Jeff wrote:

... one way to estimate the R-value of a radiant barrier based on the air
gap and the emissivities and surface temps and the direction of heatflow
from http://www.reflectixinc.com/pdf/RIMA_Handbook.pdf

How does this work out for the double bubble?

Haven't tried that. You might work it out, if you know the gap width, etc.

http://www.blueridgecompany.com/radi...ic/189#pricing

It seems to me there are two ways to go for the underfloor insulation
for staple up radiant.

One is foil backed fiberglass insulation with an airspace. That is hard
to find! The other would be double bubble stapled on the joists. It
seems to me that would minimize heat loss through the joists themselves
as they would be uninsulated elsewise. Any thoughts?

I would staple on foil or thin double-foil foamboard.

Nick