DIY Vapour barrier plasterboard

[Tim S]

OK, stupid time.

Accidently bought 6 sheets of ordinary 9.5mm PB in 3x6' sheets. Need to use vapour barrier PB for the ceilings on some bay window ceilings which will be insulated with celotex above. I don't even think the use of vapour barrier PB is that critical as there's a huge void over half the bay ceiling as the pitched roof meets the lead flat roof of the bay, but vapour barrier was suggested.

Any harm in using normal PB together with a vapour barrier or either plastic or perhaps even bacofoil (turkey sized) stuck on the back of the board.

Seems a pointless waste to dump the board as I can't use it elsewhere.

Cheers

Tim

[NT]

ali foil is a more effective VB than plastic, and sticks well with PVA. VB PB is only ali foil coated anyway.

NT

[John Rumm]

I can't see a separate barrier being a problem - just because its not glued to the board does not stop it working.

[Stuart Noble]

Is that so? Plastic would be easier

[Tim S]

Stuart Noble coughed up some electrons that declared:

Thanks all,

That's settled.

I thought foil would be a good possibility, because a) I can buy it readily in the supermarket, b) it's vapour proof and durable, c) I can probably lightly stick it with either PVA or some spray on evo-stick I happen to have a can of. Sticking it, even very lightly would ensure it stays put as I install the board.

Just glad I don't have to waste it :)

Cheers

Tim

[Rad]

Tim There is a BG wall board "paint on" vapour barrier - ? wall board top coat or the like any oil based paint will do chris

[NT]

Yes, normally. Cooking foil comes in narrow strips though, and this will need to be glued to avoid gaps between strips.

NT

[John Rumm]

You could use the ali foil tape to stick em together if you really want. ;-)

[NT]

I assume a coat of oil based paint is significantly less effective than foil though, and less effective than plastic film. May be good enough though.

NT

[John Rumm]

You can get paint on DPC products if you want. Used before laying screeds etc.

[The Natural Philosopher]

Both foil and plastic film are 100% impervious

[Stuart Noble]

Probably not as effective on a porous surface though. IIRC paraffin wax is the vital ingredient in water resistant PB

[Tim S]

Stuart Noble wibbled:

Ah - but aren't there two distinct products: vapour barrier *and* moisture resistant?

The former is useful to me to prevent excessive vapour getting to cold bits of wood. The latter would be of more use perhaps in a bathroom?

Cheers

Tim

[Dave Plowman (News)]

What do you do with the gaps between boards? Just curious.

[RobertL]

I'm interested in why a vapour barrier is needed. is it because there are no ventilation points into the void?

Robert

[The Natural Philosopher]

Normally its to prevent condensation on the far side of e.g. porous rockwool insulation.

[Tim S]

RobertL coughed up some electrons that declared:

I think in this case it is to reduce the risk of condensation on cold bridges, the rafter faces. And to reduce the amount of warm vapour leaking into the cold side. These will be nominally air tight but there's no way to make it 100% so - too many bits of random wood to seal every interface.

I have no way to overlay insulation on these, only between them.

The cold side has the requisite 50mm air gap.

Cheers

Tim

[NT]

not quite, hence some medical & food products need to be in glass jars, and some plastic piping needs a foil layer in it. I assume its good enough though.

NT