# One way insulation?

I know from basic physics lessons that a box painted black will absorb heat better than a box covered in tin foil. However a box covered in tin foil will lose heat slower than a black box.

So what is the best way to conserve the temperature of a box? (aiming for 0 degrees Celsius inside the box when it's -60 outside).

I mean would painting the outside of the box black, and having tin foil on the inside work? So the box can absorb heat better (black paint) and the tin foil making it harder for heat to escape?

• While you can make insulation that will work better in one direction than the other for some finite duration, it's important to realize that "true" one-way insulation would be a violation of the Second Law of thermodynamics, as simply putting it between two equal-temperature reservoirs would create a temperature difference between the two. – Emilio Pisanty Nov 12 '13 at 17:33

If your box (at 0-20°C) starts out hotter than the environment (at -60°C) then your best strategy is to prevent any heat flowing out of the box into the environment i.e. insulate the box.

Using foil will reduce radiative energy transfer, however in most cases the cooling is dominated by convection rather than radiation and foil is a rather good conductor of heat. You can demonstrate this by wrapping yourself (at about 37°C) in foil and standing in a -60°C wind (though I wouldn't do this experiment for very long). Mind you, painting yourself black would also do little to keep you warm when you're standing in a -60°C wind.

However, suppose it's a clear winter day and the Sun is shining brightly. In that case painting the box black would help because it would increase the absorption of energy from the Sun.

A perfectly one way insulator would violate the law of conservation of energy. You could place it in a fluid filled box and let a temperature gradient develop. You could then use it to drive machinery. Bam! Energy for nothing. Therefore by the conservation of energy (and second law of thermodynamics: the entropy would decrease) such a one way insulator is impossible (although exceptions may exist if the insulator gets used up or something)

With respect to your question, covering the box with tin foil will prevent the energy from escaping better then the black one (the box's content will stay warmer for longer), however it won't make the box any warmer.

• basically I'm trying to keep the inside of a box at a reasonable temperature (0 to 20C) while the outside drops to -60C. so is black or foil better? and why? – Jonathan. Nov 12 '13 at 17:41
• This answer is incorrect. One-way insulation would not violate the first law of thermodynamics; it would simply allow energy to pass from one side to the other without getting 'energy from nothing'. However, it does change the distribution of energy in such a way that entropy decreases in a closed system. – Emilio Pisanty Nov 12 '13 at 18:02
• But won't we be creating a temperature gradient that will want to equalise, and can drive machinery? i.e. a heat engine? – Michal Nov 12 '13 at 18:10
• Yes. Your conclusion is correct, your reasoning is not. There's plenty of energy around, the problem is harvesting it. – Emilio Pisanty Nov 12 '13 at 18:14
• To concur and clarify slightly: it's the second law of thermodynamics (increasing entropy) that would be violated, not the first law (conservation of energy). You could use it to build a "perpetual motion machine of the second kind", which converts heat at a single temperature into usable energy, but not a "perpetual motion machine of the first kind", which creates energy from nothing. – Nathaniel Nov 13 '13 at 7:16

make 1 box, with the inside and outside as reflective as possible. make a second box the same way except bigger. using magnets on all sides to 'levitate' box 1 inside of box 2. suck all the air out of box 2.