I know that this material is one of the best heat conductors we have, I’ve seen videos of people using good conductors to melt ice. (If you put an ice cube on a regular counter and you put aluminum under a second identical ice cube, the second one will melt faster). I understand this is because the conductor transfers heat from the environment to the water quicker. Then using aluminum foil to keep things (or people) warm makes no sense to me. Wouldn’t the aluminum transfer the heat from the food to the environment more quickly, making it cold faster?
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Because aluminum is shiny.
When you use it as insulation, you do *not* want it to touch the warm thing, for exactly the reason you note. It will conduct the heat away. Aluminum is used for heat sinks for this reason, but to do that you need it very closely touching the hot thing over a big area.
When you put it *over* something, like across the top of a dish of hot food, the foil isn’t touching the food. The aluminum traps the air in the bowl, preventing convection (movement of warm air), preventing conduction (air is a good insulator), and preventing radiation (aluminum is shiny so it reflects the heat radiation back at the food).
You can test this by cutting the top off a soda can (carefully! the edges will be sharp) and then sticking a finger into the middle of the can. It should feel noticeably warmer as the infrared (heat radiation) from your finger bounced off the inside of the can back to your finger.
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