Minimum Depth of insulation and screed for polished concrete floor finish

ontour2

Basically, I took out a suspended timber floor that was approximately 400mm above the external patio level.  Plan was to dig out underneath, then insulate, cover in concrete and polish to a level that was the same as the patio.  There is an architect, engineer and builder so it is not down to me!
Discovered that where 'someone' thought it would be just clay under the void it is actually a 200mm concrete base.  Potentially walls are also sitting on it.  Engineer is on holidays so I will be getting the appropriate professions in but was interested to see if anyone had similar experience.
One thing I am trying to figure out is whether I could put the insulation and screed on top of the existing slab.  Are there any low profile underfloor insulation?  What is the minimum depth I could achieve?  Any thoughts or experience is appreciated.

Dudda

Shouldn't be a problem using the existing slab. If it's taking walls it will easily take the floor. You need a min of 75mm of concrete if you are polishing. Anything less and you risk cracks. Will you have underfloor heating? What depth do you have? I'd go 85mm concrete and the rest good quality kooltherm insulation or similar with a radon membrane under the insulation on top of the existing slab. Put a lot of thought into the expansion cracks. Shouldn't have more than a 6m span in any direction. You'll also need them at door and other locations depending on the house layout.

Calahonda52

If I read this correctly, you are dropping the FFL 400mm to the same level as external patio?

Where is the wall DPC?

is patio at ground level vis a vis garden?

what level is the rest of the house?

ontour2

No underfloor heating. Space is c. 4.5x6m. It looked like the dpc was around the level at the patio, there was talk that there would be a 100m step to the patio which I believe was related to the DPC level.
The garden is the same level or very slightly lower than the patio. Site has a gradient from front to back so there will be 2 steps down from front rooms to the proposed new concrete floor in kitchen.
Thanks for taking the time to reply.

Calahonda52

Always nice to see sensible replies to suggestions so here is one for the road, given the two steps
Accessibility?
http://www.housing.gov.ie/housing/building-standards/tgd-part-m-access-and-use/technical-guidance-document-m-access-and-use-2

Just think about what might happen if the water table rises.....