It doesn’t matter.
The makers of Reynolds Wrap tin foil say it makes “no difference” which side you use for cooking, freezing or storing food.
Cooking tests seem to back that up: the shiny and dull sides of aluminium foil “insulate and conduct heat at the same rate”.
The Reynolds Wrap people say the differing appearance of the two sides comes from the “final rolling step” of production, when two layers of foil are passed through the rolling mill at the same time.
“The side coming in contact with the mill’s highly polished steel rollers becomes shiny,” they say.
In the non-stick version of its tin foil, the non-stick coating is applied to the dull side (so in that case there is a difference between the two sides).
The most interesting thing we discovered in researching this was that Reynolds Wrap was originally advertised in 1947 as performing “1,001 kitchen miracles”.
We have asked the company for the list of these miracles and we eagerly await their reply.
Update: We got the following message from Reynolds Wrap: “The complete list is likely buried in our archives; but we do have a booklet ‘Over 100 helpful household hints’ that we can send you outlining the many uses of aluminum foil.”
We now eagerly await our booklet.