Insulating against thermal conduction can be represented as an R-value because it is a linear calculation. I can add two or more R-values together and I will see a linear benefit.
Insulating against thermal radiation is more complex.: You cantcan't add two radiation barriers and get one that is twice as good. ItInsulation is based on the temperature of the barrier itself and the temperature of the environment radiating to it (occursand occurs on both sides of the barrier). It gets even more complicated when it is placed between insulation or when there are materials against it on either side. Now only the radiation that permeates the surrounding material is being reflected. The absorption of the material is doubled because the radiation is absorbed again on the way back out. Thus radiant barriers are always more effective when at least one side of them faces an air gap.
Though it cantcan't be represented by an R-value, you can reasonably calculate its effectiveness when against an air gap for a specific temperature differential. Just use the equations in the thermal radiation link above.
