Insulated slab, do I need DPM under internal insulation before screeding?

[Renegade105]

Hi dude,

 

Not sure if its too late to ask but why add a screed over the top of the slab? You could tie the UFH pipes into the Slab directly, just ask Tanners and they will sort it? Or it is a very large footprint/area, in which case lie would be a lot easier doing it your way to get a level floor.

 

 

 

 

[Thorfun]

7 hours ago, Renegade105 said:

Hi dude,

 

Not sure if its too late to ask but why add a screed over the top of the slab? You could tie the UFH pipes into the Slab directly, just ask Tanners and they will sort it? Or it is a very large footprint/area, in which case lie would be a lot easier doing it your way to get a level floor.

 

 

 

 

Not possible as the 2 slabs join the main building with a basement where there is block and beam and which was a massive void when the slabs were poured. 

 

[SimonC]

Not resurrecting the previous thread, but I've a question about your experience with insulation over the ground heave product.  I'm in a similar situation with a concrete slab and mini piles and I want to install insulation under the slab and on top of the ground heave sheets (in my case the SE has specified 160mm Clayboard by Dufaylite) but the SE is not happy.  He is concerned about supporting the insulation once the heave protection is gone and all that's left is the designed void under the slab.  Did you install any mechanical fixings to tie the 300mm insulation to the slab?  If not, how did you supported the insulation to stop it from collapsing?

Thanks, Simon

[Thorfun]

13 minutes ago, SimonC said:

Not resurrecting the previous thread, but I've a question about your experience with insulation over the ground heave product.  I'm in a similar situation with a concrete slab and mini piles and I want to install insulation under the slab and on top of the ground heave sheets (in my case the SE has specified 160mm Clayboard by Dufaylite) but the SE is not happy.  He is concerned about supporting the insulation once the heave protection is gone and all that's left is the designed void under the slab.  Did you install any mechanical fixings to tie the 300mm insulation to the slab?  If not, how did you supported the insulation to stop it from collapsing?

Thanks, Simon

we didn't use any mechanical fixings to tie the insulation to the slab. tbh i don't know what will happen if/when the heave protection is gone but our SE (Tanners) never mentioned it to me and so i'm not worried at all. maybe it's different with piles?

[saveasteading]

5 minutes ago, Thorfun said:

maybe it's different with piles?

I'm puzzled why heave protection is necessary along with piles. aren't the piles holding it down as well as up?

Anyway, its a good point that if the ground doesn't heave but the clayboard collapses  , or if it heaves, squashes the clayboard then shrinks again, the insulation won't be supported.

 

Wont it simply require some wire pushed up through the insulation into the slab space, with a hook bent in it?

[SimonC]

On 05/03/2024 at 15:56, saveasteading said:

I'm puzzled why heave protection is necessary along with piles.

 

From what I understand, once the concrete slab is set and is supported by the piles then the heave former has done its job.  For the Clayboard product you have pipes set to pour water in to erode/weaken the former in the void.

 

On 05/03/2024 at 15:56, saveasteading said:

Wont it simply require some wire pushed up through the insulation into the slab space, with a hook bent in it?

 

A mechanical fixing would be the solution;  Installed through from the underside of the insulation to be bedded into the concrete.  A wire with hook bent is probably a better H&S solution than an array of inverted insulation fixing screws that would create something like a bed of nails.

 

Next step is to check what insulation will be ok under concrete, though since the insulation wouldn't be bearing the load of the concrete - rather suspended by fixings to the underside - I expect most will be ok. 

 

[SimonC]

On 05/03/2024 at 15:43, Thorfun said:

we didn't use any mechanical fixings to tie the insulation to the slab. tbh i don't know what will happen if/when the heave protection is gone but our SE (Tanners) never mentioned it to me and so i'm not worried at all. maybe it's different with piles?

Ok, thanks.

 

The Cellcore looks different product to the Clayboard in that it seems designed to stay in situ until a ground heave event when it can be crushed.  The Clayboard product on the other hand, you kind of destroy as soon as the concrete is set by flooding it with water to breakdown the former material. 

 

Just guessing, but if the ground does heave then you have the DPM holding the insulation in place, and if the boards were tightly packed or any spray foam was used to fill gaps that may assist also.  But if in doubt ask your SE. 

[saveasteading]

1 hour ago, SimonC said:

For the Clayboard product you have pipes set to pour water in to erode/weaken the former in the void.

That seems weird to me. a cardboard honeycomb would surely crush between the slab and the upcoming clay, that being the point of it.

As I have never used such products perhaps I am missing something.

 

If a void is formed then the insulation will need support, (using hangers I suggested, which could be made of coat hangers or fence wire. ) . If there is clay heave, all is supported. But if the clay shrinks again then the insulation could settle with it...so tie it up.

 

[SimonC]

7 minutes ago, saveasteading said:

That seems weird to me. a cardboard honeycomb would surely crush between the slab and the upcoming clay, that being the point of it.

As I have never used such products perhaps I am missing something.

 

If a void is formed then the insulation will need support, (using hangers I suggested, which could be made of coat hangers or fence wire. ) . If there is clay heave, all is supported. But if the clay shrinks again then the insulation could settle with it...so tie it up.

 

 

For Clayboard by Dufaylite, the honeycomb material in the former looks like a clay-cardboard material.  Introducing water weakens / erodes the honeycomb leaving a void between the underlying earth and the bottom of the reinforced concrete slab (or as is being discussed, a void between the insulation mechanically fixed to the underside of the concrete slab).  But I agree with you that the clay-cardboard would likely crush by any ground heave forces so why go to the bother of weakening it with water?  But that is the manufacturer's installation instruction, including installing the pvc pipes (which are then later cut, capped or filled) after the one and only water pour once the concrete slab is cured.  Perhaps the clay-cardboard doesn't crush in the same way that other products like Cellcore, which is a polystyrene based former.