Installing radiant in an existing house does require installing more thermostats.

Fishing thermostat wire up through floors and trying to get to a proper location for the thermostat can be difficult. See the instructions below.
You can bring the stat wire up through the corner of a closet and drill through the wall into a room.
You can remove a the trim on the side of a door frame, drill through the floor and come up behind the trim and drill across, placing the stat next to the next stud in the wall.