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    • CommentAuthorkebabman
    • CommentTimeSep 17th 2012
     
    Looking at the column for vapour permeability in a PDF entitled Breathability Matters that I think I downloaded from the natural building website it says that gypsum plaster has a value of 50 and lime plaster 75.
    I have some areas of wall that have been plastered with gypsum and I was thinking of removing it but if the vapour permeability is lower for gypsum should I bother and why is lime plaster said to be better?

    Many thanks
    • CommentAuthorEd Davies
    • CommentTimeSep 17th 2012
     
    These are values for vapour resistivity (the inverse of permeability).

    Neil May's Breathability in Buildings paper (same one? or probably quite similar) has a table with the same values. What surprises me is how little extra resistivity cement plaster has, 100 GN·s/(kg·m) vs 50 and 75 for gypsum and lime respectively.

    So, yes, I'm also curious what all the fuss is about.
    •  
      CommentAuthordjh
    • CommentTimeSep 17th 2012
     
    Dunno about those numbers. Here are some others by Straube who also quote Minke:

    http://members.westnet.com.au/ejt/pdf/Straube_Moisture_Tests.pdf

    [ng/Pa s m]

    Cement:Sand 1:3 datum 1.7
    Cement:Lime:Sand 1:1:6 datum 10.3
    Lime:Sand 1:3 Datum 18.9

    Gypsum appears to be in the range 1.62 - 3.5 according to http://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=05930938

    It may also be interesting to read:
    http://www.conservationphysics.org/wallbuff/wallbuff.php
    • CommentAuthorkebabman
    • CommentTimeSep 17th 2012
     
    Djh many thanks for the links, after reading through them I'm still not really any the wiser I'm afraid regarding the downside of Gypsum. :shamed:
  1.  
    Anyone become any wiser on this issue over the last couple of years? Seems a lot of hassle getting plasterers to use traditional plasters so is there anything really wrong with gypsum? I'm thinking for example on the inside of pavatherm used as internal insulation on brick walls.
  2.  
    Mu values and breathability. Stated values and the perfect construction
    http://www.greenbuildingforum.co.uk/forum114/comments.php?DiscussionID=9383&page=3#Item_16

    and some others
    http://www.greenbuildingforum.co.uk/forum114/search.php?PostBackAction=Search&Keywords=breathability&Type=Topics&btnSubmit=Search

    like to have a layman answer to this one too. Mu values seems to differ depending on who is writing about it.
    • CommentAuthorjamesingram
    • CommentTimeJun 6th 2014 edited
     
    here's some tables from the above discussions
    • CommentAuthorjamesingram
    • CommentTimeJun 6th 2014 edited
     
    So fermacells Vapour Diffusion Resistance EN ISO 12572 μ = 13

    So
    vapour diffusion coeffient
    vapour resistance
    Vapour resistivity
    vapour resistance factors

    Anyone care to remind me how they interrelate ?? :shocked:
    • CommentAuthorjamesingram
    • CommentTimeJun 6th 2014 edited
     
    fRom links above
    http://builddesk.co.uk/wp-content/uploads/2013/01/vapourResistances.pdf

    -value (“mu-value”) of a material is also known as its “water vapour resistance factor”.

    To convert a m-value to a vapour resistance (MNs/g)
    1. Multiply by thickness in metres
    (this gives the “equivalent air layer thickness” in m)
    2. Divide by 0.2 g.m/MN.s
    (this is a typical value in the UK for the vapour permeability of still air)

    To convert a m-value to a vapour resistivity (MNs/gm)
    Divide by 0.2 gm/MNs
    (this is a typical value in the UK for the vapour permeability of still air)
    • CommentAuthorEd Davies
    • CommentTimeJun 6th 2014
     
    First table: “Calculated Mu = Vapour Resistance x 0.2". I think “Resistance” should be “Resistivity” here. It's what makes sense logically, given that µ factors are stated for materials in general rather than particular thicknesses of layers, and how the numbers seem to be calculated.
    • CommentAuthorEd Davies
    • CommentTimeJun 6th 2014 edited
     
    The µ value of a material is its vapour resistivity relative to that of still air. The vapour resistivity of still air is said to be 5 MN·s/(g·m). So multiply the µ value by 5 to get the resistivity (or divide by 0.2 if everything in your life is so simple that you need to make the arithmetic look at tiny bit more complicated).

