Timber frame- airtight? breathable?

[elmsoft]

I am making up timber frame panels by nailing OSB on to C16 studs with the intention of then covering the OSB with Tyvek breathable membrane. I have seen recommendations that the OSB joins should be taped and sealed to make them airtight. "Airtight" seems to a desirable idea in building these days.
So I am curious as to why I should use (expensive) breathable membrane on something I am trying to make "airtight"?
Just curious.

 

[freddiemercurystwin]

http://www.trada.co.uk/news/view/14A2543E-0849-4C00-8843-025E99E1C9BF/

 

[elmsoft]

Thanks for the reply. But I am still puzzled.

As TRADA say:

[the membrane] i allows water vapour to escape from the construction; and it can also contribute to air sealing the wall.

But if the construction is airtight- sheeted in visquene, say, there will be no water vapour- which is presumably why a vapour control barrier is fixed to the inside of the timber frame wall[/b]

 

[freddiemercurystwin]

Fundamentally the breather membrane protects the timber frame from any rain penetration that gets through the outer skin, whatever that is. External skins be they timber cladding or brickwork or whatever are rarely 100% rain proof hence the need for something to protect the timber frame behind.

With respect to airtightness of the timber frame, all it needs is a vapour barrier behind the plasterboard, which can be foil backed plasterboard if you wish or a 500G membrane behind the plasterboard. There will still be the potential for air leakage through the vapour barrier though, with all those fixing penetrations and joins. Your average Joe builder knocking up a timber frame wall and ‘sealing all the gaps up’ will not leave it 100% air tight and even the most discerning of DIYer’s or builders will probably have a few leaks here and there especially after a few years when the building has settled etc.

 

[ban-all-sheds]

But if the construction is airtight- sheeted in visquene, say, there will be no water vapour- which is presumably why a vapour control barrier is fixed to the inside of the timber frame wall[/b]

There will be vapour, coming from people and their activities in the building.

The VCL promotes air tightness, but the reason you need it is to stop water vapour getting to the cold side of the insulation and then condensing on the structure. Imagine if you had a brick-built house and wanted to insulate an external wall - air tightness would be irrelevant, but you'd still need a VCL.