Hi all
I'm looking to improve upon my usual OSB3 sheathing board as we struggled to get our AT on test on our last project and have put it down to the OSB despite being carful handling and taping I suspect the OSB itself was our let down.

So has anyone found a reliable cost effective alternative?

1 must have similar racking resistance
2 excellent AT properties
3 cheap
4 available.

Thanks in advance.

Andy, I had a big chat with a Smartply boffin when we were both on the WUFI course in Dublin. They are aware of the need for reliable, stated airtightness, working on it but can't quality-control it yet. However Peter Warm, PH Certifier, states categorically that OSB is fine as air barrier. Next job I will ask Smartply to test some batches off the line, that they can supply as an order certified direct from the works.

Despite Scottish Sterling Board, Irish Smartply ex Cork is still the 'locally sourced', minimum airmiles brand for most of England - and also 'no added formaldehyde'. Worth sticking with them, and with OSB, if you can. Still, looking forward to alternastive suggestions.

Some European suppliers offer coated OSB of guaranteed airtightness - but then no longer cheap.

That's why it's remarkable that

Posted By: fostertomPeter Warm, PH Certifier, states categorically that OSB is fine as air barrier

I'm not ready to throw the towel in yet - more to this than meets the eye.

However, it's more crucial than most people realise. It's precisely this kind of very low-intensity, widely dispersed leakage that risks a v big effect of interstitial condensation. Larger, concentrated air leakage, like around windows, and at the taped joints (yuk!) between boards may be a big heat leak but aren't liable to cause interstitial condensation.

This paradoxical fact is because faster, concentrated air-leak flows have enough thermal oomph to warm up the walls of the crevice, making condensation unlikely. Whereas slow, dispersed leaks never warm up all the material they're dispersing through, so condensation is more likely.

This is the conclusion of recent research at Fraunhofer Institute, parent of WUFI software. In fact this knid of slow leakage turns out to be causing as much, if not more of interstitial condensation, that the traditional focus on vapour permeation (which is what WUFI models).

WUFI has therefore had a module added as first stab at accounting for interstitial condensation caused by air leakage - but it's incredibly crude, guesswork, compared to the sophistication of WUFI's vapour permeation modeling.

So we need answers!

Frustratingly, in two discussions - arguments more like - with WUFI boffins on WUFI forum, they don't admit the need to add airtightness data to the many physical properties of materials, that WUFI requires. And they don't intend to start testing for airtightness, when manufacturers ask them to test their materials for inclusion in the WUFI database.

Instead they rely on emprical correlation with blower door test results, as input to the 'incredibly crude' module I mentioned above. Apart from the fact that by the time you've got blower door results it's too late, blower door results tell you nothing about what proportion of the measured leakage is caused by the 'harmless' big leaks, and what by the dangerous small, dispersed leaks. The latter could be a tiny, or a huge proportion of the whole. The empirical corelation they've discovered must be only from testing certain types of construction, with no guarantee of relevance to other types.

In fact ignore the blower door tests - all we need to know about, from the interstitial condensation POV, is the small, dispersed leaks. Not the joints between OSB boards, but the centre of the board. This is in fact eminently testable, without reference to blower door tests.

<blockquote><cite>Posted By: tony</cite>I can't see air leakage through the sheets, friend just got .69 for non ph build using them.

Joints, holes, gaps etc must be the problem, most likely window holes too.</blockquote>

Achieved slightly better results on a resent non PH project, but then spent most of that day chasing sub 0.6 until at a suggestion of our tester we polythene a 1m2 on OSB used as AT layer on ceiling the polythene very slowly billow.proving that sheet wasn't completely AT


<blockquote><cite>Posted By: fostertom</cite>Next job I will ask Smartply to test some batches off the line</blockquote>

You must a made a good contact Tom to able to ask them for that service


what of these coated sheets? has anyone used/tested them? how much dearer are they? any thought of coating standard osb3 with something cheap to improve its performance?

How about if treated OSB moved to inner skin of timber frame and a more permeable sheathing such as timber vent or panel vent on external side thus applying the rule increasingly permiable layers as you move out wards ?

Everything I've learned with messing with WUFI says that 5:1 (some say 3:1) rule is a red herring. A gradient from in to out neither helps nor hinders. I could never find where this idea originated - it was mainly put out by Warmcel but their tech dept cd throw no light. I think it was based on the traditional, erroneous belief that moisture moves mainly from inside to outside (tho I still don't see how a gradient would help that) - in fact moisture originating from outside and ebbing and flowing diurnally and annually completely swamps the little bit originating from cooking, washing etc.

It's the same misconception that led to internal VCLs - which IMO are best left out altogether (even 'intelligent' ones) - and which again IMO in fact 'worked', for many years, not as vapour barriers but as air barriers preventing just the slow all-over air leakage mentioned in post above. Now, high higher insulation levels, they don't 'work' any more.

What matters is low enough vapour resistance throughout. OSB and EPS are both only just vapour-permeable enough, so we shouldn't do anything to deteriorate either of them

Like is there a vapour membrane on the warm side of the insulation?

How important is vapour permeability through the ceiling layers verses air tightness - if the ceiling is completely air tight excess vapour would be drawn out through the MVHR!
Is it feasible to use a polythene sheet above the OSB to create the air tight layer?

Posted By: fostertomWhat matters is low enough vapour resistance throughout. OSB and EPS are both only just vapour-permeable enough, so we shouldn't do anything to deteriorate either of them

But if we are to use these two products together and the osb ISNT airtight then this system doesn't work?

Posted By: Mike GeorgeHi Andrew, can I ask what is the construction make up from inside to outside?

Roof
Plaster board fixed to 25mm counter batten fixed through Airtight layer of OSB3 to 241mm or 300mm steico Ibeams pump filled with diamond bonded thermabead sheathed over with 11mm panelvent or timbervent with 25mm downslope counter batten, breather felt and tile.

walls
Plaster board fixed to 25mm counter batten fixed through Airtight layer of OSB3 to 145mm MOS timberframe filled with 140mm xps with 11mm panelvent or timbervent with 25mm downslope counter batten,12mm cements board and render


Im trying to use the structural sheathing as a airtight layer to save on time and money

Posted By: Mike GeorgeWhy the counter -battens on the warm side? Is it for a service void?

yes

Posted By: Mike GeorgeIs it an option to just trap a polythene sheet between plasterboard and OSB? I've done this on loft conversions at they've been so airtight you cannot get the internal doors to close without keeping a decent gap under them

would that not work with the batten? what about junction? Tapes?

I think the trapping method works best as the tightness of the layers offsets the air infiltration through any punctures in the polythene. Though I guess trapping the polythene against the OSB may be almost as good

Have you considered foil backed plasterboards? foamed at edges and taped and skimmed?

Anyone seen or better still used this system? http://www.huberwood.com/zipsystem/home-zip-systemAny idea of cost and availability. It looks a bit unattainable.
What about its permeability Tom?

if one cant rack then neither can the other! unless hit by something that could destroy the building

I came across this when looking for vapour barrier inner board http://www.ecologicalbuildingsystems.com/products/spano-durelis-boards/spano-durelis-vapour-block/

In essence they are a pre-coated structural board that is nail gunned to the inner timber frame and then the joints taped over. As easy as fitting OSB with the only extra work needed to make it air/vapour tight being to tape the joints, I believe it is not much more expensive than osb.

Anyone got cost comparison, coated vs standard OSB?