In basements, garages and under heated driveways a layer of closed cell R10 insulation panels are laid down, into which usually 1/2″ Oxygen Barrier PEX tubing is inserted at 9″ or 12″ spacing. On top of that the concrete is then poured. Each loop of PEX tubing is 300′ long at the maximum.
The rough-in
On upper floors there is no need for insulation and the tubing is stapled onto the subfloor.