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Fundamentals: How airtight is EWI when done right?

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    • CommentAuthorringi
    • CommentTimeMar 7th 2017
     
    Let assume that all the gaps between EPS sheets are sealed with expanding foam, and that the renter is put on with care, how close to being airtight does EWI make a old building even if there are gaps in mortar joints etc?

    What if the EPS was put on in the normal way without sealing with expanding foam, but the render was still done to a reasonable standard?
    • CommentAuthorMikC
    • CommentTimeMar 7th 2017
     
    Depends largely on if and how the adhesive is applied I would have thought. Unless it is continuous adhesive between the EWI and the wall air could move within the gap and escape at the eaves or footing, depending on how these areas are detailed.
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      CommentAuthordjh
    • CommentTimeMar 7th 2017
     
    The fast way to lose airtightness is to have holes. So I wouldn't be looking for problems in a rendered wall particularly but much more likely to be at connections of that wall with other things - the roof, doors, windows, the ground, pipes & wires, drainpipes, porches, clotheslines etc.
    • CommentAuthorgravelld
    • CommentTimeMar 7th 2017
     
    Depends 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.
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      CommentAuthorfostertom
    • CommentTimeMar 7th 2017
     
    Not having a lump to hand to suck at, is EPS itself airtight, never mind the joints?
    •  
      CommentAuthordjh
    • CommentTimeMar 7th 2017 edited
     
    Posted By: fostertomNot 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?
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      CommentAuthordjh
    • CommentTimeMar 7th 2017
     
    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.

    If the render cracks, you simply repair it. But external render is susceptible to far more influences that are likely to crack it than is internal plaster, which is one factor in the preference for internal airtightness barriers. But again, it is the connections that are likely to be most problematic.
    • CommentAuthorgravelld
    • CommentTimeMar 8th 2017
     
    Hmmm but the repair becomes a new weak point right. I'm looking at this in terms of the overall system air tightness, taking into account things like not-getting-around-to-fixing-it and so on and also the dimension of time...

    Good point as to another reason why internal plaster is preferred.
    • CommentAuthorgravelld
    • CommentTimeMar 8th 2017
     
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      CommentAuthorfostertom
    • CommentTimeMar 8th 2017
     
    Posted By: gravelldPaul Jennings on this subject
    A gd link, what with other comments (not properly answered there)!
    •  
      CommentAuthorfostertom
    • CommentTimeMar 8th 2017 edited
     
    Posted By: ringiassume that ... the renter is put on with care
    Yes, tenant behaviour can outwit best intentions!

    Posted By: djh
    Not 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?
    Yes, I think they are supposing that.
    Ringi's OP Q is in two parts - the second part is about just the finish render alone, as contributor to airtightness.
    The first part is also about the contribution of the EPS - and improving that by 'all the gaps between EPS sheets are sealed with expanding foam'.
    I'm suggesting (suddenly realising?) that sealing the gaps (my longtime insistence) may in fact be futile if the EPS itself is air permeable.

    My 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!
    • CommentAuthorMarkyP
    • CommentTimeMar 8th 2017
     
    assuming the render creates an air tight skin, is convection possible through pore spaces in EPS and a fag paper gap between boards? I thought that EPS was intentionally gappy, little air pockets being insulative. A sheet with lots of little air spaces reducing conduction loss over a solid board of the same stuff?
    • CommentAuthorDarylP
    • CommentTimeMar 8th 2017
     
    still air is an insulant; lambda 0.024 IIRC? However, air that moves = convection currents...:cry:
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      CommentAuthordjh
    • CommentTimeMar 8th 2017
     
    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!

    Things don't have to be airtight to resist convection. Think about wool insulation, of all types. As another example, there is reason to suppose that there is some convection in straw bales, but it's not much (different k-values for different straw orientation in the bales).

    FWIW, 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!
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      CommentAuthorfostertom
    • CommentTimeMar 8th 2017
     
    I find sucking is an easier technique - can generate much more pressure and it's a bit self-sealing!

