I have a suspended concrete slab with radiant tubing over a wood subfloor as a first floor in a single family residence in Climate Zone 5 in New York State. The basement is completely underground with 2" of rigid insulation on the outside walls. The only opening to the outside is a Bilco Door. The basement will be minimally heated. I originally had specified 1" white thermax (with a reflective side) for under the first floor joists but am now wondering whether a bubble reflective radiant barrier would work in this situation. The advantage is cost. Thanks....

Siegenthaler recommends the following MINIMUM R-values:

Slab-on-Grade Floors: Rmin = 0.125(Ti - To)
Floors over heated spaces: R-11
Floors over partially heated basements: R-19
Floors over unheated spaces (e.g., crawl spaces or unheated basements): R-30

Sounds like you may be closer to being unheated than partially heated... Fiberglass batt is a fairly common way to accomplish Above-Floor or Below-Floor tube & plate system insulation. For a unheated space, you may want consider using a vapor permeable barrier under the batt to prevent trapping condensed moisture. To optimize the insulation economics, you might want to do a floor heat loss analysis with the objective of determining what R-value will keep the downward heat loss just under 10% of the upward heat gain required to meet your building/room heat loss analysis requirements for your climatic design conditions.

the answer to your question is no. reflective bubble insulation is never an acceptable replacement for real insulation.