Radiant barriers are relatively new technology that reflect heat instead of preventing heat flow like traditional insulation. When the hot summer sun heats up the roof, a radiant barrier can help prevent that heat from getting into your attic. For existing homes, the most effective radiant barrier (from a cost and performance perspective) is specialized shiny aluminum paint sprayed onto the roof decking. This type of radiant barrier works a little differently since there is no air space between it and the roof deck to reflect heat into, instead the shiny surface transfers much less heat into the attic when it gets hot from the summer sun. While this type of radiant barrier is not quite as effective as foil sheet radiant barrier (see New Construction - Attic Radiant Barrier), it is typically less expensive to install on existing homes. To work effectively, radiant barriers need to be installed correctly and used in combination with a properly insulated and ventilated attic.