Reflectix is built around air pockets, they have no/low resistance against crush force the way 5mm of dense foam like Prodex displays, a continual sheet would be a vapor barrier and provide about R-0.7 minimum insulation before the tricks of radiative/reflectance properties, and remove the need for any/many additional fasteners to hold it in place.
Note: there were no belly wraps in place during the following experiment...
When I tore out the one-sheet-thickness-of-air-gap Prodex the moisture content underneath was saturated, smelled of mold too, I'd prepped the floor underside with kilz sealer paint and gasket sealed the Prodex tightly with vulkem... and it ended up trapping moisture anyhow.... anyhow, I made the sheet 'drain' towards the center by a double thickness standoff and used a nail to chew weep holes there to encourage liquid water to drain if it ever was present... maybe if the wraps were on I'd have seen something different when tearing it out.
EDIT: added photo