You might get some heat to the touch of the foot for some minor comfort improvement but for any appreciable room heat, which I think OP will need for his application in a garage conversion, the radiant will not provide enough heat for overall room heat it that’s what he’s after. In Ontario building code for new construction for instance, as far as radiant heat as the primary source of building heat, any vapour barrier must be BENEATH the heating source tube or cable to mitigate vapour to be less than 45 ng/Pa•s•m2 . Usually poly is best placed below any R10 insulation foam as well which the radiant heat would be tied on to or a puck panel used. Again, OP isn’t strictly following code I guess (because I bet he’s not even allowed in most municipalities to convert a garage to livable space) but in terms of just getting good bang for his buck, that cable or mat isn’t going to heat much without a thermal mass (being embedded in concrete) or with LVP above it.