Mind you, for a 60sqm bedroom wing, I'm looking at about £1.2k for the styrofloor thing, or as little as £800 if I buy xps and chipboard separately... Appreciate this is not comparable to 10cm PIR, but will probably feel better than 2mm LVT glued to concrete. I imagine at the rate we're going, this time next year we'll be paying £800 for a tank of diesel.

Biggest issue is the 4-6cm reduction in door height.

Edited October 25Oct 25 by tetris

18 minutes ago, tetris said:

carpets in a corridor won't survive a fortnight.

You do have to pay more for hard-wearing, which will last years.

If you don't mind the utility look then carpet tiles . In office quality they will last you out, and allow kids' wheely toys, and for any accidents, plasticine or damage, can be taken outside to clean.

I have some salvaged from an office after 7 years of heavy use, used in a corridor, gym, workshop area. It is on an uninsulated concrete slab. I put an underlay of 10mm down first. The theoretical insulation (by U value) is low but much better than nothing, and the effect on feet comfort and warmth is significant.

Not sure this really makes sense in your situation but throwing it out there just in case:

If you are thinking about putting thin polystyrene in the floor and boarding over it, then think about whether you could use one of the thin UFH products (pregrooved polystyrene that acts as a template for the pipes). That might solve your radiator issues (and generally UFH will be cheaper than buying a load of vertical rads).

Important note is that the insulation layer is not thick enough for an external floor (would be very expensive to run), but if all you have below the floor is another habitable room then the small amount of insulation might not be a big deal. It would certainly solve the cold feet issue.

The floor still won't be as good as a screeded ufh setup but everything is a compromise.

The biggest issue in your case @tetris is likely that you want to do this quickly while living in the place and doing that with the floor might be tricky and certainly requires careful thought.

Thank you - it's a bungalow, and I've read (on this forum if memory serves me right) that unless you have 10cm PIR below UFH, all you're doing is heating up the floor void.

Just now, tetris said:

Thank you - it's a bungalow, and I've read (on this forum if memory serves me right) that unless you have 10cm PIR below UFH, all you're doing is heating up the floor void.

Doh! You are correct. Sorry I think you mentioned that in a previous post. I was thinking this was a first floor.

Edited October 25Oct 25 by -rick-

I would spend time making sure your floor is sealed to stop drafts that may be there. Then spend you money a good thermal underlay and a wool carpet. Do the same in bedrooms and corridor.

A very quick search brought up this

https://www.carpet-underlay-shop.co.uk/collections/thermal-insulation-underlay/products/56oz-heat-insulation-wool-carpet-underlay

15mm thick, add a good carpet job done.

Sealing the floor to make sure it's airtight would be my priority.

Another layer could be thermal tile backer boards, glued direct to floor and then underlay and carpet.

Thanks.

SWMBO sets the cleanliness standards around here, as she's the one who does the cleaning. Her view is that carpets become very dirty to the point of needing replaced the moment they leave the manufacturing facility 😁.

Carpets in bedrooms might just be the way to go in our case, but no carpet is ever going into the corridor (and I have to agree - the amount of dirt that my kids will get into it within the first fortnight will make me want to chuck it out). Even for bedrooms the carpets would not be our first option. Small kids get sick all the time. When they get sick, they puke. If that gets into the carpet, even a weekend with a carpet cleaner is not going to get the smell out of it.

Here's a thought: Would 25mm XPS sheeting take care of slightly uneven flooring (within +/-2mm perhaps?), so I can just put 18mm chipboard on top and that's the top floor ready for clicking? Sounds like a huge time saver if that's the case.

Current carpet + underlay thickness in rooms is 15mm. If I raise the floor by 50mm, less 15 from carpet removal, less another 15 from raising the doors, I'm only getting a net floor height increase of 20mm. If that means fast & easy floor installation + some insulation benefits, this is starting to look quite attractive.

In that case go straight tile backer, even a couple of layers glued down and tiles. Why bother with the chipboard at all

chipboard to spread the load of the click flooring?

