The Laws of insulating hot foods
Insulating food before Shabbos
Not to insulate with materials that increase heat: One which desires to remove a pot from on top of a Kirah and insulate it in order so it not become cold, it is forbidden for him to insulate it in a material which increases heat, even if one wants to do so from before Shabbos.
The reason for this restriction is because: it is a decree made due to that [if this were to be permitted] then one may come to insulate the pot in ember, which is ashes which has coals mixed into it, and then afterwards on Shabbos one [may] forget and stoke the coals.
If one transgressed and insulated in material which adds heat: If one transgressed and insulated [in material which increases heat], [then] even [if this was done] inadvertently [without prior knowledge of the prohibition], [nevertheless] the food is forbidden both to him and to others until after Shabbos. [However this only applies] in a case that the added heat to the food made it condense in a way beneficial to it, or if the food was cold and got heated through this insulation. However if it remained the same temperature as it was originally, and it’s a food [which has been cooked to the point] that further cooking will cause it to condense in a way detrimental to it, then it is permitted [to be eaten on Shabbos], being that there is no benefit being gained from having done the prohibition.
Insulating food on Shabbos
Insulating food on Shabbos itself: On Shabbos itself it is forbidden to insulate the hot foods even with material that does not add heat.
The reason for why this is permitted is: because pots are usually still hot by Bein Hashmashos, as close to Bein Hashmashos the pots are removed from the fire and thus [at Bein Hashmashos] there is no need to worry that perhaps the food has cooled off and one will come to reheat it.
May one insulate on Shabbos food which further cooking will cause it damage: Even food which is completely coked and further cooking will condense it in a way that is damaging to it, is forbidden to be insulated from when it becomes dark [after Bein Hashmashos, by the definitive beginning of Shabbos], even with a material that does not add heat.
Placing an upside down pot over a hot pot on Shabbos: It is forbidden to place a vessel as a covering over a hot pot for the purpose of retaining its heat (if the pot is entirely insulated underneath the vessel), unless the vessel is wide enough to the point that its walls do not touch the sides of the pot, in which case there is no [prohibition of] insulation being done at all.
Covering the pot in order to guard it: However it is permitted to place a vessel on top of food in order to guard it from mice, or so that it not get dirty from the ashes, as this is not considered insulating for retaining of heat, but is rather is a measure of guarding and [is like] placing a cover on top of a pot.
Different details relevant to insulating on and before Shabbos
Which materials are considered to add heat?
The following materials always add heat even when dry: The following are materials which add heat [whether they are wet or dry]: Waste of olives and of sesame seeds [that is left] after their oil has been removed, and fertilizer and salt and lime and sand, whether they are wet or dry [they add heat].
The following add heat only when they are wet: Straw and sediment, which is grape waste [that is left] after their wine have been removed, and Mochin, (which is a general term for any soft material such as cotton and strings [made] of soft wool of an animal, and the scrapes of worn out clothing) and herbs, when these four materials are moist [they add heat], even if they are wet with cold water which fell onto them.
The following materials do not add heat: However all types of garments and fruits and feathers of all birds and thin residue of flax and sawdust of carpenters, and skins whether they have been processed or not processed, and the fleece of wool do not add heat in any situation.
Insulating with ash: Ash which does not have coals mixed into them, even if it is hot, do not add heat.
If one insulated in this ash a pot from before Shabbos, and on Shabbos one found mixed in it a few fiery coals surrounding [the pot] or underneath it, then [nevertheless] the food is not forbidden after the fact, being that the insulation was done with a permissible material.
Insulating with stones: It is permitted to insulate with stones from before Shabbos even though they add heat, as it is extremely uncommon to insulate with stones, being that they break the pot, or they ruin the food, and anything that is not common the sages did not decree against. However something that at times is used to insulate in, even though it is not so common, [nevertheless] the Sages decreed not to insulate in it if it adds heat.
May one re-insulate on Shabbos a pot that had been insulated before Shabbos?
Insulating on Shabbos food which has been transferred to a different pot:
It was only made forbidden to insulate on Shabbos with material that does not add heat, when one is insulating the actual pot in which the food was cooked in, as then the decree that one may come to heat it up is applicable. However if one removed the food into another pot, it is permitted to insulate it.
[Furthermore] even if one goes back and returns the food into the original pot, it is permitted to insulate it (being that the food has already cooled off a little in the walls of the second pot, and the Sages only forbade insulating if the food is [still] hot in the original pot that it was cooked in.)