    To get the resistance of a particular layer multiply the resistivity of the material by the thickness of the layer.

    Posted By: jamesingram(this is a typical value in the UK for the vapour permeability of still air)
    Will the vapour permeability of still air in Scotland change in the event of a “yes” vote?

    Seriously, is this why people use µ factors - because they scale with some other parameter? Temperature I would guess?
    • CommentAuthorEd Davies
    • CommentTimeJun 6th 2014 edited
     
    Posted By: Ed Davies: “Seriously, is this why people use µ factors - because they scale with some other parameter? Temperature I would guess?”

    Seems it depends on temperature and pressure: http://www.wufi-wiki.com/mediawiki/index.php5/Details:WaterVaporDiffusion
  3.  
    just reading that myself :)
    • CommentAuthorEd Davies
    • CommentTimeJun 6th 2014
     
    Not that it makes a terrible amount of difference. Over a temperature range of -30 °C to +40 °C and a pressure range of 900 to 1050 hPa (millibars) the resistivity of still air according to that WUFI formula varies from 4.28 to 6.13 GN·s/(kg·m):

    >>> import math
    >>> def δ(t, p):
    ... T = t + 273.15
    ... p *= 100
    ... return 2.0e-7 * math.pow(T, 0.81) / p
    ...
    >>> 1/δ(20, 1000)
    5019115462.354611
    >>> 1/δ(20, 1000)/1e9
    5.019115462354612
    >>> 1/δ(-30, 900)/1e9
    5.255987841486506
    >>> 1/δ(-30, 1050)/1e9
    6.1319858150675906
    >>> 1/δ(40, 1050)/1e9
    4.995740215061416
    >>> 1/δ(40, 900)/1e9
    4.282063041481214
  4.  
    On that bombshell I'm off to the pub :bigsmile:
  5.  
    Can I come too?

    HaplessDIYer wrote:

    Anyone become any wiser on this issue over the last couple of years? Seems a lot of hassle getting plasterers to use traditional plasters so is there anything really wrong with gypsum? I'm thinking for example on the inside of pavatherm used as internal insulation on brick walls.

    From observation:

    Gypsum overloaded with moisture becomes like porridge, and if I remember rightly, the more it has been saturated, the more hygroscopic it becomes (no idea where I learned that - if someone confirms for me that it is wrong, I will try to forget it!!). It appears to 'manage moisture' badly. Lime seems to tolerate more moisture-related 'give-and-take'. As an example, a cottage in Derbyshire which receives most of its rain horizontally. Gypsum patches - moist and salty. Old and (now) new lime - fine. The wall does get wet, and the colour-change attests to this, but the plaster finish tolerates it.

    I would not pay out for Pavatherm/Pavadentro and use gypsum on it. If you are not a plasterer too, the Baumit lime plasters are *much* easier to work with than gypsum.

    Edited for tripe-o-graphical errors
  6.  
    Posted By: Ed DaviesFirst table: “Calculated Mu = Vapour Resistance x 0.2". I think “Resistance” should be “Resistivity” here. It's what makes sense logically, given that µ factors are stated for materials in general rather than particular thicknesses of layers, and how the numbers seem to be calculated.


    Yes, you are right Ed. My mistake :) The conversion figures in the tables are correct but resistance should read resistivity
  7.  
    Nick, my experience with gypsum is that it often (but not always) fails where used over historic lime mortar (undercoat) In my area this is usually *black* mortar containing a lot of coal dust or ash from the era of king coal. Areas of failure are generally solid walls with cement render outboard - ie where moiture tracks across after being trapped within the fabric - as you say the lime seems to be able to deal more readilly with the moisture retaining it's form and function - whereas the gypsum spalls or exhibits salts or both
  8.  
    Mike, in the particular case to which I refer, the wall had been about 90% re-skimmed with gypsum, but the vast majority was still lime behind - only 2 'eyes' about 400 dia had been re-plastered right back to the wall in gypsum. These 'eyes' 'held the damp'. The householders thought they were being watched!

    The (c3mm) gypsum skim seemed to be able to breathe enough to allow the lime to still 'do its lime thing', while full-thickness gypsum just 'sucked'.
    • CommentAuthordelprado
    • CommentTimeMay 3rd 2017
     
    could I add to this debate? It seems to me there is nothing wrong with gypsum, per se, but that you have to apply a load of pva to get it to stick, which is basically impermeable?
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