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Green Building Bible, Fourth Edition
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    • CommentAuthorslow
    • CommentTimeMar 14th 2018 edited
     
    Hi all

    I've not posted in a long time, but am back in the throes of a timber frame extension. I'm debating whether I need a VCL in the wall build-up or not.

    Wall build-up, inside to outside is:

    skim
    12.5mm p/board
    25mm service void
    18mm OSB (structural as anti-racking)
    140mm deep studs (infilled with either Frametherm 32 slabs or Steico Flex, at 140mm deep)
    EWI 60mm Steico woodfibre boards
    Lime Green's lime render finish (their Warmshell system)

    So, since I have the OSB layer on the internal face of the studwork, would taping the joints be sufficient for this to perform as the VCL, or ought I to install a separate VCL behind the OSB?

    If I do install a separate VCL behind the OSB, would I then tape over the heads of each of the fixings (as these will obviously pierce the VCL)?

    Many thanks for your thoughts & experiences
    • CommentAuthortony
    • CommentTimeMar 15th 2018
     
    VCL to protect against future damage from moisture in the building diffusing through the walls and causing mould or rot problems
    •  
      CommentAuthorfostertom
    • CommentTimeMar 15th 2018
     
    VCL obstructs half of the vital seasonal re-drying process.

    What part of UK (?) are you? What's the local climate? On the face of it, your build-up should work fine as 'breatheable', less well with internal VCL.

    Except you need an airtight layer, which can be airtight but vapour-open - doesn't have to be a VCL.

    The OSB if glued and screwed could be your airtight but breatheable layer, but I wd put it on the outside of the studs (no penetration by joist ends), and not bother with the inboard service void (just plasterboard on the studs, freedom to puncture for wiring etc).
    • CommentAuthordelprado
    • CommentTimeMar 16th 2018
     
    I dont think either of the above comments are right are they?

    A VCL simply slows down moisture transmission, so you dont get spikes and otherwise allows substrates to buffer and release in a more controlled fashion?

    A very good VCL can do this in both directions depending on how humidity shifts (usually to outside in in the summer(
    • CommentAuthortony
    • CommentTimeMar 16th 2018
     
    No a vcl stops vapour and moisture moving through it, ie polythene
  1.  
    Out of interest, if you did decide to use a VCL, why would you put it *behind* the OSB? Simply because of the risk of damage during works?

    You asked: ''So, since I have the OSB layer on the internal face of the studwork, would taping the joints be sufficient for this to perform as the VCL, or ought I to install a separate VCL behind the OSB? ''

    As others have said, the OSB is, on its own, highly unlikely to do the job as a VCL but, taped at all joints and perimeters, it may do the job as an air-tightness layer. The big issue (to which I think you are trying to get an answer) is whether your lay-up will work as a 'safe', fully-breathable, moisture-managing structure and thus not *need* a VCL. The attraction of this (no VCL) route is that if your lay-up does not require a VCL, then there is no VCL to install incorrectly or to get damaged, at the time of works or at any time in the future life of the building.
    •  
      CommentAuthorfostertom
    • CommentTimeMar 16th 2018 edited
     
    Several kinds of membranes -
    Vapour Control Layers (VCLs) e.g. polythene are complete vapour barriers (as long as the tapes hold and the nailholes don't leak etc etc);
    Breather felts are vapour-transparent but liquid-water proof and may also be airtight;
    Air barriers are airtight but may be vapour transparent;
    'Intelligent' membranes vary their vapour resistance from near-VCL to fairly vapour tansparent, depending on surrounding Relative Humidity (RH).

    A membrane can be airtight but vapour open but not the other way round.

    A sheet of OSB, or many other solid materials, can function as different types of such membranes, usually in a more durable way.

    The joints and junctions are usually much more of a challenge, cost and risk, than the sheet of material itself.

    Much misunderstanding and much mis-naming of products - the manufacturers being as guilty as anyone, of ignorance and misleading. So research which type (if any) you need and carefully check what you're buying.
  2.  
    As FT says, most VCLs, intelligent membranes, air barriers etc perform (probably) 'as described on the tin' *in their sheets*. The joints are only as good as the joints! I know many are concerned re tapes, and I use a lot of them, but I do not now use foil tape. I have found the strength and adhesion so variable, and that they have a tendency to rip when put under any sort of strain - at a 90 degree ceiling-to-wall junction for example. Even with strong tapes, engineer in some form of 'slack', if you can, at joints which may 'pull'.
    • CommentAuthorCWatters
    • CommentTimeMar 16th 2018
     
    So, since I have the OSB layer on the internal face of the studwork, would taping the joints be sufficient for this to perform as the VCL, or ought I to install a separate VCL behind the OSB?


