"Foil is one of the best barrier films available,” states food engineer Timothy Bowser.
“It resists and offers great protection to light, moisture, gases, bacteria, and odours."
There is a small problem associated though—it is easy to tear and poke holes in. That is why many commercial food packaging include an aluminium layer sandwiched between plastic layers (e.g. juice boxes and chip bags). The plastic protects the aluminium, allowing it to retain its excellent barrier properties.”
For cooking, lining pans and covering food, foil is obviously the choice. Heavy-duty aluminium foil is recommended for packages that are going to be opened and closed frequently (for example, for campfire cooking).
Relax. You have nott been doing it wrong no matter which way you’ve been using it. Generally it doesn’t matter. The foil has a dull and a shiny side just because of the way the product is made. The side coming in contact with the mill’s highly polished rollers becomes shiny, and the other side comes out with a matte finish.