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Comments
Joints, holes, gaps etc must be the problem, most likely window holes too.
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.
Others can see such leakage:
http://www.greenbuildingadvisor.com/blogs/dept/musings/osb-airtight
Both in the US and in Ireland. The big message is the variability of sheets - some give good airtightness, others don't.
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.
Note that Air Permeance of Building Materials http://www.cmhc.ca/publications/en/rh-pr/tech/98109.htm is v old - 1999 - materials will have changed much since then.
Joints, holes, gaps etc must be the problem, most likely window holes too.
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
Posted By: fostertomNext job I will ask Smartply to test some batches off the line
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?
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
Is it feasible to use a polythene sheet above the OSB to create the air tight layer?
Vapour could also cause problems anywhere it could leak or diffuse.
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
Is it an option to just trap a polythene sheet between plasterboard and OSB? I've done this on loft conversions and 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?
Have you considered foil backed plasterboards? foamed at edges and taped and skimmed?
What about its permeability Tom?
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.