advice on build-up for UFH

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Hi all.
I came across this forum searching for info so my first post.

I'm renovating 1890's Victorian house with suspended floors. Joists are traditional 16" centres.
I want to install wet UFH to bathroom on 1st floor and finished floor to ceiling height is tight so trying not to lose any space unnecessarily.
Finished floor will be 10mm porcelain tiles

Looking at using 22mm pre routed UFH chipboard for structural integrity fixed to joists.
Supplier of said system states a layer of ply over the chipboard is required. I asked the purpose of the ply and they couldn't really tell me just saying thats "what the manufacturer recommends".
They suggested 9mm or 12 mm ply and tile on to ply with appropriate preparation.
I have 2 objections - a) not happy to tile on to ply sub base so would add tile backer anyway so adding an additional 12mm to build up and b) the extra ply would surely interfere with my UFH output and heating up time.



I thought I could use a 6mm backer board screwed and glued/ adhered straight on to the chipboard. Tile on to backer board giving an overall height from joists to top of finished floor 38 - 41 mm - inside my target.
Questions:
1. Do you see any potential issues with installing 6mm backer board between chipboard and tiles? If yes what do you recommend?
2. When fixing backer board should I avoid screwing through chipboard and pickup joists to keep backer board independently fixed to just chipboard?

An alternative to the above is to use aluminium spreader plates that hold UFH pipes between joists and then 18 or 22mm TG4. Downsides for me is additional costs and there's a lot of plumbing in joist spaces that would need to be lowered to accommodate plates thus more costs.


Any advice gratefully received

thanks
 
Hardie backer glue and screwed sounds like it would work but would be easy to screw through pipes, wouldn’t advice a wedi type board as this will insulate the heat the wrong way
 

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