Insulating cold food on Shabbos:[1]

It is permitted to insulate completely cold foods on Shabbos with materials that do not add heat, in order so it not become too cold, or in order to remove its coldness. ([Furthermore] if the food cooled off to the point that it is not Yad Soledes, it is permitted to insulate it even while it is still in the original pot.[2]) Materials that add heat:[3] It is forbidden to insulate even completely cold foods using material that add heat to the food. [This applies even if the material is not defined as Hatmana Shemosif Hevel, nonetheless, so long as it adds heat to the food, it is forbidden.[4]]

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[1] Admur 257:9 “(If the food cooled off to the point that it is not Yad Soledes, it is permitted to insulate it even while it is still in the original pot) and certainly it is permitted to insulate completely cold food in order so it not become too cold, or in order to remove its coldness”; 318:23; Michaber 257:6; See Piskeiy Teshuvos 257:9

^[2] Admur ibid [parentheses in original]; see also Kuntrus Achron 1 that leaves in Tzaruch Iyun; M”A 257:14 “Its possible…”; M”B 257:31 [Allows in case of need].

May one rely on this even initially? Some Poskim rule one should only be lenient to insulate food, which is still hot but not Yad Soledes, within the original pot, if it is a pressing situation. [Mishneh Berurah 257:28; Admur here places in parenthese and in Kuntrus Achron 257:1 he leaves this ruling with a Tzaruch Iyun] Rav Farkash rules likewise being that even the Alter Rebbe placed this ruling of the Magen Avraham in parentheses. [Shabbos Kehalacha Vol. 2 page 53] Other Poskim are lenient even initially. [Chazon Ish 37:31; Shevet Halevi 8:166; see Piskeiy Teshuvos 257:10]

^[3] Admur ibid “However, all the above is only [allowed] when using insulation material that does not add heat, however material which adds heat is forbidden to insulate with. This applies even with completely cold food, and even before Shabbos.”

[4] Mahadura Basra for chapter 259; p. 878 in new Shulchan Aruch; M”B 258:2; Sheivet Halevy 5:31; Shabbos Kehalacha Vol. 2 page 54-55 and 65 concludes as Mahadurah Basra that it is forbidden; Piskeiy Teshuvos 258:2

Ruling of Admur in Shulchan Aruch: In 318:23 Admur rules that one may immerse a bottle of cold water completely in a Keli Sheiyni. Likewise in 254:4 Admur implies that if a food is fully cooked one may return the insulation even if it is on top of a hot pot, hence implying that one may add heat. However, see Tehila Ledavid 254:4 that establishes this case to be referring to a pot that is on the fire and the insulation was done over a wood board and is hence not defined as insulation at all. Based on this one cannot bring a proof from 254:4 regarding how Admur would rule regarding actual insulation. 

^[5] Mahadura Basra for chapter 259; p. 878 in new Shulchan Aruch; M”B 258:2; Sheivet Halevy 5:31; Shabbos Kehalacha Vol. 2 page 54-55 and 65 concludes as Mahadurah Basra that it is forbidden; Piskeiy Teshuvos 258:2

[6] M”B ibid

^[7] M”B ibid “if top is revealed”; Sheivet Halevy 5:31 and other Poskim based on the rules explained in this chapter.

[8] M”B 258:2 regarding if one’s intent is to heat up the food and Shaar Hatziyon 258:7 regarding if one’s intent is simply to remove its coldness; Sheivet Halevi ibid

Other opinions: Some Poskim rule it is forbidden in all cases to submerge the bottle even if one’s intent is simply to thaw it down and not add heat. [Chazon Ish 37:17]

^[9] See Piskeiy Teshuvos ibid