Self Level compound or board first?

[Pritch]

Hi everyone .
I have a question to ask about my bathroom.
I'm about to install electric under floor heating in my bathroom and lay new tiles .
I've lifted the old tiles that were laid directly on top of the chip board flooring but the old adhesive is extremely difficult to get off.
Is it ok to install/ screw the 6 mm insulation board over the top of the old adhesive then use self level compound or should I self level, lay insulation board and self level again?
Thanks

 

[Bond]

You could skim over the existing adhesive , with a flexiable fast set adhesive and set your boards into that, or , as you say fibre reinforced slc first , when dry ,set your boards in tile adhesive, screw and scrim, fit ufh and latex over. Ready to tile.

 

[Blunt Tool]

As above if you have the depth if not diamond disc grinder with hood attachment and dust extraction will get rid of old adhesive in no time. You can hire them.

 

[Flintstone]

Take off any high spots and stick your Hardie backer down with a suitable trowel and screw, they carry out any levelling you need to do