Holding Food Frozen Through a Thaw: Insulation Sizing for an Outdoor Winter Cache Whose Only Refrigeration Is the Season

In cold regions, food frozen hard outdoors in deep winter can be stored in an insulated above-ground box whose only refrigeration is the season itself — provided the box can ride out a warm spell without the food spoiling. This paper sizes that box. A stated mass of mixed frozen food starts at −30.0 °C and must survive a stated design spell of 7 days at +8.0 °C. The food's thermal buffer is 57.0 kJ/kg of sensible heat up to 0 °C plus the latent heat of its water fraction, of which a stated hard quality limit allows 10% to melt (80.4 kJ/kg absorbed in total). A lumped enthalpy model with spherical-shell conduction through expanded polystyrene (thermal conductivity 0.035 W/(m·K)) gives required insulation thicknesses of 0.232 m for 10 kg of food, 0.097 m for 25 kg, 0.064 m for 50 kg, 0.046 m for 100 kg, 0.035 m for 200 kg, and 0.027 m for 400 kg. The requirement falls steeply with mass — sixteen times the food needs only 0.28 times the insulation — because stored energy grows with volume while heat leak grows with surface. Two cautions come with the sizing. The stricter high-quality limit of −12 °C is crossed after only about 1.4 days at the design thickness; the remaining protection is the latent plateau at 0 °C, not deep cold. And a plain 25 mm wooden box holding 100 kg reaches the hard limit in 0.94 days under the same spell, so an uninsulated or lightly built cache does not survive a week-long thaw.