Underfloor heating - Backer Boards

[MSB Tiling]

Hey Everyone.

Ive got a job coming up monday tiling a 12m² concrete floor. The floor is having backer boards down which I am going to lay on tile adhesive, my question is Do I use flexi adehsive to lay the backer boards?

I no that flexi is used on the underfloor heating because of the movement from the heat etc.

My gut instinct is to use flexi on both but I thought asking here may give me better options.

Thanks.

[Sir Ramic]

Use the Flexi as it wont cost much more.