Bar-b-que on what side of aluminum foil?

The shiny or dull side of aluminum foil does not matter when cooking.

The difference between shiny and dull aluminum foil is due to the manufacturing process. Shiny side is the side that has been in contact with the rollers during the manufacturing process, while the dull side is the side that has been exposed to the air. Some people think the shiny side should be placed facing up to reflect heat while others think the shiny side should be facing down to absorb it. Neither is true. There is no significant difference in the reflectivity or absorption of heat between the two sides.