Putting aluminum foil on windows can keep heat out. Aluminum is highly reflective, which makes it effective in keeping out the radiant heat of the sun.
Up to 40 percent of undesired heat in a home comes in through the windows. However, while foil may keep out unwanted heat, it is unsightly and essentially defeats the purpose of having a window to look outside. There are several less intrusive window treatments available, such as semi-transparent reflective films and reflective coatings that operate on the same principle. Aluminum foil can also be used in the attic to reflect radiant heat that is attracted by the roof.