I'm looking at adding the maximum feasible amount of insulation to a floor in an existing building.

We can raise the floor level by a bit - doors and so on will be replaced anyway - but not by too much, because then we will run into headroom problems.

It's an existing (damp) ground bearing concrete slab. Digging it out is not feasible. We are looking at an engineered timber floor finish. No underfloor heating. As far as I can make out the basic options are:

1) An insulated underlay straight onto the slab on top of the dpm, and the most insulative option seems to be those which are 5 or 6mm polystyrene. Of course this is not going to provide any kind of substantial insulation but hopefully "better than nothing".

2) A minimal layer of XPS70 or XPS100 insulation (say 25mm) but then you need a layer of board on top of that, then an underlay, then the flooring board itself. To bump up the insulation layer from 5 to 25mm you have to add an extra 40mm or so of buildup, because of the extra layer of board.

My question: are there any other effective options that I've not thought about?

In a similar situation I have just laid 20 mm XPS, VCL then click fit floor boards. The total thickness 32 mm. The click-fit flooring was 12 mm thick and rated for bathrooms.

The concrete floor was flat enough that the XPS went straight down and there was no need for any other boarding or underlay before the click-fit. The VCL was to cover off the click-fit guarantee requirements. There is no DPC in either the floor of the walls (a barn conversion with stone/earth walls).

I did the same in another part of the same building but used 50 mm XPS and no VCL. The flooring was click-fit engineered wood. 10 years on and no problems. And in another room I used 50 mm XPS, 18 mm T&G flooring chipboard then 60mm x 60mm industrial carpet tiles (sale offer) Again 10 years on and no problems

<blockquote><cite>Posted By: Peter_in_Hungary</cite>In a similar situation I have just laid 20 mm XPS, VCL then click fit floor boards. The total thickness 32 mm. The click-fit flooring was 12 mm thick and rated for bathrooms.

The concrete floor was flat enough that the XPS went straight down and there was no need for any other boarding or underlay before the click-fit. The VCL was to cover off the click-fit guarantee requirements. There is no DPC in either the floor of the walls (a barn conversion with stone/earth walls).

I did the same in another part of the same building but used 50 mm XPS and no VCL. The flooring was click-fit engineered wood. 10 years on and no problems. And in another room I used 50 mm XPS, 18 mm T&G flooring chipboard then 60mm x 60mm industrial carpet tiles (sale offer) Again 10 years on and no problems</blockquote>

Advice I've had from suppliers is that if you put click fit / t&g type flooring directly onto XPS that's more than about 5mm thick, you are likely to find it a bit "bouncy". I assume you've not had problems with that?

Does XPS come in different ratings like EPS? (i.e. EPS 70 vs EPS 200) The rating affects the bounciness a lot.

I wonder if the bounciness is something that becomes a problem when the subfloor is uneven.

A friend put 20mm flooring-grade PUR under boards (much like Peter) and it was fine and a big improvement over plain concrete.

I've been looking at this here, and found I could get an extra 30-40mm by chipping off the screed on top of the slab. Obviously you then need some levelling compound to get back to flat but there should be room for at least 20mm more insulation for the same thickness

XPS does come in various flooring grades: 300, 500, 700kPa. e.g from Kingspan, Unilin or Soprema. So it being strong enough to support boards without undue bounce is not a problem.
I put 50mm XPS in my garage with 9mm MgO boards on top. That was standard XPS so probably 200/250kPa? That did result in differential movement at board joins because they were not T&G. I routed space for joining pieces underneath, screwed in, which worked well but was a faff. Also MgO is not scuff-resistant enough to make good flooring without some sort of cover (paint/vinyl/carpet). It is very strong though - no problem with heavy 50s pillar drill or angle-iron steel bench feet.

So 50mm depth of standard-grade XPS does need a T&G floor. But it doesn't feel at all 'bouncy'. Less of an issue at 20mm depth. If you can get higher-strength XPS that would be better, but as Peter says standard XPS+click-together flooring is OK in practice.

XPS is very high embodied carbon, so I'd try to get the Unilin low-carbon 'ECO360' stuff (hmm, that only seems to be PIR currently, and more signifcantly for you, no thinner than 100mm for their flooring-grade)

For anyone wanting a solid screed-type floor floating over XPS or PUR in the thinnest form, the best available option I've found is Cellecta 'screedboard' which is flooring-grade cementitious boards with T&G edges in 20mm and 28mm thicknesses. That will work over UFH and a vinyl or marmoleum decorative layer will only add about 2-4mm. This isn't any use in lineweights case.

I copied Wookey's approach to our garage-which-is-now-a-workshop floor - I used standard 50mm foil faced PIR with 9mm Magply (MgO board) on top. Before starting I painted a bitumen DPC within 500mm of slab edges, and the wall nearby, just in case. Didn't bother elsewhere, it's never shown any sign of damp. Everything was wetted a little and glued down with expanding foam glue, and squashed flat (using dozens of paint tins/bricks/anything heavy per sheet) for a good fit all over. Joints then taped. Between sheets I pushed in "Sabrefix builders band" from screwfix - especially at corners, and made sure this was glued in place. After fitting, I made nice with some sealant between sheets.
It seems generally good, I've wheeled our shiny new 15kWh battery (120kg on 4 castors) about on it, over joints, no problems.

Only faults so far:
Magply is water resistant, not waterproof. Spilling tea on it results in a stain you can't get off, as it soaks in.
Magply is cold underfoot, as it conducts fairly well, just like a tiled floor. I'm planning to cover it with something like corkboard, as I dislike this.
I didn't glue one slightly bowed sheet down properly, and it's a bit bouncy in the middle which is annoying. The rest is fine.
Magply is tough, but when I dropped a heavy metal bar on it (would have broken a toe/cracked a tile type impact) it locally (2x2cm) dented the Magply.

I had similar issue. I have a room converted from a garage years ago and it must have been done badly because the floor is much colder in this rom than the others. I started using that room for work last year and couldn't stand how cold the floor was, to the point I needed to increase temperature by 1 degree in whole house. So, I lifted carpet, remove the bubble wrap underlay and put 25mm EPS with 6mm backer board and carpet back. It was done only where I have desk and chair and I can feel small difference. But bear in mind what I did was only a temporary solution because the room will be renovated soon and wanted to have a play with how much insulation it requires. The biggest problem is durability of such solution. Ideally it requires screeding or 18mm T&G boards but that's another 20mm at least. For this winter I'm testing another idea which seems providing exceptional results!
I went to a shop and bough just sheepskin slippers. The only problem now is with Mrs complaining that the temperature in the house is too low.