Radiant barriers reflect radiant energy transferred via radiation. Radiation is the transfer of heat (infra-red radiant energy) from a hot surface to a colder surface across an air space. All surfaces including roofs, ceilings and basic insulation radiate heat in varying degrees. Radiant heat is invisible and has no temperature, just energy. When this radiant energy strikes a secondary surface, it is absorbed and increases the temperature of that secondary surface.
This concept can be understood with the following common examples:
- On a bright sunny day, radiant heat from the sun travels through a car's window, strikes the steering wheel and is absorbed, causing the steering wheel to rise in temperature.
- Radiation from the sun strikes the outer surfaces of walls and roofs and is absorbed causing the surface to heat up. This heat flows from the outer side of the material to the inner side of the material through conduction heat transfer which is then radiated again into attics and living spaces increases the temperature inside the building.
A radiant barrier is designed to "block" the radiant energy that strikes it thereby reducing the amount of heat that would normally be radiated into attics and living spaces without a radiant barrier in place.