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Discuss Is a polythene membrane necessary between biscuit mix and floor boards? in the Underfloor Heating Forum area at UKPlumbersForums.co.uk.


Would you use a a poly membrane?

  1. Yes

    0 vote(s)
  2. No

    0 vote(s)
  1. Lynsey Clark

    Lynsey Clark New Member

    Hi! I am looking for some feedback regarding the use of a polythene membrane between my UFH in drymix and the CaberDek floor above. We have no such thing installed, is it essential? I have only seen it suggested once as a recommendation on a website. The floor is: 200mm celotex, osb, pipes stapled to osb and buried in a sand/cement biscuit mix then topped with chip board floor boards. Any insight here would help. The supplier and fitter of our UFH both say that based o. The build up of our floor they have never used polythene membrane as suggested. Only in larger commercial builds where a concrete slab has been laid would they consider it. Your help is much appreciated! Thanks
  2. snowhead

    snowhead Plumber

    Polythene membrane is to stop rising damp.
    That should have been on sand under the Celotex, not that it would get past the Celotex.
  3. Lynsey Clark

    Lynsey Clark New Member

    Thanks for the reply. I’ve checked my pictures from the build and there is a dpm below the concrete solum, then an air gap then the celotex between joists. Seems right to me. Thanks again!
  4. townfanjon

    townfanjon Trusted Plumber GSR Top Contributor!!

    I have seen a few with a membrane but also a few without , seems to be a “matter of opinion “
  5. chris watkins

    chris watkins Plumber

    It is installed as a vapour barrier to stop condensation at the point were warm & cold meet.
    • Agree Agree x 1
  6. chris watkins

    chris watkins Plumber

    Sorry, above isn't strictly correct, it is a vapour barrier but normally placed on top of the insulation just in the same way insulation on chilled water pipework is covered with a sealed foil outer finish which stops the passage of warm moist air passing through to condense on the cold pipe surface below.