Sizing the Walls of a Low-Cost Insulated Fish Box: Expanded Polystyrene versus a Sawdust-Filled Double Wooden Wall

An insulated fish box is usually built with whatever sheet insulation is at hand, at whatever thickness fits, and its performance is discovered on the first hot trip. This paper instead sizes the walls from a stated duty: a box of fixed external size 1.00 m × 0.60 m × 0.60 m must hold 40 kg of fish, loaded at 5 °C, with a 25 kg ice charge, at or below 4 °C for 48 hours at an ambient temperature of 35 °C. Because the contents remain at about 0 °C while any ice remains, the duty reduces to one condition — the ice must not be exhausted — and the wall thickness follows from a conduction balance. Two low-cost constructions are sized. Expanded polystyrene sheet (thermal conductivity 0.035 W/(m·K)) requires a wall 69.5 mm thick, which in practice means building to 70 mm; it leaves an internal volume of 182.9 litres, 50.8 percent of the 360-litre gross volume. A double wooden box with dry sawdust fill (0.09 W/(m·K)) requires 136.2 mm, built to 140 mm, and leaves only 78.1 litres — 21.7 percent of gross — which is barely more than the 76.5 litres the fish and ice themselves occupy, a spare volume of just 1.6 litres. If the fish are loaded warm at 30 °C instead of 5 °C, the sawdust design ceases to be workable inside the same envelope: it would need 190.5 mm walls and its 29.7-litre interior could not hold the load. The paper gives the sizing rule, a thickness table across trip lengths and temperatures, and the volume each wall costs, so a builder can choose with the trade-off in front of them. The study is simulated, not measured.