radiant floor heating

For radiant under floor heating how close should the plastic pipe be to the underside of the subfloor? Is the reflective bubble insulation better or fiberglass bating? My plumber installed the piping 5"-8" below the subfloor and I have open web joists. It will be very difficult to insulate with the foil-bubble insulation now. Should I insist that he re-attach it closer to the subfloor? Thanks for any opinions.
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