Where using foil for its heat transfer properties makes sense is when you tent food. To do it properly, there must be a small air gap between the foil and the food, otherwise the heat will conduct from the food to the foil and the foil does nothing.
But if you have a small air gap, you block the natural convection that would normally come off of the a hot food item and if you do it right you turn the air gap into a small insulation layer just like in double pane windows. Air is a very poor conductor of heat, unless it is moving, if you stop the move you massively reduce the heat transfer. So now that you have stopped (significantly slowed) the convection off of your food, you also get the added benefit of a radiative barrier. So a little math:
Assume a food item with a skin temperature of 200, a room temperature of 70, and a room and food emissivity of .8 (typical for light colored latex paint), also assume a full view factor for either condition. So with no radiative barrier the heat transfer from the food would be ~120 BTU/hr-ft^2. With the foil radiative barrier, you knock the emissitivity of the cold body down to 0.01 instead of 0.8, which would reduce to radiative flux to only 1.5 BTU/hr-ft^2.
In conclusion, if you tent the food with a about a 1/2 inch air gap, foil will significantly reduce heat transfer away from your food. If you wrap your food tightly with foil it will still reduce the radiation loss to the room (assuming you don't add too much surface area) but it will not do anything for convection, which is the majority of the loss.
Edit: BTW: Shiny side in would reduce radiation to 1.5 BTU/hr-ft^2, while dull side in would be about 7.5 BTU/hr-ft^2.
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