Insulation in renovation, Suspended timber floor

[andreas]

We have to replace some rotten floor joists and floorboards in a downstairs room on a renovation.

If I am reading correctly, if you take up all the floorboards you have to install insulation in that room? The joists are 100mm depth on sleeper walls, the distance between the bottom of the joist and the concrete oversite is 120mm.

What would be a good option for PIR install and at what thickness, balancing cost and performance? I have read multiple conflicting measurements, some say leave at least 150mm between underside of PIR and concrete, so would be limited to 70mm if correct?