Insulating a food or pot that is on top of a stove top, blech, electric plate, oven, or hot pot:
Any item that is properly insulated according to Halacha [as explained in Halacha C-D] is forbidden to be placed even before Shabbos on top of any item which contains a live heating source and continuously elevates and increases heat to the inside of the pot that is on top of it. [Thus, one may not place an insulated pot on top of a stove, blech or electric plate even before Shabbos, if the fire or heating element is on. If, however, the heating element is off, then even if the surface is still hot, then it is permitted. Thus, one may place an insulated pot inside of a hot oven before Shabbos, if the oven has been turned off.]
May one place an insulated pot before Shabbos on top of a hot pot? It is permitted before Shabbos to place a fully insulated pot of food on top of a hot pot that is off the fire, in order so it remain warm.
According to the Michaber may one place a minute amount of insulation on the pot that is found on the fire, blech or electric plate?
May one on Erev Shabbos place an insulated pot onto a blech or an electric plate?
May one place a pot insulated with material that does not add heat inside of an oven that has been turned off, or an electric plate that is now off?
Yes, as since there is no more heat being added it has the same law as a Kira which has had its coals removed from it.
May one on Erev Shabbos place an insulated pot on top of a pot which is on a flame or blech or electric plate?
No. Thus, on Erev Shabbos one may not place a towel on top of the pots sitting on top of pots on the Blech if the towel will be touching majority of the side walls. It is a Mitzvah to publicize this matter.
May one on Erev Shabbos insulate food inside a hot pot of water that is off the fire?
Yes, as since it is off the fire, it is no longer considered to add heat.
May one on Erev Shabbos, or on Shabbos, place food which is wrapped in tinfoil on top of a pot that is on a flame, Blech or electric plate?
If the wrapping is placed in order to guard the heat of the food, then it is forbidden to do so even before Shabbos. If done in order so the food does not spill or fall or get dirty, then it is permitted to do so, although one must be careful to not wrap it more than needed for this purpose. Thus one may not wrap his challah in tinfoil even on Erev Shabbos and place it on any source of heat, unless it is being wrapped to prevent it from getting dirty and the like.
May one on Shabbos place a pot cover over food that is resting on top of another pot that is on the fire?
If this is done in order to insulate the food and the pot cover touches the food, seemingly it is forbidden. Otherwise, it is allowed. It is permitted to turn over a pot of Kugel and place it on top of a pot which is on the fire.
 Admur 257:10 “A pot which has been insulated with Mochin or other materials which do not add heat, is forbidden to be placed before Shabbos on top of olive waste, as olive waste elevates the heat above them into the Mochin, and the Mochin then adds heat into the pot, which is insulated within it, and thus it is just like insulating with material that adds heat. However, it is permitted to place it on sesame seed waste, as they do not elevate the heat above them. Similarly, it is permitted to place it into the Kira that it was cooked in, and insulate it there with material that does not add heat. Now, even though that the surface of the Kira which the pot is standing on is very hot, nevertheless since the heat of the Kira is not from it itself, but from the fire [that is in it], and [thus] as time passes it gets colder, [therefore] it is not considered something that adds heat, and is not similar to olive waste and the like which add heat from their own very substance. [Admur ibid; M”A 257:18; M”B 257:43 in name of M”A ibid; Shaar Hatziyon 257:41] Nevertheless [in order to be allowed to do this], the Kira 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. [Admur ibid; Michaber 257:8]
Other opinions: Some Poskim rule that if the element is still hot then it is forbidden due to Hatmana Shemosif Hevel even if it was turned off. [Rosh; M”B ibid]
 The reason: The reason for this restriction is because even though the clothing does not add heat due to themselves, nevertheless, due to the coals underneath them which elevate the heat, the clothes add heat to the pot, and the pot is thus [considered] insulated in a material that adds heat. [Admur ibid]
 Admur 258:1 “It is permitted to insulate, [a pot, with material that does not add heat,] before Shabbos, whether [the food in the pot is] hot or cold food, [and to then place it] on top of a hot pot [that is not on the fire].”; Elya Raba; Atzei Shitim; M”B in Shaar Hatziyon 258:6; Piskeiy Teshuvos 258:2
Other Opinions: Some Poskim rule it is forbidden to do so due to Hatmana Shemosif Hevel. [Taz 258:1; P”M 258 M”Z 1; 259 M”Z 3] Admur negates this opinion of the Taz. [Kuntrus Achron 257:3; Mahadurah Basra 259]
 The reason: This is not considered like one is insulating on top of something that adds heat, which is forbidden to do even from before Shabbos, being that the hot pot is not considered to be adding heat, being that its heat is diminishing as time goes by. [Admur ibid]
 Piskeiy Teshuvos 257:11; Shabbos Kehalacha 2 p. 19 and p. 34; Or Letziyon 2:17-10
 Piskeiy Teshuvos ibid; Shabbos Kehalacha ibid p. 34 based on Beis Yosef; Chut Shani 28:4 based on Chazon Ish
 Ketzos Hashulchan 71 footnote 29; Or Letziyon ibid
 Mahram Ben Chaviv; Eretz Chaim 257; Birkeiy Yosef 273; Zechur Leyitzchak 74; Or Letziyon ibid; Yabia Omer 6:33; Yalkut Yosef 3
 Shabbos Kehalacha pages 28-30; Piskeiy Teshuvos 257:1 and 11
 The reason: as the heat of the blech and pot consider this to be insulating with an item that adds heat, as the heat is continuously added, just like is the law by a Kira which did not have its coals removed.
 Shabbos Kehalacha Vol. 2 page 39-40 based on Admur ibid; M”A 257:18; M”B 257:43 in name of M”A ibid; Shaar Hatziyon 257:41
Other opinions: Some Poskim rule that even in such a case it is forbidden due to Hatmana Shemosif Hevel. [Rosh; M”B ibid]
 Implication of Admur 258:1 and Kuntrus Achron 1; Peri Megadim 258 A”A 1; Tosefes Shabbos; Machatzis Hashekel; Menorah Hatehora; Avnei Nezer Choshen Mishpat 106; Mishnah Berurah in Biur Halacha 258 “Mutar” and Shaar Hatziyon 258:6; Igros Moshe 4:74-3; Shabbos Kehalacha pages 31-32; Piskeiy Teshuvos 258:4
Other opinions: Some Poskim rule it is permitted even in such a case and is not considered Mosif Hevel. [Tehila Lidavid 257:8-9; 258]
 The reason: In such a case it is forbidden to insulate the upper pot with any material, just like it is forbidden to insulate a pot when placed in a Kira that has not been swept of its coals. So rules the Peri Megadim and the Avnei Nezer and the Mishnah Berurah, and the Igros Moshe. Unlike the ruling of the Tehila Lidavid
 Shabbos Kehalacha Vol. 2 page 41; Piskeiy Teshuvos 258:4
 Igros Moshe 4:74 Hatmana 3; Rav Shlomo Zalman Aurbach brought in Shabbos Kehalacha ibid; See Mahdurah Basra 259; Pnei Shabbos 12
 Rav Shlomo Zalman brought in Shabbos Kehalacha ibid; Maor Hashabbos 1:9; Koveitz Mibeis Levi 6:36; However, see Igros Moshe ibid from which it can be learned that even in this situation it is prohibited.
 See Shabbos Kehalacha Vol. 2 page 41-42