Home  5  Books  5  GBEzine  5  News  5  HelpDesk  5  Register  5  GreenBuilding.co.uk
Not signed in (Sign In)

Categories



Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
These two books are the perfect starting place to help you get to grips with one of the most vitally important aspects of our society - our homes and living environment.

PLEASE NOTE: A download link for Volume 1 will be sent to you by email and Volume 2 will be sent to you by post as a book.

Buy individually or both books together. Delivery is free!


powered by Surfing Waves




Vanilla 1.0.3 is a product of Lussumo. More Information: Documentation, Community Support.

Welcome to new Forum Visitors
Join the forum now and benefit from discussions with thousands of other green building fans and discounts on Green Building Press publications: Apply now.




    •  
      CommentAuthorfostertom
    • CommentTimeJun 16th 2019
     
    One for our boffins:

    If the water vapour resistivity of a material is known, and the 'size' of a molecule of gasseous H2O,
    is it possible to estimate, from that, the resistivity of the material to each of N2, O2, Ar and CO2, based on the 'size' of each of those molecules?

    As air consists of 78.09% nitrogen, 20.95% oxygen, 0.93% argon, 0.04% (aka 400ppm) carbon dioxide, it should then be possible to estimate the 'airtightness' of the material.

    Why haven't I thought of this before? Or is it boloney?
    • CommentAuthortony
    • CommentTimeJun 16th 2019
     
    Boloney I am afraid
    •  
      CommentAuthordjh
    • CommentTimeJun 16th 2019
     
    There's undoubtedly some truth in it and it would be possible to derive a model based on different sized billiard balls passing through different-sized goals, but there are complications when the molecules interact with each other and with the material that the goals are made from. I expect those complications, paricularly in the case of water, make the whole exercise fairly futile.
    •  
      CommentAuthorfostertom
    • CommentTimeJun 16th 2019
     
    Why
    Posted By: djhparicularly in the case of water
    - intriguing
    • CommentAuthortony
    • CommentTimeJun 16th 2019
     
    Hydrogen bonds, weak but drag water back, then osmotic pressure, physical and chemical interactions, atomic/molecular size, dipole moment, inertness, clathrate effects, other interactions, etc
  1.  
    Tom, this works for diffusion through gases and liquids, using the Stokes Einstein equation:

    https://en.m.wikipedia.org/wiki/Einstein_relation_(kinetic_theory)#Stokes-Einstein_equation

    But air permeability through solid building materials is not actually true diffusion, its much more about the pores and cracks in the material, which though very small to the eye, are much bigger than the molecule size effects.

    If the material is so crack-free that true diffusion becomes relevant, then for practical purposes it is impermeable - thinking of poly/Ali films etc
    •  
      CommentAuthorfostertom
    • CommentTimeJun 17th 2019
     
    Posted By: WillInAberdeenits much more about the pores and cracks in the material, which though very small to the eye, are much bigger than the molecule size effects
    So then why is vapour resistivity to small H2O molecules very much less than (but not proportional to?), resistivity to larger air (N2, O2, Ag etc) molecules?.

    If the "pores and cracks" are orders-of-scale different from molecule size, then the latter should play no part - but something like that does seem to cause a difference in resistivity to those different gasses.
    •  
      CommentAuthordjh
    • CommentTimeJun 17th 2019
     
    Posted By: fostertomSo then why is vapour resistivity to small H2O molecules very much less than (but not proportional to?), resistivity to larger air (N2, O2, Ag etc) molecules?.

    What particular circumstances are you talking about, Tom?
    •  
      CommentAuthorfostertom
    • CommentTimeJun 18th 2019
     
    There are materials that are airtight but water vapour open, but not the other way round AFAIK. So it looks like the 'sieve' principle does operate - smaller molecules get through but not larger ones. But WillinAberdeen seems to be saying that the pores and cracks are much too big for molecule size to have any effect. OTOH you say "complications, paricularly in the case of water" which might mean water vapour is specially slippery for reasons other than molecule size!
  2.  
    Tom, apparently the pores in timber are of the order of 10microns, 10^-5m. Water molecules are of the order of 0.1nm, 10^-10m.

