Please explain how foil-faced roof sheathing works

Some say that foil-faced roof sheathing can reduce the temperature in an attic. I&#;m unclear how this works. Considering two types of attic designs:

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1 &#; The attic is ventilated, and the insulation is laid on top of the ceiling drywall.
2 &#; The attic is not ventilated, and the insulation is attached tight to the roof sheathing.

Insulation R value only relates to conducted heat. Foil-facing reflects heat, so it reduces heat transfer between inside and outside via another mechanism (stops radiant heat from becoming conductive heat). Comparing foil-faced and non-foil-faced insulation of the same R value, the foil-faced will reduce total heat transfer more.

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I'm in a situation where I may need to replace foil-faced insulation with another type that is not foil-faced. I can match the R value, but would lose the foil's reduction in radiant heat transfer. That would translate to more conductive heat, which could be offset by increasing the R value.

Is there a way to figure out what increase in R value would yield equivalent reduction in heat transfer as the foil adds (i.e., maintain the same temperature difference between inside and outside as the foil-faced insulation)?

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