Floor screeding question

[pace]

Hi all, I am in the process of building an extension to my home. The next job to get organised is the floor screed.
My question is about insulation. I have about 200mm to make up to the existing house f/f/l so was planning to lay 125mm of insulation and then 75mm of screed. The price of insulation seems to vary wildly so I was wondering what my options were. Jablite, Celotex or something alse? I have had different opinions from people about the suitability of Jablite which is considerably cheaper than other options. The floor area is about 24 square meters and a dpm has been installed under the concrete sub floor and is lapped up to dpc level so I guess i dont need a 2nd dpm? although some have said I do.
Any advice would be gratefully received.

 

[noseall]

At 125mm thick, jablite (polystyrene) will be adequate.

A secondary layer of polythene is added to prevent any screed material falling between any gaps and forming a potential cold bridge. You can overcome this with say foil tape or a thinner layer of polythene placed over the top of the inso' and lapping up enough to prevent 'droppage'.

 

[pace]

Thanks for that noseall, people have said not to use Jablite as it has a poor insulation value and can compress as its not very strong. At 125mm I cant see the U value being an issue and if its fitted snugly it shouldn't be able to compress or collapse.

So I will go with that as the best option. I'm a little worried about laying polythene over it though, PVC cables react with Jablite so polythene may have the same problem? If I cut the sheets tight I shouldn't have any gaps.

 

[jeds]

Jablite is half the insulation value of PIR foam (Celotex, Kingspan etc.) So 125mm of Jablite is the same as 62.5mm of Celotex. You also need to check the screed requirements for the insulation. They have different densities and compression and some specify maximum thicknesses below screed.

 

[Norcon]

I definitely wouldn't use polystyrene.
It sinks over time and you'll be left with a gap under the skirting.

 

[freddiemercurystwin]

The right grade of Jablite will be fine. I've specified it in commercial projects. Its not sunk.

 

[noseall]

I definitely wouldn't use polystyrene.
It sinks over time and you'll be left with a gap under the skirting.

And what do you suppose causes the gap under celotex insulated floors?
I'm guessing maybe shrinkage across the the grain of the skirting is to blame.