it makes absolutely no difference at all. the two sides exist because of the manufacturing process when they roll the metal through the machines. it has nothing to do with heat reflection or special cooking techniques. people these days overthink everything and look for hacks where none exist. grandma was right to just cook and not worry about chemicals or shiny sides. just put the foil on the tray and get on with your life. things were much simpler before the internet started making up problems that dont exist. modern cooking is full of useless gadgets and unnecessary steps that do nothing but waste time.