Concrete slab in direct contact with PIR

[garyo]

I've just had my BCO around for an inspection prior to my slab pour. All very friendly. I've foil taped the seams on my PIR so that I don't get any concrete seeping and cold-bridging. Previously I've laid a second layer of DPM to achieve the same.

He said you shouldn't really let concrete come in direct contact with the foil, but he was fine with it, inspection passed.

I don't like to unnecessarily elongate conversations with my BCO's, so I left it at that. Has anyone heard of this before? It he misinterpreting guidance based on people pouring a slab directly on top of PIR without taping the seams, and therefore creating cold bridging?

Thanks

 

[wessex101]

One of my Council's with a very thorough building control department insists on it. I did look into it at the time and it is a genuine technical requirement from the manufacturers. I can't remember the details off the top of my head but it is something to do with the concrete reacting with the aluminium foil on the insulation.

 

[^woody^]

something to do with the concrete reacting with the aluminium foil on the insulation.

That sounds like waffle to me. If there is a reaction to the cement, then why is it allowed to be up against concrete and masonry walls?

 

[^woody^]

He said you shouldn't really let concrete come in direct contact with the foil, but he was fine with it, inspection passed.

He's talking crap, else why has he permitted it?

Somethimes a polythene barrier is useful to stop water from the concrete seeping under the insualtion and lifting it, causing a hollow when it dries and then potential for settlement of the slab.

Some manufactures refer to this as a VCL, but I can't see how vapour conctrol could be relevant in this situation.

 

[tomfe]

Aluminum and cement react to produce hydrogen, this is now thermalite blocks are made.
It reacts to a lesser extent with dry concrete. But can react badly if you have any steel work in there.

 

[garyo]

Somethimes a polythene barrier is useful to stop water from the concrete seeping under the insualtion and lifting it, causing a hollow when it dries and then potential for settlement of the slab.

That was my logic with foil taping the seams. With hindsight, I think I'll go back to my previous method of using polythene or cheap vapour barrier - much quicker/easier.

 

[freddiemercurystwin]

AFAIK the purpose of the barrier between the insulation and concrete is to prevent the concrete seeping into any gaps between the boards.

 

[chappers]

how are you now going to make the slab's water resistance contiguous with the DPC of the masonry

 

[^woody^]

how are you now going to make the slab's water resistance contiguous with the DPC of the masonry

 

[tomfe]

AFAIK the purpose of the barrier between the insulation and concrete is to prevent the concrete seeping into any gaps between the boards.

But it's not, you don't use a sheet when you do eps and eps floats a lot easier then pir. The sheet is to stop the aluminum reacting with the concrete. At least this is why it was originally specified when foil faced insulation was first used in floors.

 

[^woody^]

But it's not, you don't use a sheet when you do eps and eps floats a lot easier then pir

You should for any such board insulation if the risk is there.

 

[^woody^]

The sheet is to stop the aluminum reacting with the concrete.

And vertical concrete framed buildings? You don't see any such barriers specified there

 

[John D v2.0]

Well for those who didn't look either, celotex say they have many reasons for recommending a poly sheet
"
Why is there a polythene membrane between the Celotex and screed?
A polythene membrane on top of insulation below a screed or concrete oversight performs three tasks: it acts as a vapour control layer (VCL); it prevents migration of the screed / concrete which can force the boards apart leaving cold spots in the floor; and it prevents a reaction between the wet screed / concrete and the foil facer where small volumes of gases given off can weaken the screed/concrete. Celotex FI5000 features an innovative composite facer that when taped and sealed performs these functions without the requirement of a separate polythene VCL.
"
This polythene will change your life!