We have fitted an electric mat system on a concrete floor with 10mm of
insulation. It was claimed it would come up to heat in 30 mins. Not true
more like an hour and a half to get to full heat.

I would lift the boards and put 150 mm of insulation on top of a net
attached to the bottom of the beams then put your flooring back, then
insulation board and lay your tiles on the flooring. You will need a slot
for the thermostat cut in the flooring.or the insulation mat.

The thermostat in mine was faulty but I got the tile back up no problems so
do not grout that tile until all is working well.

If you leave a few tiles to get access to the pipes without heating
underneath you will feel how cold they are. It is really obvious when you
stand on them.

eddie