I had radiant floor heat installed last year. The PEX tubing runs in the joist bays on the main floor. As part of the package, I was provided with rolled foil insulation (looks like a big bubble pad). As a cost cutting measure, it was agreed that I install the insulation myself. I was told by the installer to run the foil along the bottom of the floor joists with the foil side up. The whole idea is to reflect the heat back at the floor above and to keep the dead air space in the joist bay nice and warm. Without proper installation of the insulation, you are are simply trying to heat the entire area below your floor, intead of the floor itself. Not very efficient, I might add.
With properly installed insulation you should notice a difference in the efficiency of your system - especially during the cold winter months.