    I believe OSB is considered to have medium permeability.

    What does building control say you need?
    • CommentAuthordelprado
    • CommentTimeMar 16th 2018
     
    Tom - a VCL is absolutely not and should not ever be a complete barrier to moisture.
    •  
      CommentAuthordjh
    • CommentTimeMar 16th 2018
     
    Posted By: delpradoTom - a VCL is absolutely not and should not ever be a complete barrier to moisture.

    Would you mind explaining that a bit, Paul? As Tony and Tom have said, the most common VCL is a polyethylene sheet and that is pretty vapour tight and, I believe, is intended to be so.
    •  
      CommentAuthorfostertom
    • CommentTimeMar 16th 2018
     
    'To all intents and purposes' polythene is a complete vapour barrier - though metal foil has far higher numbers, and certain 'absolute' composites even higher, the difference is negligible, for building purposes.
    • CommentAuthordelprado
    • CommentTimeMar 16th 2018 edited
     
    No point repeating what this blog post sets out well - you don't want to block everything because of "reverse condensation" risk in the summer

    https://www.backtoearth.co.uk/blogs/resources/what-is-a-vapour-control-layer

    NBT did a good presentation on this point

    If you look at the data of many decent membranes one notes they are let some stuff through

    https://proclima.com/products/internal-sealing/intello-plus

    Airtight, yes, but they are water vapour permeable
    • CommentAuthordelprado
    • CommentTimeMar 16th 2018
     
    The key distinction is the word - vapor control - its not a vapour check which blocks everything, it merely controls the speed of it passing through
    •  
      CommentAuthordjh
    • CommentTimeMar 16th 2018 edited
     
    Paul, I think you need to read a bit more widely before making such categorical statements as you did. I think you'll find both Tom and Tony have a lot more experience of building than you.

    You would then find that VCL is commonly used for all membranes that block vapour at least some of the time. Yes, it is possible to use the phrases 'vapour control layer' and 'vapour check' or 'vapour barrier' to mean different things but most people don't. So polyethylene sheet is a VCL and it is a vapour barrier. Intello is a VCL but is not a vapour barrier all the time. Go check out https://www.roofingsuperstore.co.uk/browse/flat-roofing/vapour-control-layers-vcl-1.html if you want some examples of what gets called a VCL.

    The backtoearth article is not terribly good, IMHO. It doesn't cite any sources for its claim that "it has since been found that this causes the accumulation of moisture inside the wall during the summer months", specifically to any extent that does cause damage. It's also clearly wrong to claim "The high humidity levels and warm temperatures combine to form perfect conditions for mould and rot to thrive. This is problematic to both the timber structure but also to the inhabitants of the building as mould spores are well known to cause respiratory problems and ill health." since the condensation is on the OUTSIDE of the membrane if it does form and not inside where the people are.

    Note that I don't have any problems with 'intelligent' or variable permeability membranes - I have a lot of Intello Plus in my roof. But neither do I have a problem with a polyethylene VCL - I have that in my sun room. Or no VCL at all - as in my walls.
    • CommentAuthordelprado
    • CommentTimeMar 17th 2018
     
    I'm just being a parrot and repeating best practice.

    For citation look at all the research on reverse condensation
    • CommentAuthorjfb
    • CommentTimeMar 17th 2018
     
    I think the point is it is hard to know what best practice actually is if you are a parrot!
    • CommentAuthorTimSmall
    • CommentTimeMar 21st 2018
     
    You can make various generalisations and look at known-proven designs, but there are many variables which can change things in sometimes subtle ways. Best to get a clear picture by carrying out a simulation to BS EN 15026 (e.g. using Wufi).

    In general in a UK climate, if you have a ventilated rainscreen construction, good airtightness (at inner edge, or at most 50% of the way through the insulation build-up), and approximately symetrical vapour permeability when looking across the cross-section of the wall (or more vapour resistant towards the inner edge), then with normal domestic indoor conditions, then a VCL isn't needed.
    • CommentAuthorIan1961
    • CommentTimeMar 21st 2018
     
    @slow
    British Standard BS 5250:2011 - condensation control in framed buildings is the guide that would govern the approach of most Building Inspectors when looking at this issue.

    On page 55 it says that a VCL with a vapour resistance of at least double that of the [external] sheathing should be provided on the warm side of the insulation.

