A 3-litre stainless steel pot containing 2 litres of boiling water was placed in a Small HotBag.

After 5 hours, the temperature of the water dropped from 93.3 °C (simmering) to 54.4 °C. The water was still scalding hot.

After 12 hours (overnight) the temperature of the water dropped to 38.4 °C, which is lukewarm.

What makes a HotBag so efficient in heat retention?

Heat retention is related to three factors: conduction, convection and radiation.
The insulation in a HotBag includes the following:

• Polyester batting: Still air that is trapped is a poor conductor of heat, and transferal of heat through it is slow. Therefore heat does not escape easily.
• Metalised polypropylene:  The silver plastic foil serves as a steam barrier, reducing the possibility of convection cooling, and the aluminium coating provides a shiny reflector of heat that radiates back onto the contents of the HotBag.

With high temperatures such as boiling point in water, radiation is the most important of the three factors, making the HotBag uniquely more efficient than other models of retained-heat cookers.

What factors affect the efficiency of a HotBag?

Related to the above, when using a HotBag one needs to bear in mind the following:

• The pot should be at least half full. The fuller the pot, the better the performance.
• Do not place on a cold surface, as heat will be lost. Wood or newspaper are recommended as they are poor conductors or heat. Formica kitchen counters are similar to wood, but granite is cold.
• The steam barrier ensures no moisture loss, so a slow-cooked stew needs only the required amount of liquid to start with.
• The larger the thermal mass (boiling hot food), the better the heat retention, regardless of the slender, light-weight insulation of the HotBag.