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I have a property with a 150mm (6") thick concrete slab throughout. The slab has a step down of approx 300mm / 1 ft, running the width of the house. This means the front rooms are at a higher elevation than those at the rear.

I want to connect a 3m^2 room adjacent to a front room, without a step between them. This requires raising the level of the connected room by 300mm.

The finished buildup over both slabs will be: 150mm PIR rigid insulation, 50-60mm screed (with wet underfloor heating) then tiles.

An added complication is there is a timber stud wall parallel to the step, with the base plate on the lower part, thermally decoupled from the slab it sits on using thermoblock. It is decoupled from the higher slab by 50mm PIR sandwiched between the stud wall and the higher slab to a height level with the upper slab.

What is a recommended way to raise the lower rooms floor level?

A couple of options I have thought of:

  • On top of the PIR, add 300mm of concrete. This would mean the upper layer of concrete was inside the thermal envelope of the building. Seems like a lot of concrete? And will cause lagging effects on the heating / cooling of the space - this might be a benefit though?

  • bound the borders/flanks of the connected room by 50mm PIR to a height of 300mm, then fill with compacted aggregates / hard core / rubble, possibly cap off with some concrete blinding and then have the 150mm PIR over that, level with the rest of the PIR in the front room.

  • I'm totally unfamiliar with the construction techniques in your area (what is the location?), but can you just build a wooden floor with joists setting on top of the existing foundation? That has to be easier than truck loads of concrete or other fill. Also, I assume you have 30cm of additional ceiling clearance when you do raise the floor? – JPhi1618 Apr 4 at 15:44
  • It's in the UK (England). My understanding is that created an unventilated void is something to avoid? Ceiling clearance is not an issue. – neuronotic Apr 7 at 18:27
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Without thinking too deeply about it and just basing it off of similar circumstances I would fill with about 8" of compacted crush, lay 2" insulation, lay radient pipes and pour 2" of gypsum concrete. It's not as strong as standard concrete so the compactness of the substrate is critical. It's thermal characteristics is far superior. It's not suitable for basement slabs or slab on grade applications because they require a structural component. Another great feature of gypcrete is that is pretty much self leveling.

  • Aside from the upstands, we want to have 150mm (6") insulation at the thermal envelope boundary. Your suggestion sounds similar to my second option in terms of using compacted crush, except maybe I wasn't clear on the total buildup. Let me clarify: The area to be raised, needs its slab to be raised by 300mm / 12" to be in line with the already higher slab. Then there will be a further build-up over the raised slab and existing higher slab of: 6" insulation, UFH encased in 2" liquid screed (sounds like your gypcrete). Does this match up with your model? – neuronotic Apr 7 at 18:34
  • Some of the terminology is a little different so I'm having a difficult time with the details. Googling liquid screed shows the same product as Gypcrete. Sounds like the same process. For slab on grade or basements we usually use a 2" high density foam for insulation, providing an r14 rating and a thermal break. – Joe Fala Apr 7 at 20:27

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