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2. Not to insulate with materials that increase heat:
A. The prohibition; its reasoning and if it applies by cooked foods:
Not to insulate with materials that increase heat: One who 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.
May one insulate with a heating material, food which further cooking will cause damage to? Initially it is forbidden to [insulate with material that increases heat] even if [is fully cooked and] further cooking of this food causes it to condense in a way that is detrimental to it.
The reason for this restriction is because: it is a decree made due to that [if this were to be permitted] 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.
The reason for why it is allowed to leave half cooked food over a fire from before Shabbos, but is not allowed to insulate a food with a material that increases heat: [The above scenario of insulation] is not similar to leaving a pot on a flame over Shabbos, which is permitted to be done when left on top of a Kirah that is not swept or covered, as long as the food has been cooked to the point of the food of Ben Drusaiy from before Shabbos, as usually insulation is done for the need of [food which will be eaten] the next day. As one insulates the food and covers it so it does not cool down by the next day, and therefore it requires to be stoked more [than food left on a flame], in order so the food not cool off over the entire night. However, by leaving food on a flame, usually this is done for the [food to be eaten by the] night meal, as one leaves the pot there without insulation, it therefore does not need that much to be stoked if the food has already cooked to the point of the food of Ben Drusaiy from before Shabbos. [Furthermore] even if one were to leave this food [on the flame until] tomorrow, there is [still] no suspicion that one will come to stoke the coals, as the little bit of stoking that can be done will not be of help for it to stay hot until the next day being that the pot is not insulated. However, when the pot is insulated, a little bit of stoking can help [it retain heat until the next day] and therefore we are worried that one may come to stoke the coals if the pot is insulated in ember. [Now although the above suspicion only applies to ember, being that it can be stoked, as opposed to other materials which add heat, nevertheless] the [Sages] decreed [against using] any material that increases heat because of [the suspicion involved with] ember. [Furthermore, this prohibition applies] even if one insulates [the pot] for the need of the night [meal], being that the Sages did not differentiate in their decree.
Other Opinions by foods cooked to the point of Ben Drusaiy: [However] there are opinions which argue on this and say that even to insulate from before Shabbos in materials which add heat is permitted as long as the food has cooked to the point of the food of Ben Drusaiy [from before Shabbos] just as it is permitted to leave it on a Kirah which is not swept or covered, as explained in chapter 253.
The Final Ruling: In a community that it is the custom to be lenient like this [latter] opinion, one should not protest against them doing so. However, this custom should not be followed in other places.
Insulation which does not add heat:
Before Shabbos one may insulate with any material that does not add heat to the food.
Insulation which adds heat:
Before Shabbos it is forbidden to use materials which add heat as insulation.
May one insulate fully cooked cold food with material that adds heat?
See Chapter 2 Halacha 7
B. Insulating completely raw meat right before Shabbos begins:
Based on the above reason it is forbidden to insulate even completely raw meat close to Shabbos, as even though it is allowed to leave it [over a fire] being that one will [anyways] not think about it until the next day, [and there is thus no suspicion that one may come to stoke the coals at night, and even the next day there is no suspicion] as the meat is able to [become fully] cooked throughout the entire night without stoking. Now [even if] the meat will be able to cool off afterwards [nevertheless there is no worry that one may come to stoke the coals to prevent this as] a little stoking will anyways not be effective [to keep it warm for the next day] since the meat is not insulated. However, when the meat is insulated, we are worried that perhaps one will stoke it after it is cooked in order to retain its heat, so that it not cool down, and for such a purpose [even] a little bit of stoking is effective.
Other Opinions: [However] there are opinions which argue on this and say that even to insulate from before Shabbos in materials which add heat is permitted as long as the food has cooked to the point of the food of Ben Drusaiy [from before Shabbos] or if [the meat] is completely raw literally right before Shabbos, just as it is permitted to leave it on a Kirah which is not swept or covered, as explained in chapter 253.
The Final Ruling: In a community that it is the custom to be lenient like this [latter] opinion, one is not to protests against them doing so. However, this custom should not be followed in other places.
