A common question that arises with regard to Hatmana is whether it is permitted to wrap a food in aluminum foil and place it on the blech or hotplate. Although, the foil does not generate its own heat, it is still not considered davar she’mosif haval; Maran (257:8) already established the rule whereby the combination of the foil wrapping in direct contact with the heating element below constitutes a status of davar she’mosif haval-A heat generating insulation system), which is prohibited even on Erev Shabbat. If so, we would expect the halacha to prohibit placing foil-wrapped food on the blech not only on Shabbat, but on Erev Shabbat as well.
Nonetheless, we find two different approaches in the poskim to permit placing foil-wrapped food on the blech. The first approach assumes that such a case is indeed considered Hatmana. Yet, even within the parameters of Hatmana, there is room to be lenient based on a principle established by Maran (257:2). He rules that the Halachic status of an act of Hatmana depends on the kavana (intent) for which it was performed. If the covering was placed with intent to retain heat, then it is prohibited. However, if one places a covering on a pot to protect the food from dirt or bugs, it is permitted. Even though the action in both cases is technically Hatmana, the outcome is defined by the kavana.
Therefore, in our case of a foil wrapping, we must determine why a person wrapped his food with foil. If his intent was merely to prevent the food from falling apart or drying out, then we can be lenient. However, if the kavana was to enhance the heat retention, it is prohibited even on Erev Shabbat.
There is a second approach to this question adopted by Rav Shelomo Zalman Orbach, Rav Eliashiv and many other poskim. They say that wrapping a food in tin foil, no matter what the intention, is not defined as an act of Hatmana. The first layer of material containing and covering a food is not considered insulation. It is no different than the pot in which food is cooked. Nobody would consider the pot insulation; the pot that encompasses and "covers" the food is simply the normal way of cooking. Hatmana occurs when one adds an additional layer of covering to insulate the extant container.
Therefore, according to those opinions, the issue of foil-wrapped foods on the blech is no different than the halacha of heating up foods in a pot. Just like on Erev Shabbat and Shabbat, it is permitted to put a pot on the blech under certain circumstances, so too, with reheating foil-wrapped foods. According to this more lenient approach, it would be permitted even if one’s explicit intent was to retain heat.
Since the halacha of Hatmana is D'rabanan (Rabbinically ordained), one can be lenient in accordance with the second approach.
It is permitted to place foil-wrapped foods on the blech or hotplate, both on Erev Shabbat and Shabbat, according to the same halachot that govern reheating foods in a pot.