Radon underneath EPS only? No DPM (Insulated raft)

[SuperJohnG]

I follow an instagram account who are currently building a passive certified house in Ireland. 

 

One thing that has intrigued me is that they lay the  Radon barrier on the sand blinding which is above the hardcore, then put the EPS raft on top, put in the steels and then poured the raft (seemed to do ring beam first?)

 

All of the others I have seen have the DPM on top of the EPS then steels and UFH. I know the design was done by tanners. Is it normal to do this? it would seem to me then water can get between the radon barrier and EPS and also to the concrete which would then cause issues?

 

I'm sure it is OK and I'm not questioning the approach, but rather I'm quite intrigued and want to understand more.  

 

 

 

 

 

Edited March 3, 2020 by SuperJohnG
Added pictures

[Jeremy Harris]

There's an Irish building regs requirement to install a radon sump, barrier and vents underneath a slab, I believe.  The Irish guys that built our slab were going to fit one, until I told them that it wasn't a requirement where we live.  It is a requirement in some areas, mainly those over granite, where radon can be a problem.

[Conor]

Another method over here is to put the radon barrier between the first two layers of EPS and lap over the sides of the raft. Radon is assumed in Ireland. I'll be using a tanking membrane that also acts as a gas barrier.

[Mike]

In some parts of the UK it's necessary to have a sump too, which could be behind the decision.

[SuperJohnG]

The radon barrier makes sense, I was more wondering what they are doing for DPM? I assume it will go between the soleplate and slab, but surely water can still pass through and come up to the finished floor?

[Jeremy Harris]

  On 03/03/2020 at 12:16, SuperJohnG said:

The radon barrier makes sense, I was more wondering what they are doing for DPM? I assume it will go between the soleplate and slab, but surely water can still pass through and come up to the finished floor?

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The arrangement that our guys were going to use was to fit the radon sump and vents into the Type 3 base layer and blinding, then lay the radon barrier, then the EPS, with the DPM being where it is, above the first two layers of EPS and below the last 100mm layer.  The DPM is lapped over the perimeter insulation and folded back down.

 

We had to install a radon sump, vents and barrier in our last house in Cornwall, as it had high radon levels.  Pretty much anywhere over granite in the UK may be a radon risk area, whereas where we are, on top of ~40m of gault that's on top of hard sandstone there's no radon risk at all.

[Judy C]

EPS is actually waterproof - so dpm not always needed 

[Jeremy Harris]

  On 03/03/2020 at 16:47, Judy C said:

EPS is actually waterproof - so dpm not always needed 

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The snag is that you need something to seal the EPS joints and form a tray to pour the concrete into anyway, and it's easier to just use a sheet of DPM to do this.  It also saves having to have a debate with building control, plus it also gets around the problem with EPS being vapour permeable.