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Green Building Bible, Fourth Edition
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    • CommentAuthorEd Davies
    • CommentTimeDec 31st 2013
     
    From:

    http://www.greenbuildingforum.co.uk/forum114/comments.php?DiscussionID=11571&page=1#CommentBody_192463

    Posted By: fostertomCob is ideal for safely absorbing/sequestering any water vapour peaks from cooking/showering, until able to re-dry - tho of course if surface temp is such that the vapour condenses, it won't get sequestered in that way.
    Sorry, that's so surprising that more explanation is needed. I can't see why cob would store any more water *as vapour* than an equivalent volume of fresh air - rather it'll store less, surely? It would only be if the water condenses that more water could be stored, i.e., I'd have thought that only if the surface temperature was such that vapour condenses would such sequestration happen.

    Posted By: fostertomEverything I've learned lately, from playing with WUFI, says that internal (or any) vapour barriers cause trouble, because they prevent drying-out inward. 95% of the water vapour in play originates from outside, ebbing and flowing thro walls; only a trivial %age from kitchen/showering etc.
    Is that specific to masonry walls with no cladding or with render or does it also apply to walls with a properly ventilated rainscreen? In other words, is the external water which dominates come from direct rain impingement on the wall or from plain-old water vapour which is hanging around anyway.
    •  
      CommentAuthorfostertom
    • CommentTimeJan 2nd 2014
     
    Gd Q Ed. If it was organic material, then I cd visualise the little sacs within the fibre structure greedily absorbing water vapour (but how then converting to liquid within themselves - and why willing to release the water under drying conditions?). Anyway, a wool jumper can absorb masses of sweat and still feel dry - like my fabulous Icebreaker ultra-fine merino wool thermal top, which doesn't even get smelly after days of use!

    But unfired clay (cob), which is almost unique in its similar hygroscopicity, amongst non-organic materials, must do the trick by other means - the labyrinth of capillary pASSAGES between the particles. Condensation within such 'solids' doesn't happen bang at dewpoint, but is present to some extent all the time.

    First individual water vapour molecules become attached to the pore walls; at higher RH these start to clump together into a liquid film; at still higher they may bridge right across the pore's width, at which point capillary transport by surface tension will begin. So there's quite a lot of 'liquid' within a porous solid way below 100% RH. At 95% RH, cumulative effect makes the condensation curve suddenly goes steep - but even at 100% it's not all over because solids can become super-saturated.

    Maybe, maybe not, such hygroscopic materials, whether organic or unfired clay, don't allow liquid condensation to form on their surface because water molecules, whether vapour or liquid, get whisked away into the interior as they arrive.

    Organic materials must have the capillary labyrinth thing happening, as well as their active sacs mechanism.

    Either mechanism does convert vapour to liquid, so can store masses volumetricaLLY - JUST NOT SURE WHETHER CONDENSATION AT THE SURFACE IS PART OF THE MECHANISM, WHETHER IT wd promote or impede absorbtion.
    •  
      CommentAuthorSteamyTea
    • CommentTimeJan 2nd 2014 edited
     
    I think that is a complicated way to find out what is going on.
    Why not just look at the dew point temperatures and the energy needed to phase change. Then look at what happens when you change the temperature of the material under test.
    Probably best to assume a starting point with some liquid water in the material than start with no water, either as a gas or a liquid.

    Shall have to try and think up a small experiment, maybe something similar to "Ed's Freezer". Does rely on an accurate set of scales.
    • CommentAuthorEd Davies
    • CommentTimeJan 2nd 2014
     
    FT, yes, I think we have pretty much the same understanding of what's going on. All I'd say that your original version I quoted above is a bit misleading/confusing as condensation is happening at “the surface” at well below the dew point unless your meaning of “the surface” is restricted to the flat bits of the outer surface - which is what I now think you meant.

    ST, trouble is dew points only apply over flat surfaces of pure water whereas, as FT describes, we're not talking about flat surfaces here. I think you're right, though, that the phase change energy is an important consideration as it's the relatively high conductivity and thermal mass of the cob which allows it to dry out again quickly when the RH drops - it can supply heat to evaporate the water promptly.
    •  
      CommentAuthorSteamyTea
    • CommentTimeJan 2nd 2014
     
    Posted By: Ed Daviesit's the relatively high conductivity and thermal mass of the cob
    Have I ever mentioned thermal inertia. There should be a humidity inertia. Wonder if I can see it in my weather data.
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