P/b is moisture-permeable but plaster doesn't breath. How ?

[mointainwalker]

Just thinking about all the insulation questions and answers that say p/b is moisture -permeable, but then also thinking of all the things I have read that say plaster on walls doesn't breathe.

Can someone explain this for me ? The p/b is highly compressed, has two layers of paper covering and is permeable but plaster applied with the pressure of a float becomes impermeable ??

Enlightenment welcomed !!

 

[noseall]

Water vapour and water droplets are very different.

This is the reason that gore-tex works.

 

[Deluks]

Like Noseall, I don't have the answer But I suspect that gypsum based plaster does breathe to some extent, but not as much as lime, which is also somewhat 'plastic'

 

[mointainwalker]

@ noseall

Sorry , do not understand your reply. In neither case are water-droplets in evidence.

We are talking about moisture migrating neither through p/b or plaster and in neither case about evident droplets but environmenmental-borne moisture.

 

[noseall]

As Deluks has pointed out - i,m talking outa me erse.

 

[stuart45]

Air pressure inside a room can force the moisture through the P/B, whereas the moisture in a wall trying to escape is not under that pressure.
The gypsum will however soak up moisture, whereas lime mortar has a different structure and will allow air and moisture to pass through it.

 

[Deluks]

Seems you're not the only one Nose

 

[mointainwalker]

Thanks for all your comments: I have to say noseall that your reference to Gore-Tex raised a smile as being a passionate skier, I'm interested in that kind of clothing, but have never been absolutely convinced of everything they say.

Stuart45

I am certain that the air-pressure will always be equal inside and outside the house. There is likely to be a difference in moisture content ( hygroscopic pressure ?? ) and I can see this forces a migration of water-vapour from one environment to the other but this would equally apply to moisture trapped in a wall.

 

[Deluks]

Air expands as it heats up, so warm air in the house will be of a higher pressure, and 'expand through' the structure, taking any water vapour with it.

 

[Nige F]

Air expands as it heats up, so warm air in the house will be of a higher pressure, and 'expand through' the structure, taking any water vapour with it.

So that`s why me ears pop when I go outside I`ll have to be careful. I`m a Fat Bastard . I`ll end up like Goldfinger