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ICF and suspended (warm) slab


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So, our build is at the SE/detailed drawing stage. Our plan is to have a warm concrete slab and have the powerfloated finish as the final surface. Our SE has detailed a ring of marmox thermoblock around the perimeter and at intervals running across the slab on footings (the slab will be about 18x9m in total). I understand this is what makes it "suspended" - as it is essentially supported by the thermoblock strips and the insulation inbetween is not actually loadbearing. Does this make sense? Any reason why we can't just have 200mm of insulation under the entire slab and this be the loadbearing element, without having to introduce this marmox stuff? This is what happens with an insulated raft after all. We are having strip foundations and probably going for Nudura, so the insulation could directly abut the Nudura block and prevent cold bridging. Has any one any experience of strip foundations+ICF+warm concrete slab?

Thanks all!

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2 hours ago, Tom said:

Any reason why we can't just have 200mm of insulation under the entire slab and this be the loadbearing element

If your ground conditions allow, no reason why not. But if that's the case why bother with the strip footings as well. Do the whole foundation as an insulated raft.

 

The size of your footprint requires some intermediate support and cannot rely on perimeter footing alone hence the strips running through your slab.

 

I'd guess your SE is just doing it the way they know best. Without changing SE's, it might be a bit late to redesign as full raft. 

 

What insulation is currently specified? You might need to change to EPS 300 if you want it to bear on the ground and support the structure above.

 

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Thanks Will, the ground is half made-up as the site was originally leveled for the barns by just scooping out of the hillside - I think this is why the SE has been cautious re the use of an insulated raft. One corner of what's there now is happily slipping down the hill as it is! I'm thinking that with strip foundations and the first few courses of ICF essentially forming a retaining wall this will allow us to properly compact the oversite and make a stable base for a groundbearing slab. Still, no great shakes if it's suspended though I suppose. Does any one know what the U value of marmox thermoblock is? On their site they state the thermal conductivity as 0.047W/mK - so am i right in thinking the U value would be, say for a 200mm block, = be 0.047 / 0.2 = 0.235? Not too bad I suppose. If we use the slab suspended on the perimeter like this I suppose we can use insulation under the rest that has a better U value than could be achieved with eg EPS 300?

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If I were in your position, I'd be asking your SE as to the thinking behind the approach - just out of curiosity as much as anything else, always keen to learn. It might be that having the strip foundations allows using a thinner/simpler (and hence cheaper) slab??? and cheaper spec. insulation as well maybe?? I'm sure I saw a diagram of a loadbearing slab somewhere (maybe on here) and it had thickening up round the edges as well as a pretty thick layer of concrete on top. 

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