Fitting vapour barrier / VCL

[dave_104]

Hi

I'm always confused by vapour barriers....

I have a kitchen with a flat roof (warm deck as it's insulated above), and an adjoining blockwork wall. The roof joists are currently exposed, and the internal wall will be battened up the blockwork and PB on top (no insulation) as it's a very rough wall. The external wall will be insulation within studwork and all completely foil taped up - so I assume no vapour barrier needed because of the foil?

Because of the humidity in cooking etc I guess that the roof needs a vapour barrier/VCL for certain, and I think that while doing it I might as well drop the barrier down the internal wall too - even if that might not be strictly necessary. Does this sound right?

I also think that a VCL would be better to use rather than a plastic sheet/barrier, as the VCL allows a certain amount of breathing for the structures. Is that correct??

Any advice on where/what type of layer I should be putting on these please?

Thanks, Dave

 

[Wayners]

Tin foil or plastic is a vapour barrier.
Water can't get through.

So foil backed plasterboard won't let water vapour through to a cold loft where it would condense.

Our kitchen and bathroom has foil backed plasterboard

 

[^woody^]

A warm roof had the vapour barrier above the deck, not at the ceiling level. Fitting one at ceiling level when there is another above it can create damp issues not prevent them.

Likewise, fitting a vapour barrier to a masonry wall is a complete waste of time.

"VCL" is a generic term for the layer. Plastic sheet and foil board is a VCL, not an alternative.

You should deal with kitchen humidity at source, and use a suitable fan and ventilation. Not try and deal with the resulting humidity via secondary measures.

 

[robinbanks]

Put non breathable membranes as near to the moisture source, and put breathable membranes near to the ventilation.

A construction that separates the heated envelope from the world thus has one or two membranes; one that prevents moisture from the warm area getting into the construction potentially another membrane that will permit any moisture in the construction to get out, but provide some other benefit - eg tyvek housewrap protects a timber frame from external elements during construction, and is breathable, but the inner faces of the walls might be covered with tyvek airguard which is doing the work of stopping house generated moisture entering the wall when the house is in service