A refrigerator door has a height and width of H = 1 m and W = 0.65 m, respectively, and is situated in a large room for which the air and walls are at Too = Tsur = 25°C. The door consists of a layer of polystyrene insulation (k = 0.03 W/m · K) sandwiched between thin sheets of steel (s = 0.6) and polypropylene. Under normal operating conditions, the inner surface of the door is maintained at a fixed temperature of Ts,i = 5°C.