Insulation with material that does not add heat:
Before Shabbos, one may insulate food with any material that does not add heat to the food, and there are no restrictions in this regard. [Restrictions against insulating even with materials that do not add heat only begin to apply on Shabbos itself, as will be explained in the next chapter. Thus, before Shabbos one may completely wrap a hot pot of soup in a towel or blanket and leave it on the counter, so it remains warm, as a towel does not add heat to the food and just conserves the heat, so it is permitted to be done.]
May one insulate foods with material that does not add heat for the need of Motzei Shabbos?
May one place hot water or hot soup into a thermos for the sake of Shabbos?
 See Shabbos Kehalacha 2 11:27 [p. 48-51]
 M”A 257:2 based on Rama 609:1
 Elya Raba; Levush; M”B 257:4; Shaareiy Tziyon 609:1; Piskeiy Teshuvos 257:5; See Shabbos Kehalacha 2 11:27 [p. 48-51]
Ruling of Admur: Admur omits the entire discussion of the M”A ibid and does not relate either way to the law of Motzei Shabbos. This implies that he does not hold of any prohibition involved. [Shabbos Kehalacha ibid p. 51]