611. Rabbinic Restrictions on Insulating

Shabbos 4:2

Under Biblical law, one may insulate food with things that increase the temperature before Shabbos begins so that the food is covered and the heat will be preserved on Shabbos – this is certainly no worse than leaving food to cook on a fire on Shabbos, which is permitted. The Sages, however, ruled that one may not cover food before Shabbos with things that increase the temperature out of concern that the pot will boil on Shabbos and one will uncover it until the boiling stops. If he then re-covered it on Shabbos, he would be covering food with something that increases heat on Shabbos, which is prohibited. Therefore, it is permitted to cover food with things that increase the heat during the twilight hour (bein hashmashos) because by that time most pots will have already boiled and stopped. Once they have stopped boiling, it is unlikely that they will boil again.

Shabbos 4:3

Similarly, under Biblical law one would be permitted to cover food on Shabbos with things that will not raise the temperature but the Sages prohibited this so that one would not come to cover food with a mixture of ash and coals that has sparks in it and then come to stoke the fire. To avoid this possibility, the Sages prohibited insulating food on Shabbos using anything, even something that will not raise the temperature of the food.