General Summary of Hatmanah:

It is a Mitzvah to insulate hot food before Shabbos so one can eat hot food on Shabbos, in its honor.

The following are the laws involved in how one may insulate his food:

Definition of insulation:

It is only considered insulation if all the following conditions are fulfilled:

1. The insulating material is actually touching the walls of the pot.
2. The insulating material surrounds majority of the walls of the pot by insulation that adds heat, or surrounds the top of the pot by insulation that does not add heat.
3. The material was placed there for purpose of insulation, and not for other purposes, such as to guard the food from getting dirty or from rats getting to it.

• If any of these conditions are lacking, then doing so is not considered insulation and is permitted to be done even on Shabbos.

Insulation which does not add heat:

• Before Shabbos, including Bein Hashmashos: One may insulate at all times with any material that does not add heat to the food.
• On Shabbos from after Bein Hashmashos: One may not insulate even with material that does not add heat unless either:

1. One had insulated the food before Shabbos, and then on Shabbos it got removed whether by hand or on its own.
2. The food was removed from the original pot that it was cooked in and was rather placed in another pot. This latter pot may be insulated on Shabbos.
3. The food in the pot is no longer Yad Soledes.
4. The top of the pot is not covered and the pot is not on a source of heat.

• Examples of insulators which do not add heat: Garments, fruits, feathers, sawdust, skins, fleece of wool, ash that does not contain burning coals, do not add heat in any situation. The following do not add heat when dry: straw, soft yarn or other pieces of cloth, herbs.

Insulation which adds heat:

• Before Shabbos-until sunset: It is forbidden to use materials which add heat as an insulation unless:

1. The material is never usually used for insulating, such as stones and the like.
2. The food has been placed inside an oven which has been plastered shut.

• On Shabbos-from Bein Hashmashos: It is forbidden to use materials which add heat as insulation in any situation.
• Examples of materials that add heat: Fertilizer, salt, lime, sand. The following add heat when wet: straw, soft yarn or other pieces of cloth, herbs. The following never add heat: 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.

Placing insulated food on top of an item which adds heat, such as a stove:

Any item that is properly insulated is forbidden to be placed even before Shabbos on top of any item which continuously elevates and increases heat to the inside of the pot that is on top of them. Thus, one may not place an insulated pot on top of a blech even before Shabbos. If the pot is not properly insulated, then according to our custom it may be placed even directly on top of a fire inside of an oven.[1]

What is the law if one transgressed and insulated before Shabbos in a way prohibited to be done?[2]

Then the food is prohibited until after Shabbos unless either[3]:

1. Before Shabbos one insulated with material that adds heat, but the food was already cooked to the point of Ben Drusaiy, [and according to Rav Farkash only if it is a pressing situation. If the food is fully cooked then according to all it is allowed even in a non-pressing situation.[4]]
2. One insulated right before Shabbos raw meat with material that adds heat [and according to Rav Farkash only if it is a pressing situation].
3. Before Shabbos one insulated food that he does not plan to eat until the next day with material that adds heat. However, if this is a recurring matter then one may not eat the food on Shabbos.

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[1] There are opinions which say that it is a prohibition of Hatmanah to place the pot directly on the coals, and certainly if the coals surround the pot. Other opinions argue and the custom is like this opinion, and thus this is allowed to be done.

Regarding removing a pot from coals on Shabbos:  If the coals surround the pot then one may not remove the pot from the coals, but may ask a gentile to do so for him. When placing the pot on top of the coals without having them surround the walls of the pot, then Lechatchilah, if practical, one should have a gentile remove it rather than a Jew, otherwise even a Jew may remove it from there, although this must be done gently in order to prevent as much movement of the coals as possible.

^[2] Chapter 257 Halacha 1 and based on the rulings mentioned in Halacha 9 Q&A 1-See There!

[3] All the below mentioned scenarios are disputed if allowed to be done even initially, and thus we are lenient after the fact to not prohibit the food on Shabbos [and at times this leniency is only in pressing situation, as explained above].

[4] See Q&A above

If on Erev Shabbos one insulated the food with a material that adds heat but the food had been cooked to the extent that further condensing is damaging for it and the food did not become hotter, but rather retained its temperature then it may be eaten on Shabbos according to all opinions [meaning even according to the Michaber], as there is no benefit gained from the prohibition. However, if the food is not fully cooked then according to the Michaber it is not allowed on Shabbos.