    So the pores are roughly 100,000 times wider than the molecules. That's like a billiard ball rolling through a hoop 10 kilometres wide, or a car driving through a tunnel 100miles wide.

    I'm speculating that the small size of the water molecules, and their affinity for other materials, are both 'effects' with the same 'cause', the bonding Tony mentioned.

    Edit: as you know, construction timber has a lot of water in it, the pores are likely to be filled with water. As the water is stickier to itself (and everything else) than air is, the air cannot get in to dry out the flooded pores, the surface energy/tension of the water prevents this.
    You can add more water to one side of the timber, which will displace some water out the other side, which looks like the water has permeated through the timber.

    Particles are now thought of as waves, as well as billiard balls, which fits with the theme of water and flooding. :cool:
    •  
      CommentAuthordjh
    • CommentTimeJun 18th 2019
     
    Wood is hygroscopic. Hygroscopic materials contain capillary channels that lead to the hygroscopic behaviour. But the channels typically don't go all the way through.

    H2O is bigger than Ar or He by the way. But smaller than CO2. I'm sure there are tables somewhere.

    Water has some of the most complicated material behaviours of any substance we know about. It can have very different properties in many different situations. Everything that Tony and Will have mentioned and lots more. There are no general answers, every specific situation needs to be considered on its own merits.
    •  
      CommentAuthorfostertom
    • CommentTimeJun 18th 2019
     
    We know lots about the properties of N2, O2 and H20 gasseous molecules.

    Hopefully somebody somewhere also knows which of those properties (molecule size apparently not)
    have a bearing on 'resisivity' to gas transmission through materials under pressure differential
    (or perhaps for several broad types of materials).

    From that, could any estimate be made of the relative 'resistivity' of each of those gases?

    This may seem pointless - why not just directly test the air restitvity of different materials, just as their water vapour resitivity is routinely tested?

    Indeed - but the fact is that air resistivity is not regularly tested; instead the trade just uses plastic membranes of total-resistivity, doesn't see the need to test for shades of moderate resistivity.

    Not even Fraunhoffer Institute, who do massive amounts of materials testing of all the other parameters that WUFI wants to know - but not air resistivity.

    Yet data on these shades of moderate air resistivity is exactly what is needed for efficient and reliable 'breathing' constructions.

    As that hard data is not available, I'm pursuing the possibility that air resistivity might be usefully estimated, extrapolating from known water vapour resistivity, by comparing relevant characteristics of H2O vapour with same for N2 and O2.
    •  
      CommentAuthordjh
    • CommentTimeJun 18th 2019
     
    Posted By: fostertomI'm pursuing the possibility that air resistivity might be usefully estimated, extrapolating from known water vapour resistivity, by comparing relevant characteristics of H2O vapour with same for N2 and O2.

    You're on a hiding to nothing. You can't make that extrapolation, because of the different properties of the water molecule itself, and the different ways it interacts with other materials, such as membranes.

    Given what you know about the properties of Goretex, Intello, Solitex and polyethylene film for starters, can you not see that the airtightness is not connected to the vapour tightness in any useful way?

    I'm not clear why you're searching for something that is only partially airtight anyway?

    data on these shades of moderate air resistivity is exactly what is needed for efficient and reliable 'breathing' constructions.

    In what way is my lime plaster not part of an efficient and reliable 'breathing' construction, for example?
    •  
      CommentAuthorfostertom
    • CommentTimeJun 19th 2019 edited
     
    Posted By: djhI'm not clear why you're searching for something that is only partially airtight anyway?
    Well for example it seems to be reliably true that my favourite recipe of gapfilling glued-and-screwed (or nailgunned) 11mm OSB3 backed up by blown cellulose, is an excellent balance of adequate airtightness and adequate water vapour permeability, without use of any expensive fiddly fragile membranes,
    so saves their considerable costs and in contrast to those is easily done by ordinary builders on typical mucky UK sites without need for exquisite care, is strong and resistant to accidental/ignorant damage and has some in-depth self-closing tolerance of ignorant penetration.

