Does a radiant barrier have an R-value?
Perhaps the most difficult thing about radiant barrier technology is it's explanation. "R" values measure "resistance" to conductive heat only, not radiant heat. Radiant heat is measured by "emittance" (E) values. This is an important difference since the primary mode of heat transfer, and almost all summer heat, is by radiation. For summer comfort forget "R", think "E". R-value is the measurement of a material's resistance to the flow, or transfer, of conducted heat, NOT radiant energy. Insulation systems which produce R-values are designed to slow down, or retard the flow of heat from hotter air to cooler air.
By definition, radiation will drive in a straight line until it is either deflected, or absorbed. This means that the sun's radiant energy will eventually pass through your insulation, if it is not blocked by a radiant barrier.
IMPORTANT: Some aluminum foil materials can produce R-values, but in all cases they are produced in "system" installations. The materials must be incorporated in enclosed applications which create "still air spaces". If air can move on both sides of the radiant barrier it has little, or no R-value.