Tile backers description reads like they might be OK but has anyone here used them for flooring?

2 hours ago, tetris said:

has anyone here used them for flooring?

yes. under a heating mat under tiles. and it's no bother. they are a good compromise of insulation, strength and general robustness.

If I understand correctly, it's just XPS with a thin hard layer on top of it?

I'll be floating click laminate or LVT, rather than gluing tiles to it, and none of these are officially compatible with click flooring because it spreads the load differently from glued-on ceramic tiles?

And does the XPS act as a DPM in itself?

Apologies, I might be asking very daft things here.

10 hours ago, tetris said:

might be asking very daft things here.

Questions are seldom daft.

Keep asking.

Marmox etc are not eps and are robust. I've seen an eps equivalent and it isn't, but might be OK under flooring.

I'd probably paint a dpm first.

Well, I started with "Insulating suspended concrete" and ended up with "cheap & easy way to install click flooring on uneven surface while also getting marginal insulation benefits" 😬 I think this is quite far down the daft spectrum tbh.

Thanks for the MArmox heads up - seen the name mentioned while lurking around this forum, but didn't know what it was.

Marmox offer "Sound Multiboard" that's specifically stated as suitable for laying laminate on top, but it comes in 12mm thickness and is about £20 per sq m. Might be a solution for the corridor if I need to raise it just a bit to match bedrooms.

For regular multiboard the kpa is impressive (400kpa) - 3x higher than floor-rated PIR and 2x higher than regular XPS, but again, they specifically state that it is not designed to resist point pressure.

I couldn't find any mention of people successfully laying click on top though and manufacturer does not list it as designed for click flooring. The former is particularly concerning.

So, seal all gaps around the perimeter; paint DPM, (check floor unevenness is within 3 over 3m), lay marmox foam-down, lay laminate on top?

Edited October 27Oct 27 by tetris

You take out level changes and uneven areas with self levelling compound. Liquid dpm. Then an even layer of the insulation across the whole floor area.

@tetris not daft at all, and on an uninsulated slab the thin stuff punches above its weight, because the first few mm do most of the work.

Rough numbers for your 60 m² wing, assuming the floor is around U 0.8 now and a cold day puts ~20 K across it:

• as is: ~960 W

• 12 mm Marmox-type board (R ~0.35): U ~0.62, ~740 W

• 20 mm XPS (R ~0.59): U ~0.54, ~650 W

• 100 mm PIR (R ~4.5): U ~0.17, ~200 W

So 12-20 mm gets you roughly 30-40% of what full-depth PIR would, for a fraction of the height. The bigger thing you'll feel is contact temperature: LVT straight on concrete feels cold because the slab pulls heat from your feet, and any insulating layer under it changes that more than the U-value suggests.

+1 to JohnMo on sealing first. If it's beam and block, the perimeter gap behind the skirting is usually where the cold air comes from, so seal or tape that before the boards go down. Cheap, and worth more than the extra few mm.

4 hours ago, gac said:

because the first few mm do most of the work.

Thank you - that's what I was thinking. Diminishing returns each time you double the insulation thickness?

Will be sealing the perimeter with PU foam after installing new skirting grounds.

Found this thread about using XPS with osb over it. The chap used non-T&G 9mm OSB and screwed everything down into the concrete.

Then came back 6 years later to report that everything worked well.

https://www.doityourself.com/forum/solid-hardwood-engineered-laminate-flooring/320138-laminate-over-rigid-foam-insulation.html

I'm now thinking screed -> 20mm or 25mm of XPS 300, then screw down an OSB deck.

Works out about 4 times cheaper than buying marmox.

Can't find much T&G OSB under 18mm thick unless it's wholesale, so would like to hear your thoughts about using 9mm ply or OSB screwed down, like it was done in the linked thread. If I make sure laminate joins don't align with the OSB joints, does that sound reasonable?

