Vapour barrier below *and* above insulation for a concrete floor slab?

An architect has drawn up building regs drawings for a proposed extension a nd the floor detail shows a vapour barrier both below and above the insulat ion under the concrete slab. Is this normal/advised? I have only ever seen the vapour barrier mentioned as going either above or below the insulation, but never any mention of doing both (or explicitly not doing so for that m atter).
I will of course ask the architect but wanted to also get a second opinion in advance from the panel.
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Does the area have any issues with damp? Brian
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Hi Brian,
There are no issues of damp that I am aware of (and certainly I wouldn't expect the architect in this case to be either).
On Sunday, 22 July 2018 08:35:44 UTC+1, Brian Gaff wrote:

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On Sat, 21 Jul 2018 14:26:38 -0700 (PDT), Mathew Newton

We had a conservatory added to our property last year, and I'm pretty sure two 'damp courses' went down on the floor, with about 4" of concrete between, rather to my surprise.
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On 22/07/2018 09:28, Jim K wrote:

Is the correct answer. Though only necessary if 'open cell' insulation like expanded poly is used. Closed cell insulation like celotex could have a single DPM above or below it.
Infiltration of expanded poly with water and cement fines would render it useless as an insulation, as does dropping expanded poly panels loose into a cavity wall so that air can whistle past both sides (and around the edges). I've seen lots of new build houses on big-builder estate, built like this.
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On Sun, 22 Jul 2018 10:19:47 +0100, Andrew

Now it's been mentioned, I'm sure it was foam insulation between the two 'damp courses', not concrete as I said earlier.
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On Sunday, 22 July 2018 10:19:49 UTC+1, Andrew wrote:

Closed cell insulation has been specified in this case.
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Vapour barrier is required on the warm side of insulation to prevent condensation forming inside the insulation. However, in a floor, you also need a damp proof membrane to prevent moisture soaking up from the ground. So it doesn't sound like having one both sides is unreasonable.
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On Sunday, 22 July 2018 12:43:16 UTC+1, Andrew Gabriel wrote:

Yes, I did think it might have some advantages here. But then I also fear that if any moisture did find its way in then it'd likely struggle to get out again. I suppose that's not necessarily an issue.
I'll see what the architect says, and I might run it by the chap from building control too. They've already been through the drawings with a fine toothcomb but not mentioned anything about this.
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On Saturday, 21 July 2018 22:26:40 UTC+1, Mathew Newton wrote:

and the floor detail shows a vapour barrier both below and above the insul ation under the concrete slab. Is this normal/advised? I have only ever see n the vapour barrier mentioned as going either above or below the insulatio n, but never any mention of doing both (or explicitly not doing so for that matter).

n in advance from the panel.
The Damp Proof Membrane is below the insulation. The insulation is closed cell structure and has a foil covering and hence n eeds no vapour barrier,. The barrier above the insulation is to keep the wet concrete from corroding the aluminium foil on the insulation and is usually quite thin.
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On 23/07/2018 08:22, harry wrote:

Nonsense. The foil is only there to reflect long wave radiation where it it used in walls or roof constructions.
Underground, the only thing that counts is the fact that it is closed-cell, so doesn't need a vapour barrier on top.
Closed-cell insulation can go below the main DPC quite happily bwcause water should not infiltrate it (in theory).
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On Monday, 23 July 2018 21:58:48 UTC+1, Andrew wrote:

Is that so shit-fer-brains?
http://www.ecotherm.co.uk/our_products/floor_insulation/eco-versal.aspx
https://www.theconcreteproducer.com/how-to/concrete-production/how-does-contact-with-aluminum-affect-concrete_o
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