As the title asks, does the shiny side of foil really insulate or keep food warm when facing inwards? I've heard that the shiny reflective surface of the foil reflects the infrared energy. Is this true or is it just a marketing gimmick that everyone has come to believe?
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1$\begingroup$ I think you can find an answer here PhysicsSE - Does more reflective aluminum foil make a room cooler compared to less reflective foil? $\endgroup$– hxriCommented Feb 9, 2018 at 1:21
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6$\begingroup$ Possible duplicate of Does more reflective aluminum foil make a room cooler compared to less reflective foil? $\endgroup$– hxriCommented Feb 9, 2018 at 1:23
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1 Answer
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The simple physics in reflection.
A completely reflecting a light source body, creates a mirror image of the source , so that it is equidistant from the source. If the reflection were complete, all the heat energy hitting the mirroring surface would come from the imaginary space of the image of the source.
A diffuse reflecting surface acts like innumerable sources at the surface of the material, in this case aluminum foil.
This can make a difference in cooking uniformly in an oven, covering with the shiny side out will reflect the heat back to the walls, with the diffuse side out heat sources will be close to the food.
Both the shiny and the diffuse aluminum foil sides will reflect energy back. The sides should not make a difference on food on a counter.
To really insulate there should be a non conducting side on the outside ( like a space blanket), as the foil reaching the hot temperature of the container will radiate away heat according to black body radiation.
As a side note, I use aluminum foil on the wall behind the smoke exhaust pipe of a wood burning stove, and the wall behind stays room temperature, while without the foil it would be very hot. So yes, foil reflects infrared. The shiny or not side makes no difference in the insulation towards the wall.
