Duane C. Johnson wrote:

I understand there is an associated R value associated with air gaps.

Sure...

However, I would have assumed that the R value of conventional
insulation, say fiber glass, would have been better than that
of an air gap.

Add the air gap's R-value to the insulation's R-value.

What I understood was that the foil on foil backed insulation
had a different function. It was to "Reflect" infra red
radiation. This reflector can be behind sheet rock which is
able to transmit the infra red back to the source.

It could do that, with an air gap bgetween the foil and the sheet rock,
but the foil won't do much if it touches the sheet rock, except to act
as a vapor barrier.

Nick