Summary- Insulating completely raw meat right before Shabbos begins:
It is forbidden to insulate using material which adds heat even a completely raw piece of meat that was placed in right before Shabbos. Nevertheless, those which are accustomed to being lenient in such a case are not to be protested as they have upon whom to rely.
C. Insulating before Shabbos foods which one does not plan to eat until Shabbos day:
There are those who are even furthermore lenient that even if the food is not completely raw close to Shabbos, nevertheless, if one removes his mind from it until the next day [meaning he does not plan to eat it until Shabbos day], it is considered as if it is completely raw. It is thus permitted to insulate it before Shabbos [even with material that adds heat] for the use of the next day even though it has not yet reached the point of Ben Drusaiy [from before Shabbos].
The Final Ruling: One may not [initially] rely on this opinion, unless it is already after the fact [in which case one may rely on them] as long as this does not occur on a regular basis. [See Q&A]
Summary- Insulating foods before Shabbos which one does not plan to eat until Shabbos day:
It is forbidden to insulate using material which add heat even a food which one only plans to eat the next day. Nevertheless, if one transgressed and insulated such a food with material that adds heat, it may be eaten so long as it is not done on a regular basis.
How many times is considered to be a regular basis?
This needs further analysis. Although we do find by Eiruv Tavshilin, and by the laws of Shechitah, that if one had already done so one time in the past, then the second time it occurs he is considered negligent and is treated stringently. However, it requires further proof to apply that definition towards here as well.
 Admur 257:1
 Admur ibid; Michaber 257:7
 Michaber ibid
 The reason: 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.
 Admur ibid; Taz 253:1; M”A 253:3; M”B 253:5
Other opinions: Some Poskim rule that the reason for why insulating on Erev Shabbos with Mosif Hevel is forbidden is because one may come to do so on Shabbos. [Rambam Hilchos Shabbos 4:2] See Kesef Mishneh ibid that explains the source of the Rambam from the above Gemara and how he interprets it differently.
 Embers are the glowing, hot coals made of greatly heated wood, coal or other carbon based material that remain after, or sometimes precede a fire. Embers can glow very hot, nearly as hot and sometimes as hot as the fire which created them. They radiate a substantial amount of heat long after the fire has been extinguished, and if not taken care of properly can rekindle a fire that is thought to be completely extinguished and can pose a fire hazard.
 Admur ibid; M”A 257:16; M”B 257:7
Why does this not constitute a Gzeira Legzeira: It is learned from a verse that the Sages may not make a decree upon a decree. [Rashi Beitza 2b] Decreeing against using all items that increase heat due to that one may come to insulate in ember constitutes a decree upon a decree. The following answers can be used to explain this matter: 1) If both decree took place simultaneously it is permitted to make a decree upon a decree. [Beitza 3a] 2) It is permitted to make a decree upon a decree if the original decree will not be fulfilled without the second decree. [Tosafus Brachos 53a] 3) It is permitted to make a decree upon a decree when the original decree is due to a suspicion that involves the item to which the action is being done, such as insulating with items that add heat is forbidden because one may come to insulate with ember which can lead one to stoke the ember. Since the decree against ember involves stoking itself, therefore making a further decree against all material that adds heat is not defined as a decree upon a decree. [Tosafus Rid Shabbos 31a] 4) Any matter which is commonly transgressed the Sages may make a decree upon a decree. [Chacham Tzvi 75; Gra 252]
 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.
 Rashbam brought in Tosafus 47b; Rama 257:7 in name of Yeish Omrim
 Admur 257:1
 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 stays 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.
 Rashbam brought in Tosafus 47b; Rama 257:7 in name of Yeish Omrim
 Admur 257:1; Rama 257:7
 Shivlei Haleket 57; Rama 257:7 in name of Yeish Cholkin
 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.
 Admur 257:1
 Shabbos Kehalacha Vol. 2 pages 11; See Piskeiy Teshuvos 527:8