I've seen other threads on here using plasterboard and osb as separate layers in 'moisture breathable walls' so I'm guessing no. You could use duplex plasterboard but the joints and potential for scratching of the foil makes it more of a vapor check than a vapour barrier. These days I tend to use 50x50 battens then plasterboard inside a layer of taped polythene (or dedicated VCL membrane if budget allows) to create a service void - makes it much easier to avoid puncturing the VCL and the services trades can easily grasp where they can run the pipes and wiring.

You can't describe either OSB or pbd as any kind of vapour barrier - pbd has negligible resistance and OSB little-to-slight, just enough to be useful inboard in a 'breathing' construction.

Just discovered that whereas 'generic' OSB is about 175mu, the Smartply brand which I specify because no-added-formaldehyde (now 'mainstream' availability) is much lower resistance, 50mu (dry cup) declining to 30mu (wet cup i.e. as water content goes up). One to watch - or to make good use of.

By comparison pbd is about 8 mu (i.e. 8x as resistive as air), and a polythene vapour barrier about 50,000mu (before you put nails/staples thro it and fail to tape the joints impeccably and long-term durably).

WUFI data, and http://www.smartply.com/images/uploads/product_downloads/SmartPly_OSB3_Datasheet_2012_1.pdf

I have permission to publish here a brilliant diagram, which extends the BuildDesk units-conversion diagram to include American 'imperial' units, incl converting between resist-ivity(-ance) and perme-ability(-ance). Will do asap. Not to mention transmi-ssivity(-tance)(-tion) and conduct-ivity(-ance) that also crop up, for water vapour movement.

Mu (and Sd) values seems to be what we shd standardise on and get used to, because that's what WUFI and all the Europeans do, despite the ambiguity compared with e.g. Neil May's MNs/gm.

BTW, the Glaser method is totally dead, or should be, despite still being enshrined in Bldg Regs.

I think that it depends on the usage (swimming pool? Office? Grain storage shed?), and type (if any) of insulation you have outside of the studs.

I believe a becoming-common US detail is XPS/EPS/PIR outside of the studs (providing circa 50% or more of the U value), plus low-resistivity insulation between studs (Warmcel / mineral fibre / wood fibre), and PB inside (no polythene VCL).

http://www.buildingscience.com/documents/insights/bsi-038-mind-the-gap-eh

see the paragraph "Foam Sheathing Used With OSB Sheathing", and also the final detail in this:

http://www.buildingscience.com/documents/insights/bsi-049-confusion-about-diffusion/#f07

... this works with both high-resistivity rigid insulation (e.g. foil PIR), or with med/low resistivity (e.g. non-foil PIR, or EPS) - just as long as it's reasonably hydrophobic.

Tony,

The PIR (foil faced?) is your VCL in this build-up. I would not put any kind of additional VCL inboard of the PIR.
Is the osb outward of the 150studs/rockwool layer?

Cheers

I am unseen on vb part way through insulation layers, warm side for me and it doubles as the air barrier.

What Timber said.

The OSB is much better suited to the warm side of the insulation where it can remain at room temperature. This is the normal way of doing it in Germany where woodfibre is used on the outside of the studs.

If a service cavity is required then add battens or, for a more soundproof/solid feeling wall, use a double layer of plasterboard.

The insulation on the outside of the frame shouldn't be foil faced or you end up with a VCL on the cold side of the insulation. Better to use EPS/XPS or, ideally, woodfibre board.

The reason the EPS/XPS solution is becoming standard in North America is because they also need to think about the summer case where warm humid weather will lead to water vapour being driven inwards towards air conditioned rooms. So it makes sense to use an insulation which has a reasonable vapour resistance to cover the summer case & is not affected by moisture in the winter case, which is also thick enough to avoid putting the frame in the winter condensation zone.

David

Posted By: Timber
The layer(s) on the warm side of the wall need 5x (ish) the vapour resistance of whatever is on the outside of the wall.

And:

Posted By: davidfreeboroughBetter to use EPS/XPS or, ideally, woodfibre board.

This is where it gets confusing! If the outside, over-stud insulation is e.g EPS, does that then mean that it's necessary to increase the vapour resistance inboard, so OSB no longer sufficient?

If so, then the various suggestions that the between-stud, vapour-open insulations should be able to breathe (dry out) to the interior space would be at odds with this.

The Building Science article above is unequivocal about keeping inboard impermeable plastics out of the picture:

"With foam sheathing (of the appropriate thickness – R-value based on condensation control as noted above) the type of cavity insulation becomes irrelevant. Fiberglass batts, cellulose, low density foam, high density foam, ground up “blue jeans,” whatever, it no longer matters, they all work. Just don’t install an interior side plastic or foil or vinyl vapor barrier. A wall assembly has to dry in at least one direction and with that foam on the exterior the only practical drying direction is typically inwards. Remember that the assumption here is that the foam on the exterior is impermeable, which is not always the case – but once again we are being conservative."

Can somebody please clear the fog for me?!

Posted By: fostertomHow they get away with lime render on wood fibre EWI I can't imagine.

Not to mention straw bales

FWIW, there's a 380 page thesis at

https://dspace.cc.tut.fi/dpub/bitstream/handle/123456789/188/vinha.pdf?sequence=1

which sets out to explore how much vapour resistance is required. I haven't read it all yet.

Sadly it's one of those PDFs that won't let me select and quote part of the abstract.

All the timber studds in the walls are second hand and very dry, the over bosses wont accept a service void, I wanted one, I m glad to hear precedent for osb on warm side.

Thinking from in to out P board, vcl, osb, 150 rockwool studds, pir, not sealed at joins :) battens f/e boarding.

Posted By: Henry Sears

Posted By: Timber
The layer(s) on the warm side of the wall need 5x (ish) the vapour resistance of whatever is on the outside of the wall.

And:

Posted By: davidfreeboroughBetter to use EPS/XPS or, ideally, woodfibre board.

This is where it gets confusing! If the outside, over-stud insulation is e.g EPS, does that then mean that it's necessary to increase the vapour resistance inboard, so OSB no longer sufficient?

Yes, the 'rule' still needs obeying or the 'books balancing' (whichever you prefer).

We have talked about this before and a 1:1 wall will work given the correct (and limited) set of circumstances.

Agreed about climates where there may be a reversal of the warm-inside cold-outside that we are used to in the UK. It is possible that it can happen in this country with directly applied cladding systems - e.g. render on insulation on frame, but it is rare and not well documented.

Tony's proposed wall sort of looks fine - is the PIR foil faced? That will make a difference.