    But while there is hard data on water vapour permeability, there is none on air resistivity for either material, so it's hard to prove their airtightness performance and it's down to anecdotal experience and crossed fingers - perpetually uncomfortable and prob unacceptable in a large/public project. Even a well-founded method of estimating would help but I'm getting that my hope for that is doomed.

    I'd have thought that same would apply to your lime plaster, if you're using it as part of your airtightness scheme?
    •  
      CommentAuthordjh
    • CommentTimeJun 19th 2019
     
    Posted By: fostertomthere is none on air resistivity for either material

    One point is that it's not 'either' material; there are many. The properties of a sheet of OSB depend on the particular brand, factory and even batch. So the only thing you can 'rely' on numerically is what a particular manufacturer measures according to some recognized standard and is audited for. Or else you cover it with a membrane that has been tested for whatever qualities you require and meet whatever specs that way.

    We've been round this particular loop over and over again over the years. You know this.

    I'd have thought that same would apply to your lime plaster, if you're using it as part of your airtightness scheme?

    The lime plaster is the bulk of my airtightness barrier. It's airtightness was established by testing the house, not the material. It is extremely airtight. I see no reason and have no desire for it to be only 'moderately' airtight.
    •  
      CommentAuthordjh
    • CommentTimeJun 19th 2019
     
    I was saddened by this thread. Not directly, but while checking some research I discovered that Jacob Israelachvili died last year. He was a great man. He wrote a well-known book called "Intermolecular and Surface Forces". The book explains a lot of the complexity of water, among other things. There seems to be a PDF online.

    "Among all of his accomplishments, Jacob could also execute a perfect back flip off a trampoline"
    •  
      CommentAuthorfostertom
    • CommentTimeJun 19th 2019 edited
     
    Lime plaster is another material that has supreme advantages over expensive fussy membranes, for breatheable/airtightness - it's do-able by 'ordinary' builders (tho not quite as 'ordinary' as glued/screwed OSB), is robust, inspectable/repairable, longterm durable etc. It's also relatively free of the issue of leaky joints and perimeters, which are the large, uncontrolled/unquantifiable achille's heel of those membranes.

    So congratulations - I think you see what I mean about that.

    Bully for you that it's in the 'very' airtight category - but you don't need to be snobby about other materials that in different ways have similar advantages, but which are merely 'moderately' airtight, but could do with more reassurance that 'moderately' means 'adequately' airtight, and esp in combination together.

    Where 'adequate' means really good enough for e.g. PH - so why not support promotion of such materials as OSB3, blown cellulose - broaden the palette of possiblities, for the very real advantages mentioned?

    As far as
    Posted By: djhThe properties of a sheet of OSB depend on the particular brand, factory and even batch
    but nevertheless a given brand has a single warranted water vapour resistivity figure, presumably quality-controlled across factories/batches, which, I'd hoped could be the basis for good estimate of air resistivity - for that brand - of course I didn't mean for all brands of OSB3 (note not OSB2 or lesser weetabix).

    And
    Posted By: djhairtightness was established by testing the house, not the material
    is no answer when it's necessary to specify materials reliably before construction (when isn't it?)
    •  
      CommentAuthordjh
    • CommentTimeJun 19th 2019
     
    Posted By: fostertombut you don't need to be snobby about other materials

    I don't think I am.

    which are merely 'moderately' airtight, but could do with more reassurance that 'moderately' means 'adequately' airtight, and esp in combination together.

    Well there's at least one OSB that is guaranteed airtight, so the question is why aren't others? But that's a commercial question to ask the manufacturers, not something I or anybody else here can answer.

    why not support promotion of such materials as OSB3, blown cellulose

    I do support both those materials, what makes you think I don't? I have both in my house, though in my case they are not part of my airtightness strategy, although I'm happy that they provide additional assurance.

    airtightness was established by testing the house, not the material
    is no answer when it's necessary to specify materials reliably before construction (when isn't it?)

