Concrete to replace suspended floor

Volume of hardcore + volume of blinding + volume of concrete + area of dpc x2 + building control fee = cost, assuming the thickness of insulation you have meets regs, you have insulation for the perimeter and that the depth of hardcore you would require is less than 600mm

 

Oops, forgot the volume of screed.

 

Put the insulation under the concrete so you gain the thermal mass.

Use crushed concrete rather than hardcore as easier to handle and compact. Damp (not wet) material will compact better.

 

That's for you to decide but if current floor has no insulation and needs levelling anyway the added bonus of energy storage in the concrete makes a strong argument. You're going UFH so no worries about rad feeds but you'll have to think about the wiring under the floor

 

I'd stick with the suspended floor. Upgrade it if possible but even lifting boards across the ground floors could bring a few problems.
What you could be doing with going for a solid floor is getting into various expensive difficulties. unknowns that you will maybe have to deal with as they pop up.

 

I'm in the same boat as the original poster and not sure how best to proceed. Either face the poor insulation of UFH with Suspended Timber (we'll make the best of it that we can, but it will never be as good as a concrete slab), or face other risks associated with filling the void with concrete.

CCoomber, what did you do in the end?

My main concerns are:
- How do you protect against clay soil heave in the wetter months under the property if you have filled the void and have no expansion gaps.
- Similarly, I expect the concrete would expand/shrink just based on temperatures (although this could possibly be taken up with some insulation around the slab) if you can use some with some compression ability.