The heat passing through a panel is inversely proportional to its thickness: a 100 mm panel passes half the heat of a 50 mm one. The greater the difference between outside and room temperature, the thicker the panel needed; that is why thickness increases in frozen rooms. The calculation is to find the thickness that keeps the heat passing through the panel (W/m²K × area × temperature difference) at an acceptable level (about 8–10 W/m²).
Practical table: 60 mm for +10 °C ripening and dry stores; 80–100 mm for 0 to +5 °C storage; 100 mm for −5 to 0 °C; 120–150 mm for −18/−25 °C frozen storage; 150–200 mm for −30/−40 °C blast freezers and tunnels. If the surroundings are very hot (a sunlit outer wall, a roof) or the panel faces the outdoors, the next thickness up is chosen; in nested rooms (a frozen room inside a chilled room) the partition wall can be thinner.
The effect of thickness on cost is small (going from 80 to 100 mm raises the panel price by 10–15%), its effect on energy is lifelong. A panel that is too thin sweats on the outside, causes icing and mould in the corners and keeps the refrigeration system running continuously. In the cooling load calculator you can see how the conduction load changes as panel and floor thickness are selected.