    Lime was specified before my house was constructed. I'm not aware of any airtightness tests on lime as a material, are you? Indeed it's difficult to see how it could be tested for onsite application, much as onsite glueing is difficult to use in structural parts of a house. The answer is that other people have built houses with lime-coated walls before and they have been tested. You can use the same method yourself.
    •  
      CommentAuthorfostertom
    • CommentTimeJun 19th 2019
     
    Posted By: djhat least one OSB that is guaranteed airtight
    If that's Smartply's, it's a plastic coating which makes it water vapour impermeable - exactly what's not needed.

    If anyone makes OSB that's guaranteed airtight while still water vapour permeable, that wd be v gd news.

    In fact OSB3 is anecdotally airtight while still water vapour permeable, but only the latter is guarateed, no data offered on the former.
    • CommentAuthorjms452
    • CommentTimeJul 7th 2019
     
    Posted By: fostertomSo then why is vapour resistivity to small H2O molecules very much less than (but not proportional to?), resistivity to larger air (N2, O2, Ag etc) molecules?.


    Some fully fledged chemists (Tony) may pull this apart but my understanding of this issue is:

    Water H2O is really polar whilst the other gas molecules you mention (above) at least to a first approximation are not. This means water has some very positive and negative bits for such a small molecule which makes it interact with the big molecules in building materials differently.

    You could think about it a bit like solubility where water likes to dissolve in a polar matrix but not in a non-polar one.

    Speaking very generally plastics e.g. polythene aren't very polar, not much water will dissolve in them and hence the diffusion rates of water through them are low.

    Cellulose is covered in OH (water like & polar) groups, water will dissolve in it and hence the water diffusion rates through it are high.
    •  
      CommentAuthorfostertom
    • CommentTimeJul 7th 2019 edited
     
    Great you've revived this jms - just this morning I was feeling a bit sore at the general dismissal of this exploratory topic, which if illuminated could have great usefulness.

    My chemistry goes only as far as A-level, my physics same but considerably extended by reading since.
    Posted By: tonydipole moment, inertness, clathrate effects, other interactions, etc
    barely came into it back then (quantum anything not mentioned either). So I thirst to be enlightened by the wise on this forum, not merely in the form of "read some books", thank you.

    So, if for a range of trade materials their resitivity to H2O vapour is well tested and known by trade laboratories,
    but their resistivities to O2 and N2 vapours are untested by the trade, unknown,
    and it would be jolly useful to get some estimate of the latter, based on more fundamental properties (of plastics, cellulose, minerals etc, and H20, O2, N2 vapours) which are well researched within 'proper science',

    it must be possible from those properties (and their interactions) to discern at least the relative direction (more, less) of the resistivities of the three vapours, even put an estmated relative number on it?

    jms has started to do just that, for the property of polarity in both the materials' molecules and the vapours' molecules.

    Not saying it would be obvious, easy, but a proper chemist should be able to say quite a lot, straight off?
  3.  
    Posted By: WillInAberdeenTom,
    ...air permeability through solid building materials is not actually true diffusion, its much more about the pores and cracks in the material, which though very small to the eye, are much bigger than the molecule size effects.

    If the material is so crack-free that true diffusion becomes relevant, then for practical purposes it is impermeable - thinking of poly/Ali films etc


    Plastic DPM (not polar) = no pores = impermeable
    Recycled plastic wool insulation (not polar) = many pores = permeable

    Marble tile (polar) = no pores = impermeable
    Rubble wall (polar) = many pores = permeable

    OSB sheeting (polar) = many pores that are filled with water = permeable to water vapour but not much to air

    Naff OSB with lots of cracks (polar) = many pores that are not all filled with water and big gaps round the edges = permeable to water and air
    • CommentAuthortony
    • CommentTimeJul 7th 2019
     
    I am becoming uncertain of the value of this thread as the air permeability of the materials is generally not relevant as the horrendous gaps between elements of the same material and adjoining materials make the permeability on little or no consequence
    • CommentAuthorjms452
    • CommentTimeJul 7th 2019
     
    Posted By: tonythe air permeability of the materials is generally not relevant as the horrendous gaps between elements of the same material and adjoining materials make the permeability on little or no consequence


    Agree with you Tony.

