Vapour barriers

20 Nov 2010
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United Kingdom
I'm trying to understand what a vapour barrier is for in wall insulting and roof insulating and ventilating.
Is it to stop condensation in the room reaching the wall? Protect the insulation? The plaster?

What is the typical layout of a solid wall insulation upgrade to a room. Is it?
Wall-gap-insulation-vapour barrier- gap- plasterboard-skim - (tiles if bathroom)
What is the vapour barrier doing here? Have I got it in the right place?

In a roof does the VB serve the same purpose?

Is a VB the same as a VCL or a breather membrane?

Thanks all.
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The inside of a house is typically warm, with the capacity to contain considerably more atmospheric moisture than the colder outside air.

Where insulation is present, there is a temperature difference from the outside to the inside.

If there are any gaps or imperfections in the insulation, warm, moist internal air can meet cold external facing surfaces and interstitial condensation can occur.

The vapour barrier is there to prevent moist air from meeting the insulated layer, so the insulation remains dry, and condensation does not occur. It is not "to stop condensation in the room reaching the wall", it is to stop condensation forming within the wall.
It is not specifically to protect the insulation nor the plaster. You have the order right. In a bathroom, wallboards should be moisture resistant if tiles are to be laid on them. Normal plasterboard on the bathroom ceiling, below the VB.

In the ceilings below the roof space, modern practice is to include a vapour barrier above the plasterboard of the ceilings, below the loft insulation. Obviously, piercings for lighting, cables, pipes and ventilation through the ceilings should have a continuation of the vapour barrier to prevent condensation in the cold roof space.

It is not a microporous breathable layer, which offers a 2-way passage to air and vapour, but does not allow liquid water through, as in modern roofing membranes.

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