This is part of the building code in many areas: do a search for [under] "slab insulation".
* https://www.buildingscience.com/documents/information-sheets...
* https://www.finehomebuilding.com/project-guides/insulation/i...
It should be at least R-10 (RSI 1.8):
* https://www.youtube.com/watch?v=O2dNp9bicSk
* https://www.youtube.com/watch?v=XKp0wRzvR-g