It is not the polystyrene, fibreglass, polyester or wool that insulates it is the 'still air' that is captured in these products. Polystyrene for example is 98% air by volume.
By keeping the air very still, zones of different air temperature stratify across the insulation material from warm air adjacent to the floorboards to cool air at the base of the insulation. Since heat moves quickest where there is a large temperature difference and slowest where there is little difference, the thicker the insulation the slower heat is transferred through the insulation.
Heat will not flow from the base of the floor boards into the warmed air in the top of the insulation material at anything like the rate it will disperse into the cool damp air under an uninsulated floor. Similarly the slightly warmer still air in the base of the insulation material will not be displaced by the slightly cooler air in the damp cold underfloor space.
In practice however, in many New Zealand homes in exposed locations, with exposed or slat foundations, or 'gappy' tongue and groove floors, insulation performance is compromised because the air in the insulation material is not still as it is disturbed by wind gusts or drafts and the manufacturers R value claims are 'Gone with the Wind'
Fetcher Owned Tasman Insulation's Pink Batt Snugfloor, a high performance under-floor insulation Batt claims to eliminate draughts in exposed floorboards, Investigating that claim shows Pink Batts Snugfloor requires more than 99% secondary cover with a windbreak of 4mm oiled hardboard sheet to achieve performance R Values in exposed installations.