UFH overlay system advice

[Harveys dad]

I need the benefit of your experience please . I’m in the middle of building a 105sq metre extension and looking at UFH possibilities. I had to leave the job as my wife was ill and It looks like my mate  may have dropped a clanger in construction of the slab . From the bottom up we have a min 150 mm  compacted dolomite , visqueen then 120mm Celotex ,reinforcing mesh on that then 150 mm concrete. I have approx 70mm to FFL . My question is with what I have would an overlay system something like egg crates with say 40mm of liquid screed work , or should I have more insulation on top of the slab .? Or is there a better method / system , thanks in advance. 

[JFDIY]

You have enough insulation under the slab, so I'd consider bonding clip track strips to the slab that will hold the pipes, then piping it up and covering with liquid  screed.

 

You may want to talk to screed manufacturers, because I think you'd want cement based so the screed and slab have similar expansion rates to avoid cracking of the screed.

 

Or as you say the egg box type trays, if you use these with a separate DPM this will give a slip layer to prevent differential expansion being an issue

 

[Harveys dad]

Thanks for your help