Help with subfloor for Wet UFH

[Adrian of Brighton]

New to this, but a friend recommended your expertise!

 

Have a basement floor about 2m below ground.  Victorian terrace.  It is going to be 1 room and I have got wet underfloor heating system on it's way.  As you can see there are two sections, one is an existing concrete slab the other area is currently 150mm lower compacted sand over hardcore which I have been drying out. 

 

Because we have existing partial slab, I have chosen the routed thermrite boards for the whole room. 

 

For the area in the picture my plan was as follows

Lay waterprroof membrane - although not expecting much ground water as it is a well drained chalk area.

Loose lay 150mm celotex to bring up to same level as slab, using small bits of harbooard if necessary to adjust any levelling (although it is pretty level)

Then use an adhesive (not sure what, but hopefully the retailer will tell me )  to adhere the thermrite boards in place

 

Plumber seemed concerned about doing this without a poured concrete layer under the Thermrite, am I right in thinking it shouldn't be an issue?  The retailer didn't seem to be concerned!

 

Second question - I had the same friend suggest as a nice, mess free way of preparing for the final finish (oak parquet) just using cement boards - which sound a lot easier than pouring a screed (someone who has never laid screed before).  This sounded to me like a good idea and would provide thermal mass. Any reason not to do this other than it is possibly more expensive in materials.  I figured it will also facilitate me doing the project in two stages as the kitchen is in one area and the idea was to finish a section, then move the kitchen into it, whilst we finish the rest of the floor, whereas poured surface will be harder to do this way.  

 

Final thing - got a couple of manholes one at front and one towards back of property - any tips regarding these and the underfloor.  Obviously I will have to avoid putting pipes directly over them and somehow indicate where they are, but any tips for the build up (especially where the new celotex build up will be going? 

 

Many thanks in advance.  

 

Adrian