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I've searched on Google that aluminium foil can be a thermal insulator, but how does it work?

From my thought process, it should've been a conductor because its a metal, right? But, some said that because its so thin and has large surface area, the heat would be transferred to its environment quickly, hence will cool it.

If I wrap something with an aluminium foil, will it cool that thing that is wrapped? I'm thinking it wouldn't. Yes, it is logical that because its thin and large surface area it will cool it by dissipating the heat quickly, but to where? I'm thinking if I wrap it, it might dissipate it to the inside as well, transferring the heat to the wrapped object also.

Is my thought process wrong? If yes, where? I really need some explanation to this. Thank you very much!!

EDIT

To anyone who needs more context: So I have this sensor with operating temperature of 70 degree Celsius, and the measured ambient temperature is nearly that value, so I'm thinking that I need to wrap it with something to keep it cool. I have aluminium foil on hand, will it work if I wrap the foil around it to keep it cool? I'm thinking that it will eventually get hot, but will it be cooler in longer duration if I wrap it?

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    $\begingroup$ Mountain rescue use aluminium foil blankets to keep patients warm… $\endgroup$
    – Solar Mike
    Commented Sep 16, 2021 at 8:15
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    $\begingroup$ You need to find a sensor with a higher temp rating. Nothing you do (assuming you're only willing to go to the extent if "wrap it in something" vs "develop an active cooling system for it that dumps heat outside the hot environment") will stop it approaching 70 degrees in an environment that is 70 degrees $\endgroup$
    – Caius Jard
    Commented Sep 16, 2021 at 17:04
  • $\begingroup$ dissipating the heat quickly - into what? An object at 40 degrees in an environment of 70 degrees acquires heat from the environment; the environment dissipates heat into the object. Wrapping potatoes in foil and chucking them in an oven doesn't stop them cooking $\endgroup$
    – Caius Jard
    Commented Sep 16, 2021 at 17:08
  • $\begingroup$ @CaiusJard that is exactly what I'm thinking about.. it kinda doesn't make sense to just wrap it, thanks for the idea! $\endgroup$
    – el-cheapo
    Commented Sep 16, 2021 at 18:10
  • $\begingroup$ The other thing to realize is that wrapping something with an insulator doesn't "keep it cool" (or hot). What it does is slow the heat transfer. Eventually, you will get to thermodynamic equilibrium unless you are actively doing something (expending energy) to keep a temperature difference. If you go outside wearing a wool sweater and you have a balloon with 37C water in it wrapped in the same wool, you will remain warm quite a while (since you are generating heat that the wool is trapping). The balloon, on the other hand will cool, quickly at first and then slowly (as deltaT is reduced). $\endgroup$
    – Flydog57
    Commented Sep 16, 2021 at 20:35

6 Answers 6

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It really depends on what you are trying to do with it. And thermal "conductors" and "insulators" are not a hard and fast distinction in properties like electric conductors or insulators. Every material just has a certain thermal conductivity, so compared to say diamond, aluminum has low conductivity.

With foil, wrapping it with air gaps a certain way could maybe insulate something better than leaving it bare.

Can you describe the situation more clearly?

