When reheating food on shabbos, I sometimes place dry food like a kugel on top of my chulent. Is there a problem of hatmana if I have silver foil covering the top of a pan? Or is hatmana only a problem if I have an extra covering like a pot lid and then silver foil. Here I just have one covering as the silver foil is acting as the “pot lid.” Also it is not always clear why I am covering the food. I want to protect the food as well as sometimes insulate the food with the top foil.
Hatmana refers to wrapping a food, or a pot, in order to keep the contents hot. It is usually associated with an “extra” covering, because the first “cover” (the pot itself) does not mean to keep the contents hot, but only to hold the contents and permit the food’s cooking on the fire.
Poskim in general assume that a single layer of silver foil is not meant for purposes of insulation, but rather to protect the food and to keep it together. Therefore, a single layer of silver foil is permitted, but a double layer would be prohibited.
Sources: See Shulchan Aruch Harav 257:4; Me’or Ha-Shabbos 2:2 (p. 84), explaining Iggros Moshe, vol. 4, no. 74, sec. 3. See at length Machazeh Eliyahu no. 32.