Bonding underfloor heating insulation to floor

[crispibits]

Hi, I'm laying electric underfloor heating on a new concrete floor with terracotta tiles on top. We have minimal head height, so having to go for 6mm insulation board. I see the board is supposed to be bonded with tile adhesive to the concrete floor. We really need every millimetre we can get. Can anyone advise on why the tile adhesive under the board is required? Is it to prevent movement, to level the floor, to maintain a constant temperature spread or what? What's likely to happen if we just lay board on concrete, then matting then tiles on top? Thanks for any advice,
Cris

 

[tpt]

Its to bond the backer boards to the concrete. if you just lay them sooner or later the movement is likely to cause lifting of tiles or cracking