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    • CommentAuthorgoodevans
    • CommentTimeOct 24th 2017
     
    I have just spoken to a plasterer to gather info / ballpark prices for wet plastering the blockwork walls next year

    I had guessed that the undercoat would be a browning or hardwall plaster product - which he is happy to use if I asked - but his suggestion for time (and therefore cost) would be for a sand/cement screed undercoat applied at the standard 11mm thickness (no mention of lime yet).

    blockwork would be medium density blocks - what would the members here suggest for the undercoat - a plaster based product or screed.

    Secondly I would like to bed the loose edges of airtight membranes into the undercoat to for a continuous air barrier - it strikes me that a non-woven material such as Tyvek Housewrap would be soft enough to 'key' into the undercoat easily - however I can find no air permeability information for the product (except that it's supposedly wind tight) - does anyone have any air tightness numbers for Housewrap? I like this type of product because of price, it can be used inside and out, it's vapour permeable and is not easy to damage (but easy to cut when needed).
  1.  
    Over here screed is universally used at a thickness of around 15 - 20mm- with the addition of lime putty as a plasticiser. After the screed a plaster based filler is applied with a scraper to give a smooth finish for painting. The screed is put on by the builder and the filler is applied by the decorator.

    There is a lot to be said for going with what your tradesman is happy doing.
    • CommentAuthordelprado
    • CommentTimeOct 24th 2017
     
    I would use a pre-mixed baumit product for all internal work if I plastered again. Light and easy to use.

    I am doing your membrane under the scratch coat idea. NBT do tape which can be plastered over. Give them a call. Or Chris at Back to Earth.
    • CommentAuthortony
    • CommentTimeOct 24th 2017
     
    I would use sand and cement, much harder than hardwall and a LOT cheaper, i used a lot of lime in mine then sadly gypsum multifinish.
    •  
      CommentAuthordjh
    • CommentTimeOct 24th 2017
     
    Posted By: goodevansSecondly I would like to bed the loose edges of airtight membranes into the undercoat to for a continuous air barrier

    The various Pro Clima Contega tapes are designed for exactly this purpose.

    Sand and cement is the usual way to apply a parge coat for airtightness.
    • CommentAuthortony
    • CommentTimeOct 24th 2017
     
    I use eml nailedor more usually stapled over the adjoining poly membrane and wet plaster over it, this forms a hermetic seal.
    • CommentAuthordelprado
    • CommentTimeOct 25th 2017
     
    Why is everyone recommended sand and cement rather than a weaker hydraulic lime?
    • CommentAuthorgoodevans
    • CommentTimeOct 25th 2017
     
    OK - I like Tony's idea - mechanically fixing the membrane with the metal lathing to the wall then plaster over - it does not matter what the membrane material is - it will always work.

    Tapes I'd like to avoid - if the adhesive fails than the plaster on top is no longer attached - this is why I liked the idea of embedding the membrane.

    Any comments on the air tightness of tyvek Housewrap or its equivalents? I know it will leak at joints and punctures - but what about the sheet itself.
    •  
      CommentAuthordjh
    • CommentTimeOct 25th 2017
     
    Frankly I'd trust the durability of a reputable tape far more than the seal of sand and cement to the surface of a poly membrane. I'd expect the poly to break away after a few years.
  2.  
    Posted By: delpradoWhy is everyone recommended sand and cement rather than a weaker hydraulic lime?

    Because that's what most people use and it's inside their comfort zone
    • CommentAuthorRick_M
    • CommentTimeOct 26th 2017
     
    Posted By: goodevansAny comments on the air tightness of tyvek Housewrap or its equivalents? I know it will leak at joints and punctures - but what about the sheet itself.


    I assume the manufacturer has no comment? Would a DIY test by trying to blow through it with your mouth be acceptable? Is the plan to lap it from the walls to interior spaces underneath floors and ceilings?
  3.  
    If you used insulated plasterboard foamed on, how detrimental would it be not to wet plaster the brickwork prior?

    That's what I did in the shop...
    • CommentAuthortony
    • CommentTimeOct 27th 2017
     
    All about air infiltration, I reckon you had solid walls, if so will be ok so long as sealed top and bottom and at exposed edges
    •  
      CommentAuthorfostertom
    • CommentTimeOct 27th 2017
     
    Posted By: Rick_Ma DIY test by trying to blow through it with your mouth
    suck is easier.

    I've had it in mind to develop some kind of DIY calibrated rough ('order of') suck-test that an individual could apply to a range of novel or no-data samples - ranging from e.g. AAC concrete via a slice of s/cem render, gypsum plaster, or lime render, through breather felts, to polythene.

    Calibration, immediately before and after ea test, would be to pre-create and post-check a standard degree of suck in terms of height of a column of water in a small transparent tube; then subjectively judge how much air came through, which could be correlated against known figures for some material/samples.

    Note that the standard airtight test pressure/vacuum of 50kPa represents a column of water about 200mm high - and we can suck much stronger than that!
    • CommentAuthorRick_M
    • CommentTimeOct 27th 2017
     
    Posted By: fostertomNote that the standard airtight test pressure/vacuum of 50kPa represents a column of water about 200mm high - and we can suck much stronger than that!


    Also, I think it's being installed inside so I can't see how it would have to cope with much pressure
  4.  
    Posted By: VictorianecoIf you used insulated plasterboard foamed on, how detrimental would it be not to wet plaster the brickwork prior?

    That's what I did in the shop...

    That depends on the type of brick wall, inside leaf of a cavity with typical mortar will probably leak like a sieve, a solid wall not rendered outside I would expect to be fairly air tight and a solid wall rendered outside wall I would expect to be airtight. And then you are looking at gaps in the plasterboard foaming and joints
    •  
      CommentAuthordjh
    • CommentTimeOct 27th 2017
     
    Posted By: fostertomNote that the standard airtight test pressure/vacuum of 50kPa represents a column of water about 200mm high

    The standard airtightness test pressure is 50 Pa, not 50 kPa! It corresponds to 5.1 mm of water, whilst 50 kPa corresponds to 5098 mm, so I'm not sure where you got 200 mm from either! :cry:
    •  
      CommentAuthorfostertom
    • CommentTimeOct 27th 2017 edited
     
    Yep sorry for the k! Was going by memory fropm what I calc'd - but no doubt got it wrong. Thanks. Anyway, it means we all walk around with quite a powerful vacuum-tester, which can readily sniff out relative leakiness in an intuitive way.
    • CommentAuthorgoodevans
    • CommentTimeOct 27th 2017
     
    Human suck test - is it airtight? A ready reckoner.

    Assuming 0.5 m3/h @ 50 Pa is the absolute worse for an air barrier this would mean the air travels through such a material at 0.13mm per second (0.5*1000/3600)

    Humans can suck water up to about 1m (10000Pa) - that's 200 times more than 50Pa - for a linear relationship (which it isn't) that same material would have air flowing through it and out the other side at 27mm/s. I reckon that amount of flow would be noticeable to the human senses.

    Ready Reckoner: If you can sense that you can suck air through a material it's probably not an air barrier - If you can't it is!

    btw cartridge paper isn't, regular cardboard isn't, coated cardboard seems to be - has anyone sucked on some tyvek?
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