Put a Damp Proof Membrane between joists and flooring or not?

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Hi,
I've searched the forums but cannot find an answer to this specific question.
I have a 1960s bungalow that I'm refurbishing. Ground floor was:
  • Ash solum on earth, sealed (badly) with bitumen.
  • Suspended floor joists with tongue and groove timber flooring (void below floor is ventilated).

I have ripped up all floors and removed the old solum down to bare earth. The following is the current floor situation:
  • Type 1 hardcore / 20mm aggregate to build up / level solum base
  • Building sand blinding
  • 1200 gauge damp proof membrane
  • New concrete solum
Once first fix plumbing and electrics are complete I intend to insulate between joists with 100mm Kingspan or equivalent flush with joist top surfaces and fit 18mm tongue and groove chipboard flooring (joists at nominal 18" centres).

Should I fit another damp proof membrane (or more suitable alternative) on top of the joists under the t&g chipboard to prevent condensation forming on them? I'm getting conflicting advice.

Many thanks in advance.
 
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Doggit

As long as your insulation is as well fitted as possible, then there shouldn't really be any moisture coming up to cause any condensation. Use expanding foam to fill any gaps, and use aluminium foil tape over any joints in the Kingspan, and you should be fine.

But I'm a bit confused on why you're doing all this. Are you in a radon area, hence the need to put in a concrete sub base below the suspended floor.
 
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Hi,
Thanks for your prompt reply.

The reason for doing this was initially to just insulate below the floor (no crawl space access). However, on further inspection it became apparent that considerable water was collecting on the original solums. On checking all levels we discovered that most of the solums were well below ground level, hence the need to build them up (with something better).
 
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Doggit

Just how much water is collecting. Sounds as though either your water table is pretty close to the surface, or you've got an underground stream nearby. You might be able to get away with the hardcore, blinding sand and then just seal the dpm to the foundations, but concreting on top will be belt and braces. I'd say you were on the right course.
 

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