    (Interestingly the previous superceded version of BS 5250 recommended that the materials on the warm side should have a total vapour resistance of at least 5 times the sum of the vapour resistances on the cold side)
    •  
      CommentAuthorfostertom
    • CommentTimeMar 21st 2018
     
    Posted By: Ian1961(Interestingly the previous superceded version of BS 5250 recommended that the materials on the warm side should have a total vapour resistance of at least 5 times the sum of the vapour resistances on the cold side)
    And it's interesting that old rule of thumb has been rightly abandoned. I tried and tried to find any original justification for it - nothing - just a 'common sense' notion based on a fallacious understanding of how vapour flows/is resisted through a wall sandwich.

    Posted By: Ian1961BS 5250:2011 ... says that a VCL with a vapour resistance of at least double that of the [external] sheathing should be provided on the warm side of the insulation.
    That BS goes back to the 1983 World In Action exposee of emerging condensation problems in newfangled timber frame housing, which destroyed that industry for 15yrs.

    In assuming, Glaser-style, that the condensation came from inside and moved by permeation through the wall sandwich to the dew point layer, the obvious answer was to stop that by mandating an internal VCL. This did indeed reliably cure the problem, at the levels of insulation then current - but less reliably at current Building Regs levels and very dubiously at near-PH levels.

    In fact, IMO, the VCL's effectiveness was by acting much more as an airtightness membrane, that as a leak-ridden VCL. We now know that vapour transport is at least as much, if not more, a matter of bulk-air movement, than of permeation.

    This newish understanding, combined with ever-higher insulation levels, should lead to a complete re-think of BS5250. All old rules-of-thumb and coventional wisdoms need to go. IMO they led to the disaster at the pioneering near-PH Dartington Primary School, which due to 'leaks' has been torn down and being rebuilt 'conventionally' at vast insurance cost, to the ruin of the up-and-coming architectural practice involved.

    Posted By: TimSmallairtightness (at inner edge, or at most 50% of the way through the insulation build-up), and approximately symetrical vapour permeability when looking across the cross-section of the wall (or more vapour resistant towards the inner edge)
    looks like just that conventional wisdom - Tim you mention WUFI - I agree that VCLs are broadly not necessary in UK climate but the rest of it - justified in your explorations in WUFI? It's well contradicted in mine.
    •  
      CommentAuthordjh
    • CommentTimeMar 21st 2018
     
    Posted By: fostertom
    Posted By: TimSmallairtightness (at inner edge, or at most 50% of the way through the insulation build-up), and approximately symetrical vapour permeability when looking across the cross-section of the wall (or more vapour resistant towards the inner edge)
    looks like just that conventional wisdom - Tim you mention WUFI - I agree that VCLs are broadly not necessary in UK climate but the rest of it - justified in your explorations in WUFI? It's well contradicted in mine.

    Eh? I'm not sure what you think is conventional in Tim's statement? Or controversial.

    If there is an airtightness barrier (and there should be at least one shouldn't there?) then it definitely needs to be in a place that is warm enough that it isn't going to suffer from lots of condensation forming on it, thus towards the inside of the insulation, no?

    And again, having uniform insulation, or the more vapour-resistant in the warmer part, doesn't sound odd to me. What does your WUFI show?
    • CommentAuthorgoodevans
    • CommentTimeMar 21st 2018 edited
     
    Posted By: djhIf there is an airtightness barrier (and there should be at least one shouldn't there?) then it definitely needs to be in a place that is warm enough that it isn't going to suffer from lots of condensation forming on it, thus towards the inside of the insulation, no?
    Not necessarily - the air tightness layer can easily be vapour permeable, and if so it is no more likely to have condensation on it than the material either side of it. e.g. Tyvek Housewrap (vapour open, airtight, and moisture resistant).
    •  
      CommentAuthordjh
    • CommentTimeMar 21st 2018
     
    Posted By: goodevans
    Posted By: djhIf there is an airtightness barrier (and there should be at least one shouldn't there?) then it definitely needs to be in a place that is warm enough that it isn't going to suffer from lots of condensation forming on it, thus towards the inside of the insulation, no?
    Not necessarily - the air tightness layer can easily be vapour permeable, and if so it is no more likely to have condensation on it than the material either side of it. e.g. Tyvek Housewrap (vapour open, airtight, and moisture resistant).

    Oops, you're quite right, sorry.

    There were problems with breathable membranes in icy conditions, though, which led NHBC to require ridge ventilation in roofs. So I suppose that means if the membrane can get cold enough to freeze, then it's no longer vapour permeable and is subject to condensation forming.

