What's the deal with vapour barrier membranes?

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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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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.
 
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?
 
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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?
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.
 
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.

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?
 
:cautious:

Btw, are you confusing vapour check membranes with breathable membranes?
I'm new to this. Which one should be fitted when insulating between the joists of a suspended timber floor?

Condensation and excess moisture in the timber
So the membrane (vapour or breathable?) reduces the likelihood of this occurring?

Clarification of these points would be appreciated.

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

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