Edited 4 hours ago4 hr by tetris

28 minutes ago, tetris said:

Thank you - that's what I was thinking. Diminishing returns each time you double the insulation thickness?

Will be sealing the perimeter with PU foam after installing new skirting grounds.

Found this thread about using XPS with osb over it. The chap used non-T&G 9mm OSB and screwed everything down into the concrete.

Then came back 6 years later to report that everything worked well.

https://www.doityourself.com/forum/solid-hardwood-engineered-laminate-flooring/320138-laminate-over-rigid-foam-insulation.html

I'm now thinking screed -> 20mm or 25mm of XPS 300, then screw down an OSB deck.

Works out about 4 times cheaper than buying marmox.

Can't find much T&G OSB under 18mm thick unless it's wholesale, so would like to hear your thoughts about using 9mm ply or OSB screwed down, like it was done in the linked thread. If I make sure laminate joins don't align with the OSB joints, does that sound reasonable?

@tetris yes, that's it. Heat flow through the floor goes roughly with 1/R, so each doubling of thickness saves about half of what the previous step did. Bare slab to 20-25 mm XPS is the big win, 25 to 50 mm is still worth it if you have the height, beyond that it gets thin.

XPS 300 with a screwed-down deck is a well-trodden route and that thread matches what I'd expect. 9 mm is fine under laminate as long as it's fixed well enough that the sheets can't rock on the foam, so fixings every 300 mm or so and closer along the edges. At that thickness I'd lean towards ply, it's a bit stiffer and less fussy about edge swelling than OSB.

A few things I'd watch: stagger the sheet joints off the XPS joints as well as the laminate joints, leave a few mm gap between sheets and at the walls, and don't overtighten the screws or you crush the XPS and get soft spots. Get the screed flat first too, any hump telegraphs straight through a thin deck. If there's any chance of damp coming up through the slab, a DPM under the XPS is cheap insurance.

You're looking at 30-35 mm plus the laminate, so check door thresholds and skirting before you commit.

@tetris quick follow-up: I plugged your rough stack into a custom wall/floor builder (25 mm XPS 0.034 + 9 mm ply/OSB + ~8 mm finish over screed/slab) and got about U 0.87. That's the diminishing-returns picture in numbers: the XPS is doing almost all the work. Happy to tweak thicknesses if you want a 50 mm compare.

In rough terms, pir is twice as effective for twice the price. But with limited space I suggest using pir.

Thank you - floor-grade PIR is up to 150kpa, while XPS is 300kpa, so twice the hardness. Hard to picture how this translates into localized stress points, but XPS sounds like less likely to fail.

1 hour ago, gac said:

so check door thresholds and skirting before you commit.

I already committed when I bought the house 😂

I've got about -15mm from lifting the carpets+underlay and a couple cm of wiggle room that doesn't require re-building the door frames, but space is tight with current door frame clearance being 196cm.

Edited 2 hours ago2 hr by tetris

1 hour ago, gac said:

so fixings every 300 mm or so and closer along the edges.

I'd better charge the drill batteries. 🫣

How deep into concrete would you go?

1 hour ago, tetris said:

XPS sounds like less likely to fail.

Either is fine. Xps is softer unless you get special stuff.

If you walk on either with flat feet, there is no compression. With your high heels, both would compress.

Add boards and it's spread even further.

Unless you plan on dropping a grand piano you will be OK.

I am of the same persuasion in reusing something you already have.

It's up to you, and weighing up the cost/benefit over many years.

2 hours ago, tetris said:

I'd better charge the drill batteries. 🫣

How deep into concrete would you go?

For XPS into concrete I'd aim for about 40-50 mm embedment into the slab after the XPS thickness, so for 25 mm XPS that's roughly 65-75 mm overall screw length (plus whatever the board takes). Use plugs rated for the foam + board stack, and don't overtighten. If the slab is crumbly or you're near an edge, go a bit deeper or use a chemical/sleeve anchor. Spacing every 300 mm with closer centres at edges is still the key bit.