Is a polythene membrane necessary between biscuit mix and floor boards?

[Lynsey Clark]

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

 

[snowhead]

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

 

[Lynsey Clark]

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!

 

[townfanjon]

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

 

[chris watkins]

It is installed as a vapour barrier to stop condensation at the point were warm & cold meet.

 

[chris watkins]

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.