Add the insulation AND THEN add the in-floor radiant heat. Or choose another material other than tile, stone or concrete.

Bonded Wedi at 1/2" adds aprox R3 more importantly adds/ acts as a thermal break when used on ***top of basement slab, you then add radiant or hydronic heat above wedi.

This is smarter than using R-foam below slab in that you no longer have A thick mass of crete to heat up and over work the heat elements (if radiant) to reach desired temp and many times eventually forcing the sensor(s) to fail.

It is not cheap but offers a smarter approach and lifecycle by heating the tile and not the entite slab.