Yup, the lab could tell a difference! Shiney side (so mill roller facing, as opposed to the dull side which faces the other layer of aluminum) was marginally more reflective, but I believe (and a former coworker also remembered it as) it was less than a tenth of a percent (<0.1% for the visual folks)
Anyone who says it affects cooking time or something is mistaken, I’d wager.
Former process engineer in an aluminum factory. Aluminum foil is only shiny on one side and duller on the other for process reasons, not for any “turn this part towards baking, etc” reasons.
It’s just easier to double it on itself and machine it to double thickness than it is to hit single thickness precision, especially given how much more tensile strength it gives it.
Also, our QA lab did all kinds of tests on it to settle arguments. The amount of heat reflected/absorbed between the two sides is trivially small. But if you like one side better you should wrap it that way, for sure!
You know what, so will I!