# Celotex insulation under concrete?



## StevieB (6 Dec 2010)

I have some 35mm celotex I have been using to insulate stud walls and have a number of sheets left over. Its the pink solid foam type with a gauze backing to each side, not foil. Since I need to lay a slab floor as well anyone know if this stuff will survive concrete being poured over it ie will it compress or otherwise fail and leave me with a void over time? I know polystyrene will not but am unsure of celotex. It crushes if you stand on a small portion but will allow me to walk on it if you stand in the middle of a sheet without crushing. I got it from a seconds suppliers, who cannot answer my question so wondered if any of you knowledgeable chaps had tried with this before?

Steve


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## dedee (6 Dec 2010)

It just so happens that I have a copy of the Celotex Handy Guide but having read it 3 times now I am still not sure of the correct installation guidelines.

You can download the guide from here

http://www.celotex.co.uk/Literature/Fur ... ndy-Guides

Hope it helps


Andy


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## The Bear (6 Dec 2010)

Standard foil backed celotex is suitable for having a concrete slab poured over it. I have it under my workshop slab and my kitchen extension slab.

I am not sure what you are describing but if the core is standard celotex I would be happy to use it

Mark


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## StevieB (6 Dec 2010)

Thanks for the responses chaps, appreciated. After alot of searching on t'internet it would appear that what I was sold as celotex insulation is actually Kingspan Kooltherm with a tissue face on each side. There is no writing on the boards at all so I am only going from internet images. This is the best one I can find of the stuff I appear to have, taken from the seconds and co website:







Given it appears this is being laid on a DPM I am going to assume a slab is going over the top of this and give it a go unless anyone knows this is definitely a no-no. I am not using it for insulation so much as to get rid of it and decrease the amount of concrete I need to pour. My cellar is on a sloping floor so I have 2" one side and 6" the other. If I can use the insulation to reduce this to roughly 2" all the way across this saves considerable effort in preparing a slab floor in a cellar! Will put a DPM underneath and see what happens. Watch this space for a floor failure in about a years time.... 

Steve


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## Lons (6 Dec 2010)

Hey Steve

I use polystyrene usually due to cost as min 100mm is required to meet regs on domestic works but would have no hesitation using cellotex or kingspan.

I always use reinforcing mesh in the concrete however as it just isn't worth the risk not to IMO.
In your case, a 50mm floor seems very thin. Will it be a workshop and are you going to put machinery on to that?

cheers

Bob


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## StevieB (7 Dec 2010)

Hi Bob,

Its a cellar with an existing slab floor of unknown thickness. Its dry but I wouldn't bet its got a DPM underneath. Its been fine for the last 4 years but it slopes and doesn't meet up with the floor in the other cellar room and having converted that to a proper room (with 4" slab floor) I am keen to do the front cellar to match. The slope is such that I have about a 6" differential from one side to the other over a run of about 14 feet. I have put stud walls in and insulated with kingspan between the joists and OSB as a top cover. This sits on a concrete plinth around the walls I have laid to get the walls level and I need to now fill this in across the middle of the floor. Since I have several sheets of Kingspan left I was going to put these down rather than waste them and put the slab floor over the top. The slab will be variable thickness from approx 2" one side to approx 4" the other if I put 2" of Kingspan across the entire floor. Not really doing it to save heating costs or for floor insulation so buying 100mm polystyrene is not necessary. No majorly heavy machinery as its a cellar and I have to get everything down a flight of steps. Heaviest thing will be my PT I would think, which I can just about lift so probably 75kg max (much lighter than me!). Will also have a P5 chipboard floor over the slab which will spread the load a bit. My only concern was whether the Kinspan would compress with time and leave a void, in which case 2" slab would be a problem and liable to cracking. Having stood on the Kingspan it will not compress by much if at all, and its water resistant from any damp from my testing a bit in the sink for structural integrity! (I will put down a dpm underneath so not a major worry anyway). I havent found any indication from my searches that its a no-no for this application so fingers crossed it should be fine.

Cheers,

Steve


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## darrenba (7 Dec 2010)

You also need to put a vapour barrier / polythene on top of the kingspan before laying the concrete.


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## StevieB (7 Dec 2010)

Why would that be Darren - I never have done when using polystyrene under a slab - the DPM goes under the insulation as shown in the photo above?

Steve


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## Lons (7 Dec 2010)

StevieB":ui5mlo2m said:


> Why would that be Darren - I never have done when using polystyrene under a slab - the DPM goes under the insulation as shown in the photo above?
> 
> Steve



No you're correct, you don't need a vapour barrier Steve. Normal procedure is dpm, lapped under extended dpc at walls, unsulation then reinforced concrete. Insulation joints are sometimes taped but usually not

I think you'ld get away with your method especially if the existing floor is solid and no cracks but personally I wouldn't be comfortable with a concrete floor as thin in places as 2" and certainly not if it isn't reinforced :shock: 
Might be ok but if it cracks badly, could cost a lot more to put it right ! I think I'd be doing it properly and finding another use for the celotex - - do it once only is a good motto.

cheers

Bob


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## darrenba (8 Dec 2010)

According to Celotex you do need a vapour barrier over the top of the insulation as well. You didn't used to but they now recommend you do. 



> *Installation Guidelines*
> Under slab installation guidelines
> • Level hardcore and blind with sand
> • Install damp proof membrane and lap into damp proof course
> ...


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## StevieB (8 Dec 2010)

Thanks. It would seem to be an areas of discussion as to whether the VCL is needed at all, and if so what form it should take. I see you have responded to the same query here Darren

http://www.ultimatehandyman.co.uk/forum1/vapour-control-layer-to-new-concrete-floor-t29718.html

and as you state the Celotex website suggests a VCL. However, the Knauf site for their products:

http://www.knaufinsulation.co.uk/se...ground_floors/gf03-insulation_above_slab.aspx

Doesn't suggest one when insulating over an existing slab in their typical construction blurb, and only suggests one in their installation guide if there is a risk of moisture from the slab below the insulation emitting water from the drying process ie if you are going to screed before the slab has fully gone off. Given the slab I have down is dry and has been down at least 4 years (when we moved in, so probably considerably longer) I am inclined to put down a DPM below the insulation and leave it at that. 

Thanks also for the advice on thickness, a structural engineer friend of mine suggested 2" was also the very minimum thickness and 3" would be better, 4" standard. Coupled with the advice on here I will make sure I have a minimum of 3" rather than the 2" I was planning to prevent cracking.

Steve


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## Lons (9 Dec 2010)

darrenba":ed9bizjg said:


> According to Celotex you do need a vapour barrier over the top of the insulation as well. You didn't used to but they now recommend you do.
> 
> 
> 
> ...



:-k Thanks for the info - didn't know they'd changed the recommendations. I think it's a case of cover their ass in the event of any possibility however remote.
Never heard of a problem and certainly it wouldn't concern the BI even though manufacurer recommendation usually overides general regs.
cheers

bob


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## Nick W (10 Dec 2010)

The problem can be that celotex floats in concrete, particularly if the mix is a bit wet.


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## Lons (10 Dec 2010)

Nick W":16zkqp8p said:


> The problem can be that celotex floats in concrete, particularly if the mix is a bit wet.



Not if you do it the way I do Nick.

I spread around 30mm of mix on top of the insulation then lay and tie together the weldmesh before completing and tamping the total 100mm thk concrete floor.

Can't go anywhere - puts the reinforcement where it should be without weakening the concrete which is the case if you insert spacer blocks - and has never failed.

cheers

Bob


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