    It's all a very complicated subject :cry:
    • CommentAuthorgoodevans
    • CommentTimeMar 21st 2018 edited
     
    In the UK - provided you keep the rain out - no single layer common building material will have problems - i.e. a solid wood external door with a wood stain external surface is fine - we all know its fine - even if the inside is poorly ventilated dwelling.

    With multiple layers of materials things get complex fast - for example I had not appreciated the massive quantity of inter-seasonal moisture that concrete, blocks, brick and wood adsorb and release each season (completely ignored in simple condensation analysis often requested by BCO's).

    To reduce the risk of damage causing moisture - have either zero or one VCL layer.

    If you have a VCL layer ensure half of the insulation is on the outside of the layer to be safe. A VCL on the outside surface (e.g. flat roof or rain screen) must be ventilated on the dry side.

    And note that the VCL forces inter seasonal moisture from the outside to have to go back out the way it came in. In addition any leaks will have only one direction to dry out in.
    •  
      CommentAuthorfostertom
    • CommentTimeMar 21st 2018 edited
     
    Posted By: djhwhat you think is conventional in Tim's statement? Or controversial.
    Actually, novel guidelines, but of the same kind, poss same basis, as the 'conventional wisdom'.

    The first one (inside or mid airtightness), goodevans nailed - thank you.
    The other one - symmetrical vapour permeability - differs interestingly from the old 5:1 or 3:1 guideline but still not valid according to my WUFI playing. Permeability distribution can be any number of ways - but shd be checked in WUFI. One day maybe we'll have enough experience to generalise rules-of-thumb.
    Likewise
    Posted By: djhhaving uniform insulation, or the more vapour-resistant in the warmer part
    don't think I have any WUFI experience on that one.
    • CommentAuthortony
    • CommentTimeMar 21st 2018
     
    I did not like the notion that suggested that in the UK climate we don’t need vapour barriers that is not correct and will put some forms of construction at risk.
    • CommentAuthorIan1961
    • CommentTimeMar 23rd 2018 edited
     
    Posted By: fostertomThis newish understanding, combined with ever-higher insulation levels, should lead to a complete re-think of BS5250. All old rules-of-thumb and coventional wisdoms need to go. IMO they led to the disaster at the pioneering near-PH Dartington Primary School, which due to 'leaks' has been torn down and being rebuilt 'conventionally' at vast insurance cost, to the ruin of the up-and-coming architectural practice involved.


    I read that the problems at Dartington were caused by leaks rather than condensation issues.
    • CommentAuthorIan1961
    • CommentTimeMar 23rd 2018 edited
     
    .
    • CommentAuthorslow
    • CommentTimeMar 26th 2018
     
    Thank you everyone - your responses reflect the information I had researched, which left me thinking I hadn’t understood any of it, but now can see it’s that I was looking for a ‘right answer’, assuming there would be one clear, accepted & established build-up. Silly me ;)

    Taking on board what I have researched, and your explanations, I decided to go ahead with a foil VCL behind the OSB. The outside of the frame is still open, with no infill insulation yet, so the joints have been taped, as well as the foil being continued around openings and taped. It is probably not to PH standards, but I’m on site and made sure care was taken with all the details. My BC officer has some very fixed ideas and isn’t the easiest of characters to work with - he’s top-down, with no discussion - so anything he disagrees with has to go through to his technical team for approval, which causes delays each time (I’m building under a building notice, not full plans). I’m fairly comfortable with my choice now, and hoping the woodfibreboard EWI and lime render do their part in allowing any seasonal moisture to leave happily ... fingers crossed!

    Thanks again
    • CommentAuthorTimSmall
    • CommentTimeMar 27th 2018
     
    Posted By: fostertom
    Posted By: TimSmallairtightness (at inner edge, or at most 50% of the way through the insulation build-up), and approximately symetrical vapour permeability when looking across the cross-section of the wall (or more vapour resistant towards the inner edge)
    looks like just that conventional wisdom - Tim you mention WUFI - I agree that VCLs are broadly not necessary in UK climate but the rest of it - justified in your explorations in WUFI? It's well contradicted in mine.


    Yes, it's largely informed by wufi, this is a generalisation based on the scenarios I've looked at. The vapour resistance statement is pretty uncontroversial isn't it? The location of airtightness layer is not from wufi modelling, but down to the risk of convection (e.g. around defects in the insulation) leading to condensation on the inner side of the airtightness layer. That's not to say that airtightness further out is impossible, it's just more risky IMO.
   
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