    Posted By: djhThings don't have to be airtight to resist convection
    Why haven't I thought this through before? So then why have I been insisting on filling the gaps between EPS blocks, even fag-paper thickness, to prevent inside-to-outside convection - when the EPS itself has plenty of such gaps - albeit convoluted, not straight-through? What does the Brains Trust think?
    •  
      CommentAuthorfostertom
    • CommentTimeMar 8th 2017
     
    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?
    Convection'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.
    • CommentAuthorEd Davies
    • CommentTimeMar 8th 2017
     
    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!
    Tried with vacuum cleaner sucked up against a block of EPS with a pressure sensor in the block, wrapped in plastic sheet. The pressure sensor is quite capable of measuring the pressure increase, at about the level of its own noise, when I bring it downstairs yet didn't see any decrease as set up here. Still, there wasn't an obvious decrease when I put the vacuum cleaner nozzle right in the box either, which did surprise me. OK, the plastic sheet was hardly sealed round the edges but I'd still have expected some drop in pressure. It's quite a powerful vacuum and I suspect it sucks more than Tom or Dave.
      dsc02134-small.jpg
  18.  
    Convection in a closed space needs space for the convection to work. Think 2G or 3G, the space between the glass is bigger the better - up to about 19mm after that convection currents can be set up transferring heat from the warmer glass to the colder glass. Below 19mm there is not enough space for convection to work so you get the insulation effect of the air.
    With EWI if there are gaps behind the EPS and a path top to bottom or gaps in the EPS wall then convection current can be set up from the bottom to the outside somewhere. (or from a lower gap in the EPS wall to a higher gap). But in any event IMO there won't be sufficient space within the EPS for convection to work as a closed volume, an exit for the warmer air as well as an input will be needed.
    So IMO any breathability of EPS should not be a concern for heat loss but any gaps that allow through flow of air would be a concern. The normal thin coat render with the glass mesh embedded I would have though would be sufficiently robust and flexible not to crack with normal conditions, and even then a crack would have to coincide with an air passage in the underlying EPS. The thin coat render is surprisingly (to me anyway) flexible. There was a problem locally last year where several houses had the EWI render damaged by egg sized hail stones where the hail knocked off bits of render where they struck. The mesh remained intact and the biggest problem with the repair was matching the colour of the top coat (batch number differences) The same hail storm destroyed more roofs than damaged EWI.
    •  
      CommentAuthordjh
    • CommentTimeMar 8th 2017
     
    Posted By: Ed DaviesI suspect it sucks more than Tom or Dave.

    I resemble that remark! And I'm not going to get into discussions of who sucks and who blows :devil:

    Your EPS, Ed, looks like packaging, which I think is typically a finer (less expanded?) variant than EPS70. I don't know whether that would make any difference.

    Tom, convection isn't the only force that drives air movement. Air pressure changes, such as wind, also drive movement, either by forcing air all the way through barriers or by alternately compressing and expanding the air within a cavity.

    Not leaving cavities in construction helps by reducing the volume of air that can be moved by such forces, providing less opportunity for heat and moisture movement, although there have to be continuous barriers as well.
    •  
      CommentAuthorfostertom
    • CommentTimeMar 8th 2017 edited
     
    Posted By: Ed Davieswith a pressure sensor in the block, wrapped in plastic sheet
    How dya mean 'in the block' - do you mean in the bag? Why should air in the bag show any pressure change (as air is sucked from the bag?) when the bag is extremely free to deflate to equilibriate pressures?

    Does this sample of packaging-grade EPS actually have a sealed melted or tight from-the-mould surface rather than cut as building-grade EPS is?

    Posted By: Ed DaviesIt's quite a powerful vacuum and I suspect it sucks more than Tom or Dave
    In volume no doubt but in static vacuum possibly less. My muscular mouth is a very effective vacuum chamber! - far stronger than my lungs in blowing mode.
  21.  
    Posted By: fostertomMy muscular mouth is a very effective vacuum chamber! - far stronger than my lungs in blowing mode.

    Muscular as it might be, I rather doubt it's a match for a decent vacuum cleaner. Our 'Henry' wants to pick up carpets & rugs even on the half power 'eco' setting...
    • CommentAuthorMarkyP
    • CommentTimeMar 8th 2017 edited
     
    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.


    I realise convection isn't just a draught moving through the EPS. But my understanding of convection is that the air pocket size is highly signifnicant in the potential for heat to move through an air space. Like PiH said. So does convection actually happen at a level that is even remotely signficant in tiny spaces? Can a convection path really establish in a highly irrelgular, sub millimetre air pathway across an EPS board?

