Which materials are considered to add heat?
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. [See Q&A]
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, does 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, [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. [See Q&A]
Insulating with stones on Erev Shabbos which is Yom Tov: When Yom Tov falls out on Erev Shabbos, it is forbidden to insulate hot food for Shabbos with stones, as it is forbidden to gather stones on Yom Tov, as this is similar to [the prohibition against] building [on Shabbos and Yom Tov].
However if one has nothing else [available] to insulate with other than the stones, then it is permitted for him to insulate with them, as [by doing so] he is not making a permanent structure, but rather a temporary one, which is only Rabbinically forbidden to be done, and due to the [Mitzvah to] honor of Shabbos the Sages did not decree [in this situation, against using the stones to insulate], being that [in this case] it’s not possible for him to insulate in any other way.
Summary-Examples of materials that add heat:
A. The following always add heat:
- Olive waste
- Sesame waste
B. The following add heat only when wet:
- soft yarn or other pieces of cloth
C. The following never add heat:
- feathers of all birds
- Thin residue of flax
- sawdust of carpenters
- Skins whether they have been processed or not processed
- The fleece of wool
What is the status of blankets and pillows?
If they have cotton inside of them, it is proper to avoid using them if they are wet, even though that the actual cotton will not touch the pot.
What is the status of a crock pot?
It does not have the status of insulation which adds heat, even though its pot is completely surrounded by the heating frame of the crock pot as the frame does not actually touch the pot. However for this reason one must take care that when placing tinfoil on the walls of the crock pot that has more than one setting in order to be allowed to do Shehiyah/Kirah, then he may not place too much tinfoil which touches the walls of the chulent pot.
May one insulate on Shabbos with materials that do not add heat and are never commonly used to insulate with?
Logically speaking there should be no differentiation between material that adds heat and that does not add heat in this regard, as “anything that is not common the sages did not decree against“. Nevertheless the final ruling requires further analysis.
May one insulate with a dry towel?
Yes. However it is only allowed if the food is off the fire, blech or electric plate.
May one insulate with a wet towel?
 “Thus it is forbidden to insulate [food] in them even from before Shabbos due to a decree [which was decreed because if this were to be allowed then] one may come to insulate with embers as explained in chapter 257 [Halacha 1].” [318/7]
 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.]
 Shabbos Kehalacha Vol. 2 page 26
 Shabbos Kehalacha Vol. 2 page 45-47
 To note however that Rav Shlomo Zalman Aurbach rules that it does, and thus using such a pot is prohibited in his opinion.
 Being that there is always some space left between the two, and insulation is only prohibited when it touches the pot .
Rav Shlomo Zalman however says that since this is the way of cooking with this machine even during the week, therefore the space in between is irrelevant and it is forbidden.
 Shabbos Kehalacha Vol. 2 page 26
 This is permitted as only wet pieces of cloth add heat and not wet clothing.