What is the law if one insulated food on Shabbos?
If the food became hotter due to the insulation it is forbidden. If the food remained the same temperature, then if the material used is defined as “does not add heat,” it is permitted. If the material is defined as material that adds heat then even if it did not add heat to the food, there are Poskim which side that it is nevertheless prohibited [even if the food was fully cooked and Mitztamek Vera Lo]. Others argue on this ruling and permit the food [if it was fully cooked, and further cooking was detrimental towards the food] and heat was not added towards it.
 M”B 257:8, 10, 13; Shabbos Kehalacha Vol. 2 page 59-60; Piskeiy Teshuvos 257:4
 See M”B 257:8
 M”B 257:8 in name of M”A; Shaar Hatziyon 257:6
 Rav Akivah Eiger, brought in Mishneh Berurah Shaar Tziyon 257:6. However he concludes with a Tzaruch Iyun.
 Piskeiy Teshuvos 257 footnote 30; Rav Farkash argues to permit this, although he does practically rule this way in Shabbos Kehalacha.