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Posted By: fostertomNot having a lump to hand to suck at, is EPS itself airtight, never mind the joints?
Posted By: gravelldDepends on the render too, because we're concerned about long term air tightness, not just passing a test.But I guess most EWI "systems" have a modern render which, apparently (!), don't crack.
Posted By: gravelldPaul Jennings on this subject
Posted By: ringiassume that ... the renter is put on with care
Posted By: djhNot having a lump to hand to suck at, is EPS itself airtight, never mind the joints?I don't believe so, no. But nobody is supposing that it is AFAICT?
Not having a lump to hand to suck at, is EPS itself airtight, never mind the joints?
Posted By: fostertomMy priority, in sealing the gaps, wasn't to do with airtightness as such - I assume the EWI doesn't contribute to that reliably - but in preventing inside-to-outside thermal bypass convection through the joint gaps, even if they're only fag-paper width.Now maybe realising that the entire EPS face is one big convection path from inside to outside!
Posted By: djhThings don't have to be airtight to resist convection
Posted By: MarkyPassuming the render creates an air tight skin, is convection possible through pore spaces in EPS and a fag paper gap between boards?
Posted By: djhFWIW, I did just put my mouth up to a piece of EPS and blow. It's hard to be completely sure, but I think some air was passing through the EPS. In the interests of science, I suggest that somebody repeats the experiment!
Posted By: Ed DaviesI suspect it sucks more than Tom or Dave.
Posted By: Ed Davieswith a pressure sensor in the block, wrapped in plastic sheet
Posted By: Ed DaviesIt's quite a powerful vacuum and I suspect it sucks more than Tom or Dave
Posted By: fostertomMy muscular mouth is a very effective vacuum chamber! - far stronger than my lungs in blowing mode.
Posted By: fostertomConvection's not same as non-return through-passage of air.Convection can occur even when one or both faces of the sample (e.g. EPS block) are airtight, if one face is warmer than the other.Buoyancy causes denser colder air to gravitate to displace lighter warmer air, in a circular motion returning on itself.That can obviously happen in a vertical-plane inside-to-outside joint, perhaps less so in a horizontal one - and we're suggesting here that it can also happen in the intersticies of a EPS block.
Posted By: MarkyPhow are r-values calculated for EPS? How does the test avoid this convection? Or is it unavoidable and therefore the quoted R-value includes this tiddly bit of convection?
Posted By: MarkyPCan a convection path really establish in a highly irrelgular, sub millimetre air pathway across an EPS board?
Posted By: fostertomSo PiH and djh, are we saying that a) narrow joints between EWI blocks and b) the intersticies of EPS are small enought to guarantee (recirculating) convection negligible?That would mean no need to squirty-foam fill all EWI EPS joints (not an accepted industry thing anyway, just perfection (prob instigated by me) on GBF), just bigger ones (how big)?To be clear, this is a question about recirculating convection, separate from any consideration of straight-through air passage.Convection could happen even with good airtightness; straight-through only with poor airtightness.But to preclude straight-through air passage through the EWI, the airtightness doesn't have to be in the EWI - it can be inboard, outboard or as I prefer it, in the OSB tea cosy in the middle of the sandwich.
Posted By: Peter_in_HungaryIf you take the 2G /3G limits of spacing
Posted By: Peter_in_Hungarystraight through convection
Posted By: fostertomPosted By: Peter_in_HungaryIf you take the 2G /3G limits of spacingAh but that's a trade-off between better insulation (more thickness of hopefully still air) and less convection (less thickness). I'd guess there's considerable convection at those pane spacings but their effect is out-voted by the extra thickness of insulative (semi-still) air. I'm sure it's not 'no convection' at those pane spacings.That trade-off doesn't apply within EWI, when any convection at all is an uncompensated negative.
Posted By: fostertomPosted By: Peter_in_Hungarystraight through convectionFor clarity, let's not call that convection - I've been saying 'straight-through passage of air' - any better idea?
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