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General: How to make 18mm T+G OSB/3 Airtight?

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      CommentAuthorfostertom
    • CommentTimeAug 9th 2012
     
    Do the calc in BuildDesk or similar and you can keep on the right side of dewpoint, with this mid-sandwich vapour resister layout. But do the calc, don't take anyone's word for it. Then BCO shd be satisfied - why not?

    Tho, how closely BuildDesk agrees with reality is another matter, which only closer study in WUFI wd answer.
    •  
      CommentAuthorfostertom
    • CommentTimeAug 9th 2012
     
    an02ew, why 11mm nor 9mm OSB? Why not 6mm in fact, at least for walls, which aren't walk-on? Just asking.
    • CommentAuthorborpin
    • CommentTimeAug 9th 2012
     
    Posted By: fostertomThen BCO shd be satisfied - why not?
    Just because it is not something he is familiar with. At the very least it will go back and forth at least 5 times! I agree with you FT, I just did not have the stomach for the fight. Add to that the fact they will want a breathable membrane outside the OSB (why? where is it breathing to? Oh because you always have a membrane outside a TF wall rather than considering why it is required....) etc etc.
    •  
      CommentAuthorfostertom
    • CommentTimeAug 9th 2012 edited
     
    I dunno, I just don't find BCOs to be rigid and dogmatic - quite the reverse - I luv 'em! - old hands who've seen it all. May have to give gd argument in writing/drawing, for the record.
    • CommentAuthoran02ew
    • CommentTimeAug 10th 2012
     
    Well you’ve all been busy in my absence. BCO seems ok with this method; hopefully I’ve blinded him with the quality of the build?

    Posted By: fostertoman02ew, why 11mm nor 9mm OSB? Why not 6mm in fact, at least for walls, which aren't walk-on? Just asking.


    i am using 11 mm for 2 reasons, firstly for added stiffness to the walls which are much larger than our previous work Tom, i am concerned about the pack of cards effect. Secondly having read all of this discussion, i am convinced that OSB would have a better chance of maintaining its airtightness on a thicker sheet. it will be impossible to keep the sheets dry during construction especially with the bad weather. Apparently the sheets have a much better chance of air tightness if they are keep dry.
    • CommentAuthorCav8andrew
    • CommentTimeAug 25th 2012
     
    I am seriously considering using a liquid membrane to achieve an internal air and vapour barrier. I have sourced information on two such products manufactured by Prosoco and Sto. Has anyone any experience of using this form of barrier or any thoughts on its use. The roof build up is Exterior to interior;- Slate,battens & counter battens (Nulok system), breathable membrane (taped joints & perimeter edge), 18mm OSB (sealed joints), 300mm I joists (void fully filled with pumped cellulose), 12mm OSB (sealed joints), VAPOUR & AIR BARRIER (liquid or taped sheet),
    possibly a further 50mm layer of insulation (type to be decided but in which services could be buried), Fermacell boards.
    Any thoughts on any aspect of this.
    •  
      CommentAuthorfostertom
    • CommentTimeAug 25th 2012 edited
     
    If you drill holes in the outer 18 OSB you won't need any internal vapour barrier. what's it there for?
    • CommentAuthorCav8andrew
    • CommentTimeAug 25th 2012
     
    Why would you drill holes in the OSB ? Seems self defeating and puts total reliance on breathable membrane integrity which in my opinion under normal building practices can't be guaranteed.
    •  
      CommentAuthorfostertom
    • CommentTimeAug 25th 2012 edited
     
    Much discussiion in http://www.greenbuildingforum.co.uk/forum114/comments.php?DiscussionID=6890 - it's about where in the sandwich the vapour resistance has to go (inner layer(s) 5x as resistant as outer), in a 'breathing' construction that requires no internal VCL. Your 18 OSB, without holes, places most of the resistance outboard. With holes in the 18 OSB, all the resistance is inboard in the 12 OSB, as it should be, for breathing construction. If not breathibng construction, then yes you need a strong inboard VCL.
    • CommentAuthorCav8andrew
    • CommentTimeAug 25th 2012
     
