May one use a thermos on Shabbos?
Using a thermos on Shabbos involves pouring hot water into the thermos for the purpose of insulating its heat for an extended period of time, and hence touches upon the question of insulation. The following is the ruling:
It is Rabbinically forbidden to insulate hot foods or liquids on Shabbos even if the insulation does not add heat to the food, but simply conserves it. This however only applies so long as the hot food or liquid remains in the original vessel that it was cooked in [i.e. Keli Rishon]. If, however, the food or liquid is poured into a second vessel, then there is no prohibition against insulating the second vessel on Shabbos. Accordingly, there is no prohibition against using a thermos on Shabbos, and pouring boiling water into it for the sake of conserving the heat of the liquid. For the same reason, it is likewise permitted to enter hot water into a stomach pad on Shabbos.
 Admur 257:2; Michaber 257:1; Shabbos 51a
The reason: This is due to a decree that perhaps when one comes to remove the pot from the stove in order to insulate it with material that does not increase heat, he will find that the pot is cold, and will forget that it is Shabbos and reheat it on the fire, hence being liable for [the prohibition of] cooking.
Other opinions: Some Poskim rule that the reason insulating on Shabbos is forbidden is because one may come to insulate in ember. [Rambam Hilchos Shabbos 4:3; See Lechem Mishneh ibid]
 Admur 257:9; Michaber 257:5
 Kuntrus Achron 257:1 “Even if it does not cool it off”; Peri Megadim 257 M”Z 5 in opinion of Rambam; Givas Levona 25; Keren Ledavid 1:77; Zekan Ahron 2:14; Darkei Chaim Veshalom 421; Ketzos Hashulchan 71 footnote 36; Chelkas Yaakov 84; Or Letziyon 2:17-11; Igros Moshe 1:95; Betzeil Hachochmah 2:74; Beir Moshe 1:12; Minchas Shlomo 2:8; Tzitz Eliezer 11:29; Chazon Ish 37:32; Even Yisrael 9:8; Shabbos Kehalacha Vol. 2 page 66; Piskeiy Teshuvos 257:9
The reason: As the thermos is considered a Keli Sheiyni, and the Sages did not decree against it even if it does not cool off the food. [Igros Moshe ibid based on Rambam] Furthermore, doing so is permitted even according to Rashi who requires cooling down, as the entire prohibition is in covering the thermos, and it is permitted to cover it just like it is permitted to cover any vessel. [See Igros Moshe ibid; Chazon Ish ibid that there is never a prohibition in pouring from one vessel to another even if it insulates]
Other Poskim: Some Poskim are stringent based on the opinion of Rashi that requires cooling down, to prohibit the use of a thermos being that one does not intend to cool off the water. [Levushei Mordechai 2:55; Tzafnas Paneiach 2:27; Melamed Lehoil 1:45; Shevet Halevi 1:93] Even one who desires to be stringent can simply avoid filling the water until the top [Pnei Shabbos in name of Divrei Yatziv] or through pouring the water into a Keli Sheiyni and then into the thermos. [Levushei Mordechai ibid].
 M”B 326:6 in opinion of Rashi and Admur 326:5 who only records Rashi’s opinion
The reason: As there is now no suspicion that it will spill. As well, there is no medication prohibition involved here as is seen from the fact that Admur ibid makes no mention of it.
Other opinions: According to the reason of Tosafus, even a closed bottle is forbidden to be placed on the body, as it is considered Refua. [M”B 326:19 in name of Karban Nesaanel] Thus, some Poskim rule one may only be lenient on Shabbos in a time of great need or pain. [M”B 326:19; Beir Moshe 1:33-15]