DPM above or below concrete slab

[BMcN]

My floor will be made up of hardcore, 150mm concrete, PIR insulation with underfloor heating clipped and 75mm cement based screed.

 

Just wondering what is the preferred place for the DPM?  Below the main 150mm slab or on top of the slab before the insulation?

 

Thanks

Edited February 12, 2019 by BMcN

[Onoff]

Is this for a new build as in whole house?

[Triassic]

Mine is below the concrete.

 

if your DPM were above the concrete, the uFH would be heating a large damp lump of concrete. 

Edited February 12, 2019 by Triassic

[dpmiller]

both? Surely you'll need a polythene layer above the PIR regardless of where the DPM is.

And what about Radon?

[Miek]

28 minutes ago, Triassic said:

Mine is below the concrete.

 

if your DPM were above the concrete, the uFH would be heating a large damp lump of concrete. 

Depends where the UFH pipes are. If in the screed then no problem with this. 

 

DPM is usually below the concrete, then the insulation, then a polythene sheet to stop the insulation floating as the screed is poured. 

[MarkyP]

typical build up would be hardcore, 25mm sand blinding layer (to protect DPM from being punctured when concrete is poured), DPM/Radon barrier, concrete slab, insulation, polythene separating layer (500 gauge poly), UFH pipe worked clipped, screed.

 

the polythene separating layer over the insulation isn't really as a DPM, it typically serves to protect the insulation from contact with the wet screed. I believe this is because the foil facing of PIR reacts with wet cement. It is also, where a liquid pumped screed is used, to create a bath to hold the screed and stop it running down the sides or between the insulation sheets before it sets. 

 

Some have used a further DPM over the slab where there is concern it hasn't dried fully.

[Onoff]

One alternative to a protective "polythene" layer over the pir (stops screed reacting with the foil face) is Poly Panels.

 

https://www.tradingdepot.co.uk/polypipe-pb08576-ufch-floor-panel-1200-x-1000mm?tfrid=4c2ac8ed-6b49-4d94-9429-dc6a9adf3b36

 

I used them with a 100mm wet concrete slab. They act as a barrier to the foil, save on mix and make laying pipe a doddle. Here's a vid of me doing my bathroom:

 

https://flic.kr/p/Ch8rXd

 

My one reservation is the slab, even though it incorporates A142 mesh maybe feels a bit "hollow"? I'm used to I suppose a solid slab. Saying that, now it's all tiled etc it feels fine. 

 

 

[BMcN]

1 hour ago, MarkyP said:

typical build up would be hardcore, 25mm sand blinding layer (to protect DPM from being punctured when concrete is poured), DPM/Radon barrier, concrete slab, insulation, polythene separating layer (500 gauge poly), UFH pipe worked clipped, screed.

 

the polythene separating layer over the insulation isn't really as a DPM, it typically serves to protect the insulation from contact with the wet screed. I believe this is because the foil facing of PIR reacts with wet cement. It is also, where a liquid pumped screed is used, to create a bath to hold the screed and stop it running down the sides or between the insulation sheets before it sets. 

 

Some have used a further DPM over the slab where there is concern it hasn't dried fully.

Thanks, that clears it up.  

 

Yes it is new build.  Make up is to be hardcore, 150mm reinforced concrete, PIR and screed on top.  So DPM below the slab and a polythene below the screed.

 

 

[Onoff]

I've done:

 

-Hard core 

-Sand blind

-25mm EPS 

-DPM

-Polypipe panel 

-16mm pex-al-pex within panel depth 

-100mm wet concrete laid to screed rails

 

 

 

[tanneja]

I'm not sure what I am missing.

 

Intra wall structure DPC (literally plastic sheeting protruding out of the wall) seems to be close to finished floor height (is 200mm above concrete sub base atm).

 

I was always planning porcelain floor so the plan from builder was to insulate, UFH, 50mm screed and tiles.  Now I am considering a polished concrete floor, where the firms are saying they would want to lay 100mm concrete which brings my FFL up from where builder thought it would be.  He is talking about having to lip up the DPC internally to the house to prevent the 100mm thick polished concrete floor being a bridge above and below the wall DPC.  It doesn't sync at all with what this forum is saying.

 

Is my set up normal and I am just misunderstanding?  FYI this is an extension and refurb.