Chapter 258
It is permitted to place before Shabbos a cold item on top of something hot
Halacha 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.
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 diminishes as time goes by.