    However, one you start thinking about what's going on at a molecular level you get into the whole polymer engineering that leads to cool new materials like Intello etc.


    Posted By: fostertomSo, if for a range of trade materials their resitivity to H2O vapour is well tested and known by trade laboratories,
    but their resistivities to O2 and N2 vapours are untested by the trade, unknown,
    and it would be jolly useful to get some estimate of the latter, based on more fundamental properties (of plastics, cellulose, minerals etc, and H20, O2, N2 vapours) which are well researched within 'proper science',

    it must be possible from those properties (and their interactions) to discern at least the relative direction


    Barring holes/pores the diffusivities of N2 and the O2 may correlate between materials a bit (but H2O won't).


    There's a lot of info about the diffusivity of O2 through polymers in the food & beverage context where you're trying to stop tiny amounts of O2 getting into stuff over years.
    •  
      CommentAuthordjh
    • CommentTimeJul 7th 2019
     
    Posted By: jms452There's a lot of info about the diffusivity of O2 through polymers in the food & beverage context where you're trying to stop tiny amounts of O2 getting into stuff over years.

    Not to mention central heating pipe etc where the goal is to stop oxygen diffusing through and rusting tanks etc.
    • CommentAuthortony
    • CommentTimeJul 7th 2019
     
    Those kind of rates are inapplicable to air tightness considerations.
    •  
      CommentAuthorfostertom
    • CommentTimeJul 7th 2019
     
    Posted By: tonyI am becoming uncertain of the value of this thread as the air permeability of the materials is generally not relevant as the horrendous gaps between elements of the same material and adjoining materials make the permeability on little or no consequence
    Glad you mentioned that, what people like testing house Fraunhoffer inst say - but not necessarily so.

    For instance OSB3 sheathing as air barrier, gapfilling glued and screwed/nailgunned to solid backings at every joint, makes the joints the one bit that's pretty much guaranteed airtight, even with average carpentry skills. That leaves the OSB itself as the airtightness criterion.

    Blown-in cellulose is another similar - inherently gap-filling and jointless if done conscientiously and in a way where full-filing can be seen before closing in, as per the video I keep posting.

    Both these have 'moderate' airtightness which, if known and calculable, can be plenty adequate.

    Sure there's other airtight materals which are 'completely' overkill-airtight but are notoriously hard to fit and joint airtight, easy to accidentally or ignorantly damage, and uncertain long-term stability despite what accellerated ageing tests say.

    So who needs such overkill-tightness accomanied by dodgy joints etc, when 'moderate' (but adequate) tightness accompanied by foolproof installation and durabilty is availability, but hapmered by lack of hard airtightness testing.
    • CommentAuthorjms452
    • CommentTimeJul 7th 2019
     
    Posted By: tonyThose kind of rates are inapplicable to air tightness considerations.


    I think you mean immaterial.

    i.e. diffusion will transfer mass over time - it is just of a different order of magnitude to typical building leak tightness requirements.
    •  
      CommentAuthordjh
    • CommentTimeJul 7th 2019
     
    Posted By: fostertomBlown-in cellulose is another similar - inherently gap-filling and jointless if done conscientiously and in a way where full-filing can be seen before closing in

    You don't think that measuring the volume that is injected is adequate then? Where does the extra volume go if there are gaps?
    •  
      CommentAuthorfostertom
    • CommentTimeJul 8th 2019 edited
     
    Laborious to accurately calculate the reqd vol of cellulose within stud walling with odd noggings and strange nooks and crannies, to accuracy needed to ensure all are exactly filled, no more and no less - before even the uncertainty of variable compression. If that's what they claim to do, I wouldn't trust it.

    Better a system where you can visually see and feel (by prodding) where it's going, during installation, and where a bit more is needed - like in that video, where it's done behind scrim before the plasterboard or whatever facing is fixed.

    Which is good because I've seen plasterboard half-blown off its fixing screws by the pressure of cellulose filling. Better to install it behind scrim, roller that flat before fixing facing boards.
   
The Ecobuilding Buzz
Site Map    |   Home    |   View Cart    |   Pressroom   |   Business   |   Links   
Logout    

© Green Building Press