Insulating before Shabbos with non-heat adding material however on top of material that adds heat:
A Kirah is only allowed if it has been swept of coals: Nevertheless [in order to be allowed to do this], the Kirah must be swept of its coals. However if there are coals in it, then even if they have dimmed, it is [nevertheless] forbidden to insulate in it a pot with clothing, or the like which do not add heat, even if it does not touch the coals at all, such as if it is sitting on a metal grid, or is sitting on top of stones.
The reason for this restriction is because: even though the clothing do not add heat due to themselves, nevertheless, due to the coals underneath them which elevate the heat, the clothes [do] add heat to the pot, and the pot is thus [considered] insulated in a material that adds heat.
The law if the insulation does not touch the walls of the pot: If the [pot was insulated] in a way that the clothing are not touching the pot, such as if one placed over the pot of food a wider pot which [is wide enough that it] does not touch the sides of the [lower] pot and one then placed the clothing on the [external] pot, in which case it ends up that there is no insulation involved here at all, not even with a material that does not add heat, and it is thus permitted to be done. As even though the coals elevate heat above them, this is [Halachicly] meaningless being that the pot is not insulated in anything [and thus since it is not insulated, it does not fall under any insulation prohibition].
Does the above leniency apply even if one places the pot directly on the coals: [The above leniency to allow placing a non-insulated pot into the oven only applies] when the pot is not touching the coals, [such as when it is sitting on an iron chair or on stones which are inside the oven]. However if its sitting on the coals, then even if the pot is completely uncovered [meaning there is not even a pot sitting over it], there are opinions which prohibit this.
Their reason for this prohibition is: because this is considered like one is insulating with a matter which increases heat, as [here] even the pot is touching the coals, and the coals elevate and add heat into the pot.
Other Opinions: [However] there are opinions which say that as long as the pot is not insulated by its top part it is not [Halachicly] considered insulation, and even if the pot is inside the oven and the opening of the oven above has been covered, [nevertheless] this carries no [Halachic] meaning.
Their reasoning is: because the Sages only prohibited insulating with material that adds heat because of a decree that [if this were to be allowed then] one may come to insulate in embers, therefore [insulating] is not prohibited unless it is done in a way similar [to insulating with] embers, [meaning that just like embers] is moveable and is attached to the walls of the pot [so too all insulation is only prohibited if it is moveable and attached to the walls of the pot]. [Thus being that] the walls of the oven and its opening are not attached to the pot [therefore it is not prohibited to be done]. ([This applies] even if the oven is small and narrow, as it is not possible that there isn’t a little bit of air separating the [walls of the oven from the pot])
The Final Ruling: The custom is like the latter opinion.
The Shehiyah restrictions which apply when placing the pot directly on the coals: All the [discussion in the previous Halacha of placing a pot directly on the coals] only is relevant if the coals are covered or when the food has been cooked to the point of the food of Ben Drusaiy from before Shabbos, or when one placed a piece of raw meat in a pot literally very close to Shabbos, as if none of the above has been done then according to all opinions it is forbidden [to place the pot in the oven], even if there is no insulation involved at all, as was already explained in chapter 253 [under the laws of Shehiyah].
Do the Shehiyah restrictions apply when placing raw dough and legumes in the oven right before Shabbos? With regards to types of dough and types of legumes which cook quickly, it does not help at all if they are raw close to Shabbos, (being that they are quick to cook) as was explained in chapter 254 [Halacha 6 and 13].
The laws of Hatmanah in a case that the oven has been sealed shut with plaster: If the oven is plastered with clay around its covering, then it is permitted [to leave in it] in any scenario, even [if the food is] actually insulated (inside the coals). As [when the oven is sealed] there is no suspicion that one may come to stoke [the coals], as explained [in chapter 254 Halacha 1]. Therefore it is the custom to seal the opening of the oven with clay. In places that this is not the custom, [they] must verify that there is no [prohibition of] insulation [involved], as well as [in order to also avoid the prohibition of Shehiyah] they need to cook the legumes and types of dough from before Shabbos to the point of the food of least Ben Drusaiy, or [alternatively they can make sure] that the coals be covered.
It is a Mitzvah to insulate food on Erev Shabbos so one can eat hot food on Shabbos: It’s a Mitzvah to insulate food for Shabbos in order so one can eat hot food on Shabbos, as [eating hot food] is [part of the mitzvah] of honoring and having enjoyment on Shabbos.
