Advice needed on how to complete Under floor heating install

[ShyBoy]

We have a wet system installed between our suspended wooden beames. The installers placed reflective insulation and the plastic pipes over the insulation and boarded everything back with the old boards. Due to the state of the boards, i want to replace them with 18mm plywood and tile. I would say there is around 15mm of space between the piped and where the plywood will be. Should i fill that with dry mix of 8:1 sand cement? Are there any other products that will fill the void and preform better and maybe be more lightweight? Any advice will be most welcome.

 

[Norcon]

Heat diffusion plates.

 

[simond]

I don't understand why you have old boards on top of the reflective insulation, and then when you remove them, there is suddenly a 15mm gap.

Is there any reason why you cannot ply directly on top of the UFH? This would be best, introducing an air gap or a loose fill will not help heat transference.

 

[ShyBoy]

Hi, even if the pipe was right up to the plywood, it would still only be touching it with a tiny section of the cicular pipe. Most installs i have seen have a gap between the pipes and the boards but they are normally filled. I just want to know if its worth filling the gap with sand and cement? I like the idea of the aluminium plates but i wont have the time to get it done properly now as it would mean repositioning the already laid down pipes. I need a quick option to fill the gap with. Can anyone recommend or comment on the dry sand cement mix?