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.