What's the deal with vapour barrier membranes?

Discussion in 'Building' started by curium, 16 Nov 2021.

  1. curium

    curium

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    Some people swear by them. Others just use polythene sheeting.

    They seem to vary massively in price.

    I'm insulating my suspended timber floor and various websites recommend I fit one between the insulation and the flooring.

    Interested in peoples' experience. Is it just about complying with building control?
     
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  3. catlad

    catlad

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    I wouldn't fit one on a suspended timber floor.
     
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  4. ^woody^

    ^woody^

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    A vapour check membrane is polythene. What else is there?

    You would not fit one on a timber floor. What websites have you been reading? Probably best to stay away from them.
     
  5. Mr Chibs

    Mr Chibs

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    Out of curiosity, why would fitting a vapour membrane underneath the final flooring be a bad idea?

    If the op has put insulation in between the joists and has good ventilation underneath, what’s the issue?
     
  6. curium

    curium

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    This is what I'm trying to clarify.

    The prices seem to vary and I'm wondering if there a specification that is causing the different prices? Then there are the people who say they don't fit one.

    All very confusing for a DIYer.
     
  7. curium

    curium

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    When I said polythene I meant basically a plastic sheet like a DPM rather than something which purports to allow vapour to pass in one direction but not allow liquid to pass.

    I take it you're of the opinion that one should be fitted when insulating a suspended timber floor?

    Is there a specific one you've used before?
     
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  9. ^woody^

    ^woody^

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    :cautious:

    Btw, are you confusing vapour check membranes with breathable membranes?
     
  10. ^woody^

    ^woody^

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    Condensation and excess moisture in the timber
     
  11. curium

    curium

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    I'm new to this. Which one should be fitted when insulating between the joists of a suspended timber floor?

    So the membrane (vapour or breathable?) reduces the likelihood of this occurring?

    Clarification of these points would be appreciated.

    Thanks
     
  12. ^woody^

    ^woody^

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    None. There is no barrier fitted to suspended timber floors

    The floor void should be cross-ventilated to prevent any humidity issues, and that is it
     
  13. jeds

    jeds

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    Breathable membrane and a vapour check are completely different things. A vapour check is normally a poly sheet placed on the WARM side of the insulation. Cheap poly - usually 1000g - nothing special. The poly prevents vapour from permeating to the cold side where it might condense to water. Where there is good ventilation on the cold side (like a normally ventilated floor void) a VC is not normally required.

    A breathable membrane allows vapour to pass through (so would be useless as a vapour check) but not free water. These are normally placed on the cold side of the insulation such as in a roof or within a timber framed wall. These are often specified with a VC as well, but not always. It depends on the configuration of the wall/roof? Whether a VC is required is determined by a 'condensation risk analysis', which is usually done as part of a u-value calculation.
     
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