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      CommentAuthorfostertom
    • CommentTimeFeb 26th 2015
     
    I don't see anyone's problem, making OSB very, even absolutely airtight, with liquid or sheet application over it. As long as you don't mind losing vapour permeability in the process. As lots of people don't seem to be worried about the latter, then I'm surprised this whole OSB question isn't just my little minority problem.
    • CommentAuthorgravelld
    • CommentTimeFeb 26th 2015
     
    Is there no record of existing Phs to see how well they have maintained air-tightness of the long term, with details of their construction?
    • CommentAuthoran02ew
    • CommentTimeFeb 26th 2015
     
    Posted By: gravelldIs there no record of existing Phs to see how well they have maintained air-tightness of the long term, with details of their construction?


    i doubt we`d find one that hasn't had to augment the osb with additional layer to start with so all you'd be testing is that?

    Posted By: fostertomAs lots of people don't seem to be worried about the latter


    Its because you've sown a seed of doubt, using words like drenched, soaked and law-suit, very powerful words on the week minded Master O B 1
    •  
      CommentAuthorjoe90
    • CommentTimeFeb 26th 2015
     
    I think Tom has hit the nail on the head. How can we make OSB airtight but maintain its vapour permeability?. What coating would do this?
    •  
      CommentAuthorfostertom
    • CommentTimeFeb 26th 2015
     
    Don't worry Andy - in fact it's you that's raised the doubts, quite right too - we're not sure yet but we will get there. Sorry if you've got an imminent project to worry about!
    •  
      CommentAuthorjoe90
    • CommentTimeFeb 26th 2015
     
    what about painting with acrylic gloss paint to make OSB air tight but vapour permiable?

    http://www.paintquality.com.au/press/newspaper/boapt01.html
  1.  
    delete
    • CommentAuthoran02ew
    • CommentTimeFeb 26th 2015
     
    Posted By: joe90what about painting with acrylic gloss paint to make OSB air tight but vapour permiable?


    i really like this idea, seems to be based on sound scientific testing. well done Joe, that's the 3rd time you've help me on this job, i owe you a drink. anybody else like or dislike this he says holding his breath
  2.  
    Where exactly is the OSB in the construction element?
    • CommentAuthoran02ew
    • CommentTimeFeb 26th 2015
     
    undecided at the moment, but probably going for inside side of structural timber work with 25mm counterbatten service void and PB and skim
  3.  
    In that case wouldn't any paint be punctured in the same way a vapour barrier would be? Though I can see it would be better if there is a void as my preferred method involves trapping the membrane between OSB and Plasterboard (no void)
  4.  
    What are you trying to achieve? You don't save any energy in a building by reducing the airtightness from 1.5ACH to 0.5ACH, you just get a sticker and a pat on the back for your effort, with the fellow who sold you all the tape telling you you're great! The point I'm making is that OSB is fine, leave it alone! You're just playing into the arms of the guys selling the more airtight board, they started all this propaganda!
  5.  
    Posted By: Viking HouseYou don't save any energy in a building by reducing the airtightness from 1.5ACH to 0.5ACH
    Eh?
    •  
      CommentAuthorfostertom
    • CommentTimeMar 3rd 2015
     
    Down from 10 or 3 ac/hr - that's about heat.
    On down from 1.5 - 80% (for MHRV) of a fraction (for real-life pressures, mainly stack effect) of 1.5ac/hr doesn't carry a lot of heat. It's more to do with:
    1) according to PH, uncomfortable draught sensations, leading to people turning the heat up; and
    2) according to Fraunhofer/WUFI, transport of water vapout to condensation zones by bulk air movement.
  6.  
    So are you saying you don't save any energy or you don't save much?

    If you follow that logic couldn't you also say you don't save any energy by reducing u-values from say 0.12 to 0.1?
    • CommentAuthorPeterStarck
    • CommentTimeMar 4th 2015 edited
     
    I got these results running PHPP on my house design.

    Pressurisation Test Result Specific Space Heating Demand
    0.2 12.1
    0.4 12.3
    0.6 12.5
    0.8 12.7
    1.0 12.9
    2.0 14.0

    I started a thread on the AECB forum a couple of years ago about this 'problem'.

    http://www.aecb.net/forum/index.php/topic,3928.0.html

    Edit: sorry about the formatting of the table.
    • CommentAuthorMike George
    • CommentTimeMar 4th 2015 edited
     
    Thanks Peter - So a law of diminishing returns then? Very much like u-values?

    I find the AECB site too agressive where questions/discussions go against their single mindedness re PH :)
    I also ran some TAS simulations looking at ACH some years ago - I will have to dig out the results to see how/if they tally with PHPP
    •  
      CommentAuthorfostertom
    • CommentTimeMar 4th 2015 edited
     
    Posted By: Mike GeorgeI find the AECB site too agressive where questions/discussions go against their single mindedness re PH
    Tended to be, yes.
    Posted By: Mike GeorgeSo a law of diminishing returns then? Very much like u-values?
    Exactly, as far as heat loss (but as I say there are 2 other gd reasons for lowest poss leakage). However, there is a difference, when pushing into that 'diminishing returns' territory:
    It's easy to do with insulation, just costly, to add more thickness.
    It's very difficult to do with airtightness - lots of hard detail work.

    Also, need to be clear about the 2 different types of air leakage:
    a) concentrated leaks, cracks missed e.g. around windows, tapes parting etc, and
    b) dispersed, slow all-over leakage - could be via a network of taped joints begining to fail, or via the permeabilioty of the sheet/board materials themselves.

    Of those 2 reasons for airtightness:
    1) according to PH, uncomfortable draught sensations, leading to people turning the heat up; and
    2) according to Fraunhofer/WUFI, transport of water vapout to condensation zones by bulk air movement,

    for 1), b) doesn't matter - only the larger more concentrated leaks a) will be felt as draughts
    for 2), a) is not dangerous, as there's enough oomph in the airstream to warm up the walls of the crack so it doesn't condense, whereas b) is the case that causes interstitial condensation.

    So the need for high airtightness still stands, and both a) and b) must be good.
    • CommentAuthorgravelld
    • CommentTimeMar 4th 2015
     
    For 1 (b) how's about dispersed leakage causing a lowering of surface temps (e.g. dispersed leakage into void behind dot and dab plasterboard). That would cause convection currents therefore draughts.
    • CommentAuthoran02ew
    • CommentTimeMar 4th 2015
     
    Posted By: Viking HouseWhat are you trying to achieve


    For me its not just about saving energy, but just as much about hitting a target. albeit self imposed aim high and get as close as you can. excepting 1.5 , 2 or what ever, is the thin end of wedge.

    That`s why its called the Green Building Forum not the General Building Forum
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