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Housing - Renovation: Gypsum V Lime breathability

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  1.  
    • 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:

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