Foil is an insulator... it's a good thing.
Aluminium foil - Wikipedia, the free encyclopedia
Aluminium foils thicker than 0.025 mm (0.001 in) are impermeable to oxygen and water. Foils thinner than this become slightly permeable due to minute pinholes caused by the production process.
Aluminium foil has a shiny side and a matte side. The shiny side is produced when the aluminium is rolled during the final pass. It is nearly impossible to produce rollers with a gap fine enough to cope with the foil gauge, therefore, for the final pass, two sheets are rolled at the same time, doubling the thickness of the gauge at entry to the rollers. When the sheets are later separated, the inside surface is dull, and the outside surface is shiny. This difference in the finish has led to the perception that favouring a side has an effect when cooking. While many believe that the shiny side's reflective properties keep heat out when wrapped on the exterior and keep heat in when facing exterior, the actual difference is imperceptible without instrumentation
[6].
The reflectivity of bright aluminium foil is 88% while dull embossed foil is about 80% [3].