If you would like to insulate something in a dark place, such as lunch box, another material such as cotton or paper could work just as well as aluminum foil. As a matter of fact, cotton is regularly used for insulation, as it is environmentally friendly. Because aluminum foil reflects light, when it is in a dark place, there is no light for it to reflect, so it will work similarly to other materials.
Installing a radiant barrier (foil) in your attic can dramatically lower your air conditioning bills and improve your comfort. You can easily install it yourself in a few hours with common hand tools.
Even if you air-condition seldom or not at all, a radiant barrier can greatly improve your comfort level, especially in second-floor bedrooms.
A radiant barrier is a material that blocks radiant heat energy. This is the kind of heat created when the sun shines on your roof. When it does, the roof can reach temperatures of 150 degrees or more. Without a radiant barrier foil, the heat radiates down to the attic insulation and through to your ceiling.
Aluminum foil has the radiant barrier properties of high reflectivity and low emissivity. That means the foil reflects much of the roof's heat back upward. The low emissivity property of the foil reduces the amount of heat that the foil radiates on through to the attic insulation below.
There are special inexpensive aluminum foil materials available for attic radiant barriers. Although almost any heavy-duty foil zTC would be effective, foils reinforced with fibers or paper are more durable. They are also much easier to handle and staple in position.