Insulating a room in the summertime with aluminum foil can help keep sunlight out of the room and lower temperatures. Completely cover all windows with aluminum foil and secure it with a heavy-duty tape such as duct tape. The shiny surface will reflect the sunlight's rays. You can also insulate cold food when it will be near the sun. Wrapping a cold soda can, for instance, with aluminum foil will keep it chilled. Adding a layer of bubble paper between the aluminum foil and soda can will keep it cooler for longer.
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.