Reflective insulation differs from other insulating materials in the manner in which it retards heat transfer. Unlike the others, it works by reflecting incident infrared radiation, thus reducing radiant heat transfer. Reflective insulation is fabricated from aluminum foil with backings applied to provide a series of closed air spaces. Its insulating value is derived from the heat-reflective surfaces separated by air spaces into which the radiation is reflected. Studies show it is more effective in keeping heat out of homes in hot climates than keeping heat in homes in cold ones. It is typically installed between roof rafters, floor joists, or wall studs. The R-value depends on the heat flow direction and is most effective reducing downward heat flow. Single foil radiant barriers placed in attics reduce heat transfer from roofs.