25mm Celotex v 50mm Expanded Polystyrene

[FresnoBob]

Hi people, sorry I'm having a nightmare finding facts and figures today, hoping someone can point me in the right direction.

Looking to replace 50mm crumbly expanded polystyrene/chipboard floating floor, with 25mm Celotex and a Screed. How does 25mm Celotex compare to the 50mm polystyrene (in terms of performance/insulating values)? I'm getting my facts straight before I buy a load of the 25mm stuff and Building Control tell me they want 50mm lol.

 

[ajrobb]

25mm of PIR (Celotex) is not going to be as warm as 50mm EPS (expanded polystyrene). As such you might be breaking the law (Building Regulations Part L) and could be forced to do it again if Local Authority Building Control (LABC) find out. Because you'll be replacing layers in a thermal element (the floor), I think you are supposed to notify LABC. LABC can instruct you to use 50mm PIR under board as that is feasible.

25mm PIR has an R-value of 0.025/0.024 or 1.04

50mm EPS has an R-value of 0.050/0.040 or 1.25

I'm no builder, but I doubt that 43mm of screed on PIR will be strong enough. I wish I had a practical suggestion for you.

 

[FresnoBob]

Darn lol.

I think I can just about stretch to 50mm of insulation and 50-65mm of screed, but it's going to be mighty mighty close.

How does 50mm Celotex compare to 50mm expanded polystyrene? Also is Jablite any better than standard expanded polystyrene? I was wandering through the local B&Q the other day, and had a look at some, and it just looks generally 'firmer'....not sure if it's any better lol.

 

[mointainwalker]

50 mm EPS = R 1.25
50 mm Celotex = R 2.00

so Celotex is 60 % more effective than EPS when new, but will lose its extra insulating properties over the following 7-10 years and will end up same as EPS.

It is not likely that Jablite will be any better than unbranded but check for labelling/spec although they will often quote a lamda value which is a spec for the material ( insulation value of 1 000 mm of the stuff ) rather than the product you are looking at ( such a 50 mm sheet ..)

 

[FresnoBob]

hmmmm, is Expanded Polystyrene okay for laying a screed over the top of? Looking at the stuff, it just doesn't look strong enough lol, I've always assumed the Celotex/Kingspan stuff is more 'solid' (probably wrongly assumed lol).

Is extruded polystyrene any better?

 

[Matstones]

What did you do in the end, I have similar situation.

What is the drawback of not have any insulation under the floor, instead just 120mm of screed. I realise the floor will be cold, but heat rises right, so it won't drain the heat from the room and I have a pair of slippers !

 

[mointainwalker]

and I have a pair of slippers !

A round of applause for the thoroughly prepared "Matstones" who obviously knows about the important things in life.

 

[Matstones]

I've done a bit more research and pulled up the old chipboard flooring and found several layers of polystyrene underneath. I understand from reading these posts I can lay sand/cement screed directly on the polystyrene, as long as there is still 65mm for screed.

This sounds ideal, saves a lot of effort mixing and buying screed, and avoids having to dump half the polystyrene. There is DPM under the polystyrene

I am surprised polystyrene is strong enough to do the job. Looking at the Jablite website they confirm it is.

Has anyone done this ? Do I really put the screed directly on the polystyrene ? ?, the website just says use tape to block the gaps.

It does seem counter-intuitive...

 

[FresnoBob]

Lol, I thought that too, but apparently so, insulation then screed on top.

I haven't done anything yet as I'm busy with other stuff. Looking at it again though, I might be pushing it getting 50mm insulation and a screed down in the depth we have available. I have building control coming out next week however, so I think I'll pick his brains and see what he advises.

 

[vman2000]

50 mm EPS = R 1.25
50 mm Celotex = R 2.00

so Celotex is 60 % more effective than EPS when new, but will lose its extra insulating properties over the following 7-10 years and will end up same as EPS.

(I'm aware that this thread is old but, for the benefit of others reading now)

I don’t believe that is entirely correct. My understanding is that celotex does deteriorate, especially in the first few years but the quoted R-Value is the average value over a 25 year period (as per European guidelines for quoting R-values). In fact deterioration happens exponentially and is pretty much flat lining after 8 years. So the quoted R-value is actually about 7 years into its life span, rather than when it is brand-new (if it was nobody would pay the premium for it and building regs wouldn’t recognise such a short-lived r-value)

 

[tomfe]

the quoted R-Value is the average value over a 25 year period

Correct, Regulation (EU) No 305/2011, you'll also find thicker insulation has a higher lambda value as it does not offgas so quick.