Why do insulated cooler bags have aluminum on the inside? I understand that aluminum can partially reflect the sun and other heat radiation if placed on the outside of cooler bags. However, most types of cooler bags only actually have the foil on the inside and are non-reflective on the outside.
Is there actually any point of having the foil on the inside (unless you intend to keep things warm instead of cold)? In particular: the types that have no air between the aluminum layer and the outside layer.
 A: It's useful because it reflects heat which penetrates from outside (it's irrelevant which layer it is), keeping food cold, while it also has an added benefit of being waterproof, so it keeps food dry (why it's always an inner layer).
However if you are transporting hot food, it reflects heat from the inside, keeping it warm (and dry).
A: I guess because there's no light inside, nothing reflects; but since both sides of the aluminum sheet reflect, all light hitting the outside reflects away. The outside layer is sometimes painted, but still reflects light away.
A: The emissivity of aluminum is very low. This slows down heat transport into the bag.
If the bag sits still, there is no forced convection, and air is a poor heat conductor. On top of that, the surface of the bag will radiate heat to the food inside.
The low emissivity of aluminum slows down that process.
Note that e.g. PIR insulation often comes with a aluminum facer, that is for this very same reason. In modern window panes, low E coatings are used - that is the “see-through” version of this concept.