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  • $\begingroup$ So I have this sensor with operating temperature of 70 degree Celsius, and the measured ambient temperature is nearly that value, so I'm thinking that I need to wrap it with something to keep it cool. I have aluminium foil on hand, will it work if I wrap the foil around it to keep it cool? I'm thinking that it will eventually get hot, but will it be cooler in longer duration if I wrap it? $\endgroup$
    – el-cheapo
    Commented Sep 16, 2021 at 2:59
  • $\begingroup$ If you did it right, it might help by creating a kind of heat sink. But it also might make it get hot faster by collecting more heat from the surroundings (more surface area). It would be much better to wrap it in something like plastic sheet or fiberglass, but as you said even this might only prolong the inevitable if it is soaking in a hot environment. $\endgroup$
    – RC_23
    Commented Sep 16, 2021 at 3:16
  • $\begingroup$ Thanks, this makes sense to me now $\endgroup$
    – el-cheapo
    Commented Sep 16, 2021 at 4:53
  • $\begingroup$ If the sensor is a 4-20mA device, you'll find the probe will still operate but become less accurate. $\endgroup$
    – spikey_richie
    Commented Sep 17, 2021 at 9:20
  • $\begingroup$ @el-cheapo, thermal insulation slows heat flow. Without something to draw heat away from your sensor, that will not prevent it from coming to the same temperature as its constant-temperature surroundings over time. The amount of time required for that depends on the characteristics of the insulation, but it will eventually happen unless you take more steps to prevent it than just insulating. $\endgroup$
    – John Bollinger
    Commented Sep 17, 2021 at 13:27
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If furnished in a thin, polished sheet, aluminum will reflect infrared radiation, as for instance in a so-called "space blanket" which consists of a thin sheet of plastic on which a thin and smooth layer of aluminum has been deposited. In this context, the aluminum can prevent heat losses by reflecting infrared radiation.

But if that thin sheet of aluminized plastic directly touches the source of the heat, it will conduct the heat away rather than reflect it, and heat losses will occur.

Wrapping your sensor with foil may protect it from infrared radiation from a hot source nearby, but eventually the air around the sensor will become warm and thence harm up the sensor by conduction.

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  • $\begingroup$ I'm not sure you intended to say "thence harm up the sensor", but that could indeed be one result. $\endgroup$
    – Eric Hauenstein
    Commented Sep 17, 2021 at 12:56
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The main difference that aluminum foil around an object makes is that it reflects incoming radiation because of its shininess and, because of thermodynamic equilibrium, does not emit significant radiation. Whether the foil touches the object itself or not does not make a large difference since it has very little heat capacity due to its small mass.

For heat exchange by convection, a smooth aluminum surface may provide less opportunity than what it encases but that's less clear. When the foil is just pressed between two heat-conducting bodies and has solid contact with both of them, it will not provide good insulation since it is rather thin and metallic, with just a thin layer of aluminum oxide due to passivation.

The main insulation is by blocking heat exchange by radiation.

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There are 3 modes of heat transfer: Conduction, radiation, and convection.

Conduction is heat simply transferring via contact with its surroundings. Aluminum is a pretty good conductor of heat this way. In this specific mode, the aluminum is worse, it will conduct more than most other things. But, you have to ask yourself, what is it conducting to? Once heat is conducted to the aluminum it then must continue elsewhere. Aluminum has very little heat capacity, meaning the heat can't really just drain into the aluminum and be stored there. Very little energy will have left the food before the aluminum foil and food are at thermal equilibrium.

Radiation is heat leaving via electromagnetic radiation. Mostly infrared. The aluminum blocks/reflects this. This makes it a better insulator than being exposed.

Convection would be circulating air currents carrying the heat away. Again, here the aluminum blocks it.

To answer the question: Yes, aluminum foil can be a thermal insulator.

And for your edit: I believe the foil could work to keep the sensor cool as you surmise.

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Thin layers of conductive materials may be incorporated into insulation to diffuse heat from any "hot spot" or thermal pathway in the insulation material. These pathways may occur at defects or non-homogeneous spots in the insulation. The technique is used for temperature sensitive electronics in an oil well. The well bore may be over 300 F and electronic equipment is necessary for one-time measurements. Gold foil layers are incorporated into the insulation to disrupt heat flow pathways allowing for somewhat longer measurement times before heat disrupts the electronics. The gold is not an insulator but dissipates any heat flow concentrations.

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One thing that hasn't been mentioned in other answers is the extent to which shiny pale bodies radiate less than 'black bodies'

So, if you take e.g. a jacket potato, and wrap it in tin foil, then you will get several benefits:

  1. The foil reflects some IR back in
  2. There is a small insulative air gap
  3. The heat which enters the foil is quickly conducted to the outside surface of the foil, but, it then radiates away into to the air much more slowly than it would have done from the rough dark surface of the potato would have radiated directly.
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