According to experts, it doesnt matter which side of aluminum foil you use when cooking. Both sides are equally effective at heating your food, and they are made of the same material. The difference between the two sides is due to a manufacturing process called milling, during which heat and tension are applied to stretch and shape the foil. Two layers of foil are pressed together and milled at the same time, because otherwise, it would break. Where the foil is in contact with another layer, that’s the ‘dull’ side. The ‘shiny’ side is the side milled without being in contact with another sheet of metal. However, if you are using non-stick foil, there is a difference between the two sides. Since the non-stick coating is only applied to one side, you should have the dull side facing up.