tilers giving different opinions on tile backer boards for timber sub-floor

[JKami84]

i have a timber subfloor - 18mm floorboards on joists. 

I'm having electric UFH in some of the area.

 

I'd like some insulating properties to make the UFH heating more efficient. 

 

1 tiler said he would only use 6mm hardiebacker cement board.

 

another tiler said he would use 10mm Delta board (which is cement coated but foam centre). he didnt just say this for insulating, he said he prefers it as it 'absorbs' some movement. 

1st tiler said only full cement board is rigid enough. he also said if i want insulating, will still need a cement board first - but this is not an option for the floor height.

 

2 views on the same puzzle.... 

 

its confused the hell out of me.