Old uninsulated slab next to new insulated slab

[NandM]

I would like to dig up the hallway floor to lay a new insulated concrete floor, with underfloor heating.

 

The original floor is comprised 70mm screed and unknown depth 1950s concrete slab. Most of the screed has been removed, with the exception of under the stairs and I've chucked down 75mm of our, with osb and lino offcuts to finish off.

 

The gas and electric mains also go under the stairs, with the meters in a under stair store room.

 

If I was to only dig out the area I know has no mains pipes, and then build up a new subfloor:

 

1) would I get a cold bridge where the old concrete meets the new, probably around 1.5sqm max.

2) if so, how could I insulate between the two? Some sort of upstand and then screed over that?

[Dave Jones]

i would be looking at 100mm hardcore compacted and blinded. dpm, 150mm celotex ufh @ 100 centres in 75mm of screed. 25mm celotecx upstands. for such a small area not sure ufh is worth it.