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why is aluminium foil a good insulator

I think the questioner is mixing up the two meanings of the word conductor, i. e. , electric conductor vs. heat conductor. Al is an electric conductor, in fact, some wires are made of Al. Its non-conductivity of heat, i. e. , its insulation is due to two reasons: One is, as Mike W. said, the tight wrapping of Al around the food stops convective heat transfer, i. e. , it stops the flow of hot air from inside the foil to outside.

The other reason is that Al's reflective (mirror-like) surface reflects the radiation (heat) back to inside. In fact, Al blankets work this way. See Space blanket - Wikipedia. Heat also may transfer by conduction. In this regard, Al is a good conductor, i. e. , a bad insulator of heat -- touch an aluminum window frame in winter and you will feel cold. As to why a good conductor of electricity is also a good conductor of heat, see: https://van. physics. illinois. edu/qa/listing. php? id=1854 ((((OR)))) https://van. physics. illinois. edu/qa/listing. php? id=1818 ((((OR)))) http://van. physics. illinois. edu/QA/listing. php? id=1305
Arlington Heights, IL The effect of trhe aluminum foil on the cooling depends on details.

Since the foil wraps around the food, the main directions it conducts heat don t matter.

The heat flow is at right angles to the foil surface. The shielding of the radiation by the foil should help reduce the heat flow. If, however, the foil sits on some cool material that s a good thermal conductor, then it could provide a path for heat to flow away. So whether it helps or hurts in keeping the food warm depends on what else it s touching. Mike W. (published on 10/05/2015)

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