In my household, the "right" way to use aluminum foil — shiny side-up or shiny side-down — is the subject of much debate. (My vote is shiny side-up.)
As it turns out, we're not the only ones who have spent time thinking about this. There's a lot of speculation on the internet, where different theories abound. I admit that I fell down the rabbit hole of considering different kinds of heat, the possibility of a coating, and clever marketing gimmicks before I finally landed on a reliable source, the Reynolds Kitchens website, and got an answer.
Let's settle things right here and now: It does not matter which side of aluminum foil you use.
Sorry to disappoint, but the two different sides are just the result of the manufacturing process: To make foil, manufacturers melt aluminum, and then cool it into slabs called ingots. These slabs are then pressed between rollers many, many times to make them thinner and thinner.
By the time they get to the final press, the foil is so thin they need to double it up to press it through the rollers. Then they separate it and roll it onto the tubes that we buy. The pieces that come out are smooth on the sides being pressed by the rollers, and matte on the sides that press against each other.
But, importantly, the sheet is made of the same material through and through, so the textures don't make a difference in how it works. Unless, Reynolds reminds us, you're using one of their products like the Non-Stick Foil, which actually does have a coating on one side.
Now, don't you feel so much better? If only we could all also agree on the proper way to load a dishwasher.
P.S.: In case you're really interested, there's a great history of aluminum foil manufacturing here. Enjoy!