The law by one who hot food is unhealthy for him: However one who eating hot food causes him damage, it is permitted for him to eat [only] cold foods. However one who does not believe in the words of the Sages and prohibits eating hot food on Shabbos, we suspect that perhaps he is a heretic.
 This is the opinion of the Michaber. However see below that we rule that if the food had been cooked to the point of Ben Drusaiy from before Shabbos then we are lenient to allow it to be eaten on Shabbos.
 The reason for this is explained in the Mahadurah Basra to be that a person would rather have his food be hot and condensed even in a damaging way, then to be not condensed but cold, and thus the same suspicion applies here too.
 Back then, before gas ovens, fuel had to be constantly added by hand in order for the flame to continue. Thus being that on Shabbos no more fuel may be added, one is only left with whatever amount of fuel was placed before Shabbos. Thus food which is needed for the next day’s meal will usually be insulated, as otherwise, even if left on the fire from before Shabbos it will become cold by the next day. However food needed for the night meal will retain heat without needing insulation. Thus the Alter Rebbe is explaining that being that the laws of Shehiyah only refer to leaving a non-insulated food on top of the fire, therefore it can be assumed that the food is needed for the night meal.
 Meaning that although based on the above explanation the suspicion only applies when one insulates with ember food to be eaten the next day, nevertheless insulating for the night is also prohibited, as once the sages decreed against insulation with materials that add heat, they made the decree inclusive to all scenarios, even those which technically do not have reason to be decreed against.
 Meaning even though that after it has fully cooked throughout the night, since it may cool down by the time the day meal arrives, there is thus room to suspect that one will come to stoke the coals so the food stay warm. Thus the Alter Rebbe explains that we do not suspect for this being that stoking will not help at all in this regard if the meat is not insulated.
 Regarding if this ruling includes also the previous mentioned cases of if the food was completely raw and the like, see Q&A 4 and the footnotes there.
 Meaning that if he has already insulated food for Shabbos day in material which adds heat, then one may be lenient to eat this food on Shabbos. To note that according to the Michaber all the above is forbidden even after the fact.
 Bein Hashmashos, in accordance to the final ruling and custom, is the time between sunset and the definitive beginning of night. The time of night definitively begins when one can begin to see three medium stars. Before these stars can be seen, it is questionable whether the in between time period is considered part of night or part of the previous day.
 Meaning that the vessel completely covers the pot, both on its top and its sides, as is the case when one places a large pot over a smaller pot that the smaller pot fits entirely into the large pot.
 Meaning there is space between the walls of the smaller pot and the walls of the large pot which is sitting over it.
 The Mishneh Berurah however learns that only if the following materials are wet from their own moisture do they add heat. However if they simply get wet, then they do not add heat.
 When flax is pressed there are small pieces of residue which fall from it.
 This refers to the entire sheet of wool as it is when it is first cut from the animal, before pieces of it are broken off to process. [see Rashi Shabbos 48a]
 Based on the definition of insulation for material that does not add heat. A pot is only considered to have been insulated from before Shabbos if its cover was insulated, whether or not its sides were insulated.
 The question here is regarding Bein Hashmashos. Meaning if the insulation unraveled by Bein Hashmashos what law do we give it. Seemingly this is the novelty in the additional words “Shabbos” that the Alter Rebbe is saying that once Shabbos has begun, which is after sunset, then if it was insulated and unraveled one may reinsulate it on Shabbos.
 Seemingly this includes even if one specifically had in mind to not reinsulate it, and then changed his mind, that he is allowed to do it. So rules the Aruch Hashulchan and is implied from the Tehila Lidavid. [Shabbos Kehalacha vol. 2 page 63-64
 The Mishneh Berurah however and others rule that 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. 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
 However if the material adds heat to the food, even if it is not defined as a material that adds heat, it is forbidden. [Shabbos Kehalacha Vol. 2 page 54-55]
 See also Chapter 253 Halacha 10 where this subject was already discussed in somewhat greater length, and gives more light onto the subject and the two opinions which are repeated in the Halacha here.
 Meaning that this type of insulation is not even considered a minor form of insulation, which is insulation with material that does not add heat.
 Thus one may as well place a wide board or metal over the top of the pot and then cover it with clothing, as the clothing do not touch the side of the pot.
 As the apostate Tzedoki clan which did not believe in the Oral Torah held that one may never eat hot food on Shabbos. Thus we suspect that one who follows this is part of this clan of heresy.