    And here's another thought - how 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?
    •  
      CommentAuthorfostertom
    • CommentTimeMar 8th 2017
     
    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?
    Good question - and throw in highly significant internal radiation across gaps.

    R or lamda values are indeed a composite - certainly not just conduction - and as such contain many assumptions, the most important fallacy being the 'steady state' one, which simply doesn't exist in natural heat-transfer situations (or even in the test hot-box) any more than it does in weather patterns.

    Once that real-world transience (incl high frequency micro-oscillations) is admitted, the real insulativeness (outside the hot-box) of conventional insulations is worse than advertised and that of anything using radiant-barrier (e.g. multifoil) or radiant-absorber (e.g. aerogel, graphited EPS) techniques in better, in reality.
    •  
      CommentAuthordjh
    • CommentTimeMar 8th 2017
     
    Posted By: MarkyPCan a convection path really establish in a highly irrelgular, sub millimetre air pathway across an EPS board?

    Not by itself no. The boundary layer drag would prevent it. But if a temperature difference establishes different pressures on either end of a narrow channel, then the air might be forced through it if there is sufficient pressure. So it might complete a convection circuit.

    The lambda values of insulation are determined using a guarded hot plate. There's a brief description at https://www.ibp.fraunhofer.de/content/dam/ibp/en/documents/Fl-174-01-Flyer-Plattanapparat-Mai2011-Etcm1021-92195.pdf
    There's an interesting article at https://en.wikipedia.org/wiki/Thermal_conductivity_measurement that shows alternate ways that could be used if the building world could be dragged screaming into the twentieth (sic) century. [No disrespect intended to those who design the guarded hot plates]
    •  
      CommentAuthorfostertom
    • CommentTimeMar 8th 2017 edited
     
    So 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.
  26.  
    If you take the 2G /3G limits of spacing where it is generally accepted that over 19mm space between the glass could have convection currents and less that that will not then you could have fairly big gaps before any recirculating convection is set up, probably gaps of such a size that you wouldn't get them in EWI - or at least it would need to be extremely sloppy workmanship to get gaps big enough to get recirculating convection.

    Of course straight through convection is another matter which is why I won't accept any 'dot and dab' fixing of EPS but insist on a continuous line top and bottom of the EPS sheets as a minimum.
    • CommentAuthorgravelld
    • CommentTimeMar 8th 2017 edited
     
    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.
    [I might have lost track of this thread] Agreed, but the OP is about how airtight is EWI, as a result I think straight-through air passage is important, because other air barriers can be breached, e.g. outbound render, as I suggested above, regardless of whether the render can be fixed or not.

    There's no problem seen in redundancy (of air tightness) is there? It's a common strategy in lots of industries.
    •  
      CommentAuthorfostertom
    • CommentTimeMar 8th 2017
     
    Posted By: Peter_in_HungaryIf you take the 2G /3G limits of spacing
    Ah 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: Peter_in_Hungarystraight through convection
    For clarity, let's not call that convection - I've been saying 'straight-through passage of air' - any better idea?
  29.  
    Posted By: fostertom
    Posted By: Peter_in_HungaryIf you take the 2G /3G limits of spacing
    Ah 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.

    As maybe but you are not going to get anywhere near 19mm gaps in EWI (I hope) maybe 8 - 10 mm between the wall and the EPS and single digit mm between the EPS sheets, and then you will have rough uneven surfaces at the wall which will inhibit convection.

    Posted By: fostertom
    Posted By: Peter_in_Hungarystraight through convection
    For clarity, let's not call that convection - I've been saying 'straight-through passage of air' - any better idea?

    I don't mind what it is called - but it is heat loss using air as the transport medium and driven by the (greater) buoyancy of the higher temperature air.
    • CommentAuthorringi
    • CommentTimeMar 8th 2017
     
    I was thinking that if the foam gun is pushed ALL the way to the back of each joint between EPS sheets then the foam would seal round each sheet to the wall, so stop air movements behind the sheets of EPS.

    We could do with someone with the correct kit building small bad quality block or brick room and seeing how much better the air tightness is after the EPS is added, then again after it is foamed, then after the render is put on.

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