RE: Why does aluminium foil keep food warm?
ircasodle > 07-06-2018, 07:21 AM
Aluminum is reflective. Simply it reflects the thermal energy back at the object within it. This is why people put reflective covers under their windshields (bounces heat back out of the vehicle). This is why survival blankets are huge, thin, reflective sheets. This is why you can put foil in the windows of your house and save money on heating and cooling. Even chemists sometimes wrap glassware in foil to keep it warm inside. Ever notice that people are switching to metal roofs? It saves money because it reflects the heat rather than absorbing it like the dark shingles on many people's houses. It is all the same.
Metal easily conducts thermal energy, but reflective metals can bounce a lot of it back. If you are familiar with IR radiation and how greenhouses work, and how mirrors reflect light, I think you can add the two concepts together and get a decent idea of how reflective metals keep things warm.
I needed to edit this, aluminum does conduct heat since it is a conductor. It reflects IR. So for it to be useful to keep in heat, don't place it directly on the object you want to keep warm. Have a few layers of crinkled foil (makes air pockets) or something else as a medium in between. If foil is touching the object, thermal energy will move through the foil. If the foil is further away from the object, then the foil works better to reflect the energy.