612. Insulating a Secondary Container

Shabbos 4:4

If a person doesn’t know whether it is before or after nightfall, he may insulate hot food. Similarly, one may cover cold food with something that will not increase its temperature in order to keep it from getting colder. If hot food that was covered before Shabbos got uncovered on Shabbos, it may be re-covered again since its temperature is not being increased by this. One may change the insulation covering food on Shabbos, such as by replacing garments with feathers or vice versa.

Shabbos 4:5

If a person moves hot food or hot water from its original container (i.e., the one in which it was cooked) to another, the second container may be insulated using something that will not increase the temperature on Shabbos, since we are permitted to wrap cold food. The prohibition against insulation only applies to hot food in the vessel in which it was cooked. Once it has been transferred to another vessel, it is permitted to cover it.