    As far as I can tell the structure we currently have is breathable and extremely airtight in a structurally robust form (unlike, in my opinion and experience, membranes). The problem we have is that we are not hitting the magic 5 to 1 ratio. I would happily have some permeability (vapour) through the structure but I need to create a greater partial resistance to the inner surface. I am yet to have the discussion with the cellulose installers as to their opinion re. moisture movement through the warmcel but I am assuming it not a problem as long as there is movement out and away.
    • CommentAuthoran02ew
    • CommentTimeAug 26th 2012
     
    Posted By: Cav8andrewVAPOUR & AIR BARRIER (liquid or taped sheet),


    i assume taped sheets as just light weight polythene would be alot cheaper that liquid? what sort of liquid and where in your build would it be applied? back of plasterboard sheet? would VC plaster be cheaper?
    • CommentAuthorCav8andrew
    • CommentTimeAug 26th 2012
     
    I guess taped sheet would be cheaper although having said that I would probably use a Proclima membrane rather than polythene so the cost differential might not that great.
    The products I have info. on are from Sto and Prosoco and would be applied to the 12mm OSB before the 50mm layer of insulation and Fermacell is installed, as in the sequence described above. I have yet to fully research these products as to their total or partial permiability. If using Fermacell plastering would not be an option but I guess if plasterboard is used VC plaster could be explored, however I am keen to avoid wet trades and Fermacell is a product I have some experience of in respect of getting an acceptable final finish.
  13.  
    We've achieved airtightness results of between 0.23 and 0.56ACH for 13 Passive Houses so far this year using mostly taped and silicone jointed OSB, so I've see no problems in practice with the airtightness of OSB.
    The house in some of these photos http://www.viking-house.ie/timber-frame-house.html cost €10k extra to build, so I'm not sure it was necessary to spend so much on Fermacell and Gutex.

    Here's 2 studies from the US and Canada where the wall build-ups are the wrong way around, being 5 times more breathable on the inside than the outside.
    http://www.viking-house.ie/downloads/Tsong79.pdf
    http://www.viking-house.ie/downloads/Spokane.pdf
    No evidence of fungus, mould or decay were discovered when the walls of close to 250 houses were opened, leading me to conclude that the 5:1 breathability rule is a bit of a red herring!

    So don't worry about using OSB for airtightness, it works fine and don't worry about the breathability of the wall just get it airtight!
    • CommentAuthorCav8andrew
    • CommentTimeAug 27th 2012
     
    Hi Viking, thanks for this, like you I am very happy with the ability of sealed joints OSB to provide a robust airtight detail, most importantly one which is able to resist the rigours of site life, to the extent that I may just lap the membrane rather than tape the joints, will see. Good to see your test results justifying this thinking and practice, particularly after all the hours of sealing any and every joint/gap I could see. I still have some concerns re. the movement of moisture outwards but the US and Canadian studies do appear to allay this. In my case the Fermacell is my finished surface and one I can manage myself without resorting to wet trades but I presume it will add some resistance to the inner surface which can only be good. Thanks again for the links and details of your own experience.
  15.  
    I would put the 18mm T&G OSB on the inside where it can provide useful vapour resistance & act as an air barrier & use something more vapour open on the outside as a wind barrier, e.g. 9mm OSB, 9.2mm Panelvent or 22mm Pavatex Isolair or Gutex Multiplex Top.

    David
    •  
      CommentAuthorfostertom
    • CommentTimeMar 12th 2013
     
    'Bubble glue' update from an02ew:
    Expanding foam gapfilling glue for glueing the OSB to studs/rafters: you may have to order it but in his branch of T-P, Sabrefix D4, used for fixing Norboard housebuilder’s chipboard flooring, £5 for 1 litre, or boxes of 12 (you’ll be using a lot), which is under half price he was paying before.
    • CommentAuthorTimSmall
    • CommentTimeJun 25th 2013
     
    Good tip re this glue - just ordered some... By way of a correction, it's "Caberfix D4". For smaller quantities, Roofshop has the 45 minute lumberjack for about £5.50 inc VAT per 0.75 kg when buying 6 or more.

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