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    • CommentAuthornick1c
    • CommentTimeDec 25th 2017
     
    Does anyone know if the longevity of different methods of achieving good air tightness has been investigated and if so what the results were? It looks as if we may be in a position to build a house in a wet and windy site. I would love to construct it to achieve at least passivehaus levels of efficiency, but am concerned about the toll Atlantic gales will take over the years.
  1.  
    I have mainly used tape and membranes in the jobs I have done (One full refurb, one large new-build and many partial jobs), but I suspect that many here would contend, probably rightly, that plaster may pass the test of time better. If you build in masonry and use hard plaster I am not certain the winds would move things enough to cause problems, but then in Sheffield I am quite a way from Atlantic gales!
    • CommentAuthortony
    • CommentTimeDec 25th 2017
     
    I like your idea of at least PH levels of efficiency, it is always good to aim at low energy demand, good levels of air tightness and very low U values.

    The problem I have with tapes and membranes are the longevity of the tapes, possible perforation of the membranes and something I call the bellows effect which will more problematic where there are gales.

    I would go masonry, good detailing, air tightness better than PH, wet plaster, mitigation of thermal bridges, etc

    In Canada they regularly achieve very good levels of air tightness less than 0.1, here we barely understand what its or the importance of having it. Scottish islanders have a better understanding of it and I often help them with suggestions, specifications and detailing, want do you consider to be a good spec in terms of the numbers, in other words how low do want to go?
    •  
      CommentAuthorfostertom
    • CommentTimeDec 27th 2017
     
    On masonry, yes, hard plaster/render as the barrier - but still junctions etc where something flexible is necessary.

    On timber framed construction, rather than membranes/tapes, airtight but breatheable board glued and screwed at all joints with gapfilling adhesive. OSB is the obvious candidate but there's disagreement as to whether OSB is airtight, not quality-controlled from manufacturer's batch to batch. I've put my faith in the words I heard from Peter Warm, one of UKs three (?) PH Assessors - 'I've never yet found a building that failed PH airtightness because of OSB'.

    Glued and screwed OSB is very hard to install wrong, is within the skill range of everyday chippies, is robust in use - all the opposite of the dreaded tapes and membranes.
    • CommentAuthorRex
    • CommentTimeDec 27th 2017
     
    My house is timber framed and had a pre-build air tightness calculation of around 9 whatevers. I was very anal in taping everything and achieved 1.3 whatevers.

    I am also concerned about the durability of the tapes, so used the self-adhesive, aluminium duct tape on everything. It is inexpensive but somewhat fragile. My thought was, if the adhesive fails (which I suspect it will in time) the aluminium tape will not.

    For example, around all window frames, I lined the inner joint with tape, knowing that when the drylining was installed, the tape will be sandwiched in place. Likewise, the timber sole plate; the screed sandwiches the tape in place. Also the inside of the timber frames wherever there was a panel joint, the Warmcell insulation will hold the tape in place.

    Seven years down the line, it would be interesting to have the house re-tested but I have better things to do with my time and money!
    •  
      CommentAuthordjh
    • CommentTimeDec 27th 2017
     
    Posted By: nick1cDoes anyone know if the longevity of different methods of achieving good air tightness has been investigated and if so what the results were? It looks as if we may be in a position to build a house in a wet and windy site. I would love to construct it to achieve at least passivehaus levels of efficiency, but am concerned about the toll Atlantic gales will take over the years.

    There have been investigations, yes, although I don't remember the names of the authors or papers unfortunately. The Pro Clima and the Siga products are good and have both good theoretical/tested results of up to 100 years and practical results exceeding 30 years or so to date. There may be other good products but there are also horror stories about some products so caveat emptor.

    Plaster is good - it's my main airtightness barrier - but you need to think about where it is likely to crack and have a backup plan. Cracking depends on the substrate and the geometry. It's very likely at corners and junctions and having membranes and tapes behind the plaster (and keyed into it) at such locations is pretty much essential. Slab to wall, wall to roof, doors and windows, corners of the building etc. Some cracks can be seen and repaired but other locations will be hidden. Also think about penetrations etc. I just made a rule that there are no services in my external walls and nothing connected to it, except by glueing. Most people will want to find a different solution, I expect. Cracks form because of differential movement as the building dries out, no matter how solidly built, as well as movement due to wind.

    Medite make OSB products that are guaranteed airtight; I don't know about other manufacturers. The guarantee applies when the panels are taped together. I don't know whether anybody has done any testing of glued panel structures, nor how they deal with the omission of expansion joints. I've no reason to suppose that the system Tom describes doesn't work though.
  2.  
    Our house has the airtightness built into the fabric of the walls and roof in the form of Icynene and OSB. We didn't use airtightness membranes or tapes at all.
    • CommentAuthortony
    • CommentTimeDec 27th 2017
     
    I got to justless than one ach/m2/m3/h at 50Pa on my masonry build, no tapes used.
    •  
      CommentAuthorfostertom
    • CommentTimeDec 27th 2017 edited
     
    Posted By: djhMedite make OSB products that are guaranteed airtight
    And other manufacturers. But these are by coating the board, not by quality-controlling the through-and-through composition of the board. Consequently they may be airtight until surface-damaged (just like membranes) or the taping fails (just like 'tapes'!) but they also become vapour-impermeable. That fallacious durability and that non-breatheability rule them out as far as I'm concerned.
    • CommentAuthordelprado
    • CommentTimeDec 28th 2017
     
    I've thought about this too.

