We chose 200mm of EPS 70 (Expanded Polystyrene) as the insulation for the floor.
We considered using XPS (Extruded Polystyrene) or Super Plus EPS 70. The latter was three times the price of regular EPS 70 and the Thermal Conductivity/Lambda was 0.030 instead of 0.038. Steve Harris did some calculations and we concluded it would take more than 100 years to recover the additional cost of the superior insulation through reduced heating bills.
We used [10mm] OSB spacers to achieve an adequate gap for the expanding foam.
We held the insulation down with concrete blocks to stop the expanding foam lifting the sheets.
The drains went underneath the insulation and the hot/cold pipes ran within the upper layer.
We cut 50mm wide strips for the perimeter to sit beneath the 2″x2″ timber defining the finished slab level.
Insulated and ready for the steel reinforcing mesh.