Does it have a shiny ,reflective side ? If so it
should work OK, especially if it is going into a downstairs ceiling. I usually use staples (rented
staple gun) or 1/2" pipe clamps to hold the PEX up tight to the flooring for max conductance of heat through floor. Then I put reflectix suspended min. 1 in. below PEX to reflect the heat upwards. Its also important to keep dust/debris off any radiant barrier as it will impair its ability to reflect. I staple up 4 mil poly on bottoms of joists during construction to prevent this.
