To blind for ground slab? (moved from Projects)

29 Nov 2011
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United Kingdom
Hi, just wondering if any one could help with a question I have. We are part way through our extension project, underfloor heating pipe work to go down this weekend.

Originally was going to have dpm, 100mm of subfloor concrete slab, 25mm blinding sand, more dpm, 100mm celotex board, then vapour barrier (and to protect foil on the celotex from the special poured self levelling screed) then pipes then 65/75mm poured screed.

My question is if the blinding sand is just to protect the dpm from sharp bit on the concrete slab can I swap it out for 25mm celotex board? (Ive got loads of it... dont ask!)

The builders have done a great job on the concrete slab, seems pretty level with no sharp stuff.

Many thanks
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The sand blinds the hardcore
then the vapour barrier/DPM is laid
then the concrete is poured - or then the insulation is placed with the concrete poured on top.

If your slab is already down then you can place the insulation with the DPM on top of the insulation.
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You wouldn't put sand blinding on a concrete slab. If you have dpm below the concrete you don't need another one above - you can lay insulation directly on the concrete - but you do need a Vapour Control Layer. As a general note; the dpm protects against penetration from below - apart from moisture, and depending on ground conditions, it might also protect against gas, radon, methane etc. But it is not a vapour control layer. A VCL cannot function on the cold side of the insulation, it only works on the warm side. It is possible to combine dpm and VCL in the same barrier but not in an underfloor heating situation. In your situation you need a dpm somewhere below the insulation and a VCL/slip membrane between the insulation and the heating pipes.

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