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Aluminium being such a good conductor, how is it possible that it is helping me keep my food warm ?? Because ultimately it should conduct the heat that is inside to the outside for exchange and should have no effect (maybe even cool it faster by increasing the surface area).

Then why is it that we wrap our food with aluminium foil ? How does it keep my food warm ?

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    $\begingroup$ Air is one of the worst heat conductors among common materials, yet you do wear clothes when it's cold, don't you? $\endgroup$ – Dmitry Grigoryev Jul 9 '19 at 9:12
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    $\begingroup$ Aluminum is the radiation/convection insulator. You use another layer to prevent conduction, for example, paper/cardboard, or very commonly a plastic bag which because is hanging is only in contact with the surrounding air, which is a bad conductor, so in total you get total insulation. $\endgroup$ – Santropedro Jul 9 '19 at 22:02
  • $\begingroup$ The convection insulation is negligible! Since aluminium is a good heat conductor, the heat will still be dissipated by convection outside the wrapped up food. You still have to store it in a bag with better thermal properties to keep the food warm for longer. $\endgroup$ – Wood Jul 10 '19 at 10:03
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Being a shiny surface the aluminium sheet reflects radiant heat and reduces the heat loss by radiation by as much as $90\%$.

Being impermeable the sheet stops the movement of hot air from the vicinity of the surface of the food into the surrounding by convection currents.
This also has the effect of reducing the rate at which water evaporates from the surface of the food, evaporation requiring an input heat from the food.

However as you point out aluminium is a good conductor of heat and so does not reduce heat loss by this mechanism although it does trap a layer of air between the sheet and the food.
This does reduce the loss of heat by conduction as air is a bad conductor of heat.

You may have seen these properties of reduced heat loss at the end of a marathon with the use of "space blankets"?

enter image description here

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    $\begingroup$ I've never heard of space blankets at the end of a marathon but they are common emergency gear for us hikers. You can carry a lot of warm in a very small, light package. $\endgroup$ – Loren Pechtel Jul 8 '19 at 21:14
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    $\begingroup$ @LorenPechtel Wikipedia - A space blanket (depending on the function, also known as a Mylar blanket, emergency blanket, first aid blanket, safety blanket, thermal blanket, weather blanket, heat sheet, or commonly referred to as shock blankets) is an especially low-weight, low-bulk blanket made of heat-reflective thin plastic sheeting. $\endgroup$ – Farcher Jul 8 '19 at 21:46
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    $\begingroup$ I suppose if this is why you're supposed to lightly cover food in aluminum foil to keep it warm rather than tightly wrap it. Covering insulates by reducing heat loss due to convection. The interior reflective surface reduces heat loss due to radiation by reflecting it back. But because it's only lightly touching the food there's little conduction happening. OTOH you tightly wrap food you're refrigerating: reduce the trapped air to avoid insulating, and good contact with the food takes advantage of its conductive properties to cool the food in the cold refrigerated air. $\endgroup$ – Schwern Jul 8 '19 at 22:30
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    $\begingroup$ I hear talk about reflecting radiant heat a lot, but I don't think that's really a meaningful factor here. A 345K hunk of beef with 100 cm^2 area and 1 kg mass emits 8 W of power, or 8 J/s. Its heat capacity is about 0.35 kJ/(kg*K) below freezing and 2.5 above, giving it about 276 kJ of energy. Dropping to room temperature of 300 K requires losing 112 kJ, which takes about 14000 seconds, or 4 hours. But in reality, the meat will be cold in like 20 minutes from conduction in air, so radiation doesn't matter much. $\endgroup$ – MichaelS Jul 9 '19 at 11:39
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    $\begingroup$ @MichaelS It's not even conduction to the air that matters so much as the evaporation of water. There's a reason that humans sweat - evaporating water removes an enormous amount of energy. By covering/sealing the hot object so that it remains enclosed in an envelope of humidity-saturated air you prevent further evaporation and stop those heat losses. For most foods, evaporation is the dominant heat loss mechanism. $\endgroup$ – J... Jul 9 '19 at 14:22
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Because while the aluminum is a good conductor, the air movement is moving a greater amount of heat, so if you stop, well reduce, the convection currents that slows down the heat escaping from the food.

If aluminum was better at cooling bodies, rather than keeping them warm, then they would not be used as "space" blankets or safety blankets in emergency situations ie when people are suffering from hypothermia etc

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  • $\begingroup$ So if I wrap my food in plastic, can the same effect be achieved ?? $\endgroup$ – user235329 Jul 8 '19 at 12:13
  • $\begingroup$ A similar effect, but not always the same - different materials, different properties. It comes down to reducing the losses through conduction, convection and radiation... $\endgroup$ – user207455 Jul 8 '19 at 12:15
  • $\begingroup$ But aluminium increases loss through conduction... $\endgroup$ – user235329 Jul 8 '19 at 12:39
  • $\begingroup$ And also when I keep my food in my lunch box, it stops the convection part is too... $\endgroup$ – user235329 Jul 8 '19 at 12:41
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    $\begingroup$ Well aluminium is good at cooling - it's used as a heat sink for many devices. In case of aluminium, it's the structure what matters. $\endgroup$ – Tomáš Zato - Reinstate Monica Jul 9 '19 at 12:04
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The main purpose of an aluminium sheet which is a great conductor, is:

$1)$ It prevents heat losses through the process of convection. The hot air is not circulated due to the presence of aluminium sheet.

$2)$ The layer of aluminium which surrounds the food also encloses a layer of air , which provides excellent insulation to the food as air is a very good insulator.

$3)$ Aluminium being a shiny surface reflects back most of the radiation from the food back and further prevents any heat loss.

$4)$ The losses are further prevented by minimising any heat loss by water vapour. As water when evaporates removes a lot of heat. The layer of aluminium keeps the saturated air inside and prevents/reduces evaporation

Note: I am posting a self answer, because I read in the community instructions that it is encouraged here to give an answer. Also many of the good points were summarised in the comments, so I wanted to state them in the form of an answer so that it becomes easier for anyone in the future having the same question to get an answer.

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  • $\begingroup$ I don't really see the reduced convection as being specific to aluminum. Any kind of clothing can reduce convection, the key point is the reduced radiative losses. $\endgroup$ – mcodesmart Jul 10 '19 at 22:09
  • $\begingroup$ @mcodesmart Yes exactly, even plastic will do well , but I think it will not have some of the other desired effects. Also I think a lot of it comes down to the economics of it $\endgroup$ – user235329 Jul 11 '19 at 3:14
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Yes it reflects radiate heat well (like other metals, and it's cheap) and contains and stops some insulating air movement (though a sealed plastic bag would do that part better), but it also crumples a lot, somewhat sturdily (which makes it easier to close than like a piece of paper that you would probably have to fold properly), so its points of contacts with the food are reduced (which traps more air between all around it and the food than other methods generally), so this gives less chance for conductive heat transfer, as opposed to like a metal bowl (which if you ever try to from you will know how hot it gets from heat conduction).

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