Very large UFH job questions...

[Element]

I've been asked to give a ball park figure for laying 75sqm of electric ufh in a flat. It currently has 18mm chipboard T&G flooring. Initially I was thinking of laying 12mm ply throughout then bedding flexy adhesive onto the primed ply then laying the matts then bedding again with flexy before the tiles. However I am daunted at the prospect of lugging 26 sheets of ply up to the first floor and all the time I'll spend fitting them and getting them ready for the UFH etc... So I've looked at the Hardie Backer board option which seems a damn site easier. My local Topps has given me a figure for the boards but I gather from other posts that maybe my local building depot may offer them cheaper. I figure a '5 system' layout is probably the best route with 5 separate circuits & thermostats.
So, if I have it right, ply is stronger but the hardie boards are simpler and quicker to use with UFH? Given the place was fully furnished and carpeted, it wasn't really possible to get a feel for deflection but it's a fairly modern building so I doubt there'd be much.
What would you suggest - ply or hardie boards?

 

[One Day]

Neither - Use Wedi or Marmox glued and screwed.
Lighter, faster and insulates so will keep electric bill down.

It seems absolutely pointless using ply or hardi in my opinion.

- - - Updated - - -

CTD trade boards made by Jackon are the best value right now btw.

 

[Rosco100]

Defo use lightweight boards like Marmox,we have great deal on them at the moment

 

[Element]

Ok, thanks for your advice. Having looked into these insulation boards they do seem a better option. I remember using them a few yrs ago but it wasn't with UFH. My only concern would be that these boards won't offer much lateral support if there were any deflection in the floors - but without getting a much closer look I'll never know that.

 

[Stef]

Ok, thanks for your advice. Having looked into these insulation boards they do seem a better option. I remember using them a few yrs ago but it wasn't with UFH. My only concern would be that these boards won't offer much lateral support if there were any deflection in the floors - but without getting a much closer look I'll never know that.

It doesn't matter what you use to overboard, if you have deflection in the floor then overboarding it with whatever isn't going to sort it out,
Sort deflection first then overboard..

Oh & overboarding isn't going to sort out lateral movement, need something like Ditra mat for this..

 

[Daz]

The majority of first floor flats are concrete floors. Is this a house that has been converted to flats?
75sqm of electric UFH is going to be very expensive to run, I wouldn't want that leccy bill.....
Either way, sort any deflection first, then glue and screw insulation in a brick bond pattern. Install wires and I would probably cover with sic too.
if you are setting five zones, remember expansion joints too