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underfloor heating screed

We had our first Artisan visit yesterday. Not bad for a project that has been running for 3 years...
any way... questions raised
so far we have sand, plastic, insulation, concrete. next we plan underfloor heating tube attached to reinforcing stuff then the screed or 'Fluide de chappe'. tiles, bobs your uncle....
because of the fluid nature of the 'chappe' we need a pumpist to get it round the tube.
the man who came says we need to put plastic between the concrete and the chappe, because of chemical reactions.
has anyone heard of this?

Not heard of that; only to be careful with expansion ("dilitation" with a French accent) & "cold bridges"

so far I've only ever seen the liquid floors poured direct onto concrete...usually any "special" stuff like insulation etc goes under the conrete

The method I have seen is compacted base, damp course, 140mm insulation below the slab, 50mm perimeter insulation then one pour of concrete on top of the insulation, (heating pipes tied to the reinforcing) But if your contractor thinks you need polythene you should use polythene, it won't do any harm.

Don't see how you would get a chemical reaction between screed and concrete when they are essentially the same thing, just different sized aggregate.
However you should use plastic membrane over foil backed insulation to prevent chemical reaction with the wet screed/concrete. Maybe that is what he meant or what has confused him.

I specify this stuff regularly, and if you are doing it from scratch it is amazingly cheap.
I have a building at the minute, approx. 200 square metres. One floor, 5 zones, all to have their own thermostat, all wireless.
2500 quid was the most expensive bid. The boys have it ripped back to a concrete subfloor, apply 100mm of solid foam insulation, then the underfloor heating guys come and staple the hose down to it.
After that, another subbie squirts in a liquid screed - two days later, we can put the heat on.
That price includes the manifolds et all.
2500 and the cheapest was just over a grand.

We had ours done last year and yes, the screed people insisted on plastic over the insulation. Frankly at ~50 for the DPM and a couple of hours work, it wasn't worth arguing about.

Thank you all for your replies.
We know its not worth the hassle for a few quids plastic, it just struck us a strange. but then again maybe its because we are heating the whole mass and most people have insulation closer to the top to provide the barrier.
we await the quote!

Re: underfloor heating screed

the man who came says we need to put plastic between the concrete and the chappe, because of chemical reactions.
has anyone heard of this?

Yes if it is an anhydrite screed (calcium sulphate) which can react with concrete.

The usual reason for putting plastic on top of insulation before pumping a liquid screed is to stop the insulation floating on the screed!

As in the photo of ours below


Thank you.
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