At the office where I work, there is no way for me to heat my lunch, becaucse I don’t use a microwave for health reasons.
So, I bought a thermos, of the kind made for food, with an opening nearly as wide as the body.
My colleague at ZoomTorah Meir, saw my hot food and the thermos and asked me, can you do Hatmona with that?
Let’s review some of the laws of Hatmona.
What is Hatmona?
The word means “burying”, and there are two separate Takanos that Chazal decreed about “burying” food on Shabbos.
Chazal were worried that if people would bury their pots of food in something heat producing, they may come to bury their food in hot coals. Then they might come to stoke the coals on shabbos in order to heat the food. Therefore they decreed that one may not bury a pot of food in anything heat producing, not on shabbos and not even on erev Shabbos [if you will leave it there for Shabbos].
Chazal were further worried that if people would bury their foods on Shabbos in something that preserves heat [Insulation], they may come to re-heat their food before they insulate it. They therefore decreed that one may not insulate foods on Shabbos.
So putting something in a thermos before Shabbos is certainly permitted, because a thermos only preserves heat and that is only prohibited on Shabbos.
Okay, now what about putting food in a thermos on Shabbos?
Let’s try and give a better definition of what Hatmona means. Tosefos שבת מח. ד”ה דזיתים, says that if the material being used for insulation doesn’t touch the pot, then even though it insulates the food from a distance, it is not defined as Hatmona in regards to hichos Shabbos. Hatmona means to closely wrap food.
By the same token, although a thermos preserves heat, it could not be defined as Hatmona. The definition of Hatmona is using something else to cover a pot of food. A thermos is just a thick type of pot. There is no separate, independent insulation taking place.
 The reason Tosafos gives is that [since the reason for the prohibition So that one doesn’t inadvertently come to bury his pot of food in coals. Therefore since] when the material isn’t touching it cannot be compared to burying in coals and is permitted. On the face of it this would only be relevant for the prohibition of burying in something that increases heat since the reason for that Takana was so we wouldnt come to bury in coals.
Where as, on Shabbos, when we have the additional Gezairah not to insulate so as not to inadvertently come to reheat the pot before insulating it, it would be prohibited.
However when the Shulchan Aruch HaRav brings this Halacha, he explains that if the material is not touching the pot, it cannot be defined as Hatmana at all. Which would permit such insulating – even on Shabbos. The Shulchan Aruch seems to explain that way as well.