everyday-lifethermodynamics
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.
First, there's no perfect reflector nor absorber. In fact - even Aluminium does absorb some radiation (by which it gets heated, can be noticed at incident high frequency radiation). One more thing is that aluminium foils are designed in a way to reflect light.
Here's the Wiki article quote...
Aluminium foil has a shiny side and a matte side. The shiny side is produced when the aluminium is rolled during the final pass. It is difficult to produce rollers with a gap fine enough to cope with the foil gauge, therefore, for the final pass, two sheets are rolled at the same time, doubling the thickness of the gauge at entry to the rollers. When the sheets are later separated, the inside surface is dull, and the outside surface is shiny. This difference in the finish has led to the perception that favouring a side has an effect when cooking. While many believe that the different properties keep heat out when wrapped with the shiny finish facing out, and keep heat in with the shiny finish facing inwards, the actual difference is imperceptible without instrumentation. The reflectivity of bright aluminium foil is 88% while dull embossed foil is about 80%.
The shiny and not-shiny surfaces are totally a favor of production technology (credit goes to the rollers). Now, to the "why" question.
As a physics parameter, we use reflectivity to address the shininess. As we can see, the reflectivity is quite high for the bright surface, compared to dull one. The unreflected light (as you say) can go anywhere. It can go inside the aluminium foil (i.e) it's absorbed and hence the 12% & 20% loss...
Response to comment (based on edit): That's a nice strange idea. With some perfection (I mean, there should be very less allowance of any sorta radiation inside), the room will be relatively cooler. But, in reality (where we can't expect idealistic things), there will always be some radiation inside. But, it keeps the room warmer compared to the outside. But, always be careful when playing with such things because, any sort of harmful radiation (if any- what about a heater or even an electric iron?) inside the room will be reflected back to you by the matte side - which can be very harmful...
Even considering the same fabric with different colours it will depend a lot on optical properties of the dye which we cannot tell only by the colour that we can see. I would need to know "how much the silver paint would reflect and in what spectral range" and also "how is the absorption coefficient as a function of the wavelength (from near-UV, Visible to near-Infrared)". I believe that this is the most important point and not the colour that we perceive with our eyes.
Nevertheless, keeping things simple, I think that the reflective coating being outside or inside would make the same amount of radiation that passes through. The only difference is that putting the reflective layer inside you would be increasing the path length of the radiation, thus increasing the absorption, and therefore the temperature
EDIT:
In order to clarify my idea I added this picture. Dashed layer is reflective and the the grey layer is the normal dark layer. I would like to divide this discussion in two: "Radiation Protection" and "Heat Protection"
Regarding the radiation protection, I believe that it depends on how the the dark layer absorbs light in all spectral range of the sun light, i.e. absorption coefficient and thickness of the material+dye. It also depends on how the reflective layer reflects the light, i.e. reflectance. I believe that the radiation protection does not depend on the arrangement, so both schemes protect from radiation as good. (I am not considering far infrared radiation)
Regarding Heat Protection, the thought is the same, except we should consider that in the first case the length of the radiation in the dark layer is longer, thus absorbing more radiation thus heating more. Also, the protection from the heat will depend on the heat conductivity of the layers and surface morphology which will affect how well the layer will cool down.
I understand that having a reflective coating outside an umbrella would be unpleasant for other people. But it would be the best choice for heat protection. However, regarding the emergency heat blankets, I do not know why they also have a reflective layer on the inside.
Best Answer
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.