    I dont understand why people dont like foams - around a floor joist in a retrofit, and not exposed to uv, its airtight and can take quite a bit of movement (apparently).
    • CommentAuthorgravelld
    • CommentTimeDec 28th 2017
     
    While the foam itself can be airtight, it didn't mean the adhesive is 100% compete and the gap is 100% filled because of the nature of the expansion. Furthermore, only certain foams accept compression and none accept a high degree of expansion beyond curing point.

    Icynene has quite remarkable sponginess. It is actually like a bath sponge.

    I'd put up my head and question this reliance on plaster or render, unless something is added to guarantee elasticity. My own walls are not a pretty sight. Membranes are made for this purpose.

    djh any references I can read to on regarding the use of tapes with plaster?
    • CommentAuthortony
    • CommentTimeDec 28th 2017
     
    Re wet plaster, the good point about it is that cracks can be seen and fixes, I have virtually no cracks on the inside of my outside walls and only one or two minor ones on internal walls avowed doors.

    I plastered the membranes and the “tony tray” into the walls by trapping them being expanded metal.
  3.  
    I filled a lot of small gaps a good few years back, eg wall/floor under skirting, with expanding foam or silicone or acrylic caulk. Many of them have opened up again as the sealant aged and the building flexed.

    The best survival seems to have been where the bead of sealant was at least 5mm wide.

    I think the airtight layer needs to be on the inside where you will see it and maintain it. Where there are eg kitchen units in front of it, the draughts can go on for years before I happen to be doing something under there and notice the crack.
  4.  
    Posted By: fostertomOn timber framed construction, rather than membranes/tapes, airtight but breatheable board glued and screwed at all joints with gapfilling adhesive.


    We build with SIPs, additional internal insualtion, internal airtight barrier and tapes. Our neighbour did timber frame with external OSB and gapfill adhesive. Having seen/done both I'd go for OSB/Adhesive if I was building again. A lot less faff and the external shape of a house is a lot easier to work with than the internal. Also seems much less chance of damage to the airtight layer from internal works in future
    •  
      CommentAuthordjh
    • CommentTimeDec 29th 2017
     
    Posted By: gravellddjh any references I can read to on regarding the use of tapes with plaster?

    Both the Pro Clima and the Siga product literature describe their use quite well.
    •  
      CommentAuthorfostertom
    • CommentTimeJan 2nd 2018
     
    Posted By: Simon StillA lot less faff and the external shape of a house is a lot easier to work with than the internal. Also seems much less chance of damage to the airtight layer from internal works in future
    Exactly, Simon.

    Also, if the studwork voids are filled with blown-in cellulose rather than insulation roll or cut boards, the cellulose itself is pretty airtight (tho not ideally enough on its own) so any accidents/later punctures of the (well shielded) OSB is backed up by in-depth supplementary airtightness - so the overall spec is very robust longterm.

    That's in addition to the great virtues of blown-in cellulose - first, being organic material, it's hygrocopic (a separate topic) and second, because it fully fills all gaps without joints and remains under slight elastic compression, the actual in-use insulation value it delivers is far beyond its modest nominal lamda, compared with the alternatives mentioned above.
    • CommentAuthortony
    • CommentTimeJan 2nd 2018
     
    Are we talking wet or dry blown in? I thought settlement was a significant issue, certainly it has been in the past.
  5.  
    ''hygrocopic (a separate topic)''

    Poetry!

    Insulation across the nation
    •  
      CommentAuthorfostertom
    • CommentTimeJan 2nd 2018
     
    A poet - and he didn't know it.

    Posted By: tonyAre we talking wet or dry blown in? I thought settlement was a significant issue, certainly it has been in the past.
    Def dry - good practice shd rule - will look out the video.
  6.  
    Is there any reason the same approach (external/sealant adhesive joints) wouldn't work on a SIP or CLT construction?
    • CommentAuthorborpin
    • CommentTimeJan 5th 2018
     
    Posted By: PeterStarckOur house has the airtightness built into the fabric of the walls and roof in the form of Icynene and OSB. We didn't use airtightness membranes or tapes at all.
    +1
    •  
      CommentAuthorfostertom
    • CommentTimeJan 6th 2018
     
    Posted By: Simon StillIs there any reason the same approach (external/sealant adhesive joints) wouldn't work on a SIP or CLT construction?
    How would that work, when the outer skin stops at the edge of ea panel i.e. doesn't bridge unbroken across joints or between elements? In other words it's full of joints to be sealed by some method, vs eliminating such joints by making the outer skin continuous. The challenge is not the general surface of the panels, but the joints between them.
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