then have the 150mm PIR over that, level with the rest of the PIR in the front room.