Jeff Thies wrote:

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

As long as we're talking about insulation, I've been trying to figure
out the radiation from the insulation and am not getting far.

From a EffHr 0.0455 (for that .05 foil Eff)

That's a radiation conductance.

Thermal resistance R = 1/C, where C = Hc + EeffHr.

Which you add to the convection conductance, which typically brings
the combined R-value of the foil down to something between 1 and 10.

It appears to me that the max effective R value would be 22...

For the foil radiation alone. But you have to add the convective
conductance Hc for the foil to its radiation conductance EeffHr.
Then take the reciprocal, then add the insulation's R-value.

It would seem that the radiation from the insulation would mean
these high R blankets would have diminishing returns.

No. Add the insulation's R-value to the foil's R-value...

It also seems that a foil barrier on the ambient side of the insulation
would be very valuable.

It's valuable on either side, but people don't like foil walls,
and the foil would weather badly outdoors, and wind would raise
its convection loss.

Nick