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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
These two books are the perfect starting place to help you get to grips with one of the most vitally important aspects of our society - our homes and living environment.

PLEASE NOTE: A download link for Volume 1 will be sent to you by email and Volume 2 will be sent to you by post as a book.

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    • CommentAuthortony
    • CommentTimeSep 10th 2009
     
    I have been a builder for 30 years and only once used foil back board -- too expensive, naff -- how do you join the foil -- foil invariably damages

    I definitely prefer a vapour barrier which doubles as an air barrier and is continuous.

    Any other opinions?
    • CommentAuthorTimber
    • CommentTimeSep 10th 2009
     
    Separate membrane every time.

    Timber
    •  
      CommentAuthorfostertom
    • CommentTimeSep 10th 2009
     
    A construction that doesn't need a vapour barrier at all - that's the only robust solution - any membrane is guaranteed to spring a leak or part at the joints sooner or later (those who test houses for a living, say really sooner)
    • CommentAuthorJohan
    • CommentTimeSep 10th 2009
     
    Definately seperate if you're having one, 500g polythene if timber frame.

    Tony, what are you using in your masonary build?

    Tom,

    That depends how you join it. If you use double sided vulcanising tape with polythene and then squeeze it with a batten on top it won't leak in the joints for sure! There are other places for it to leak though I agree and a brethable wall might be a better option for a good indoor environment. :bigsmile:
    •  
      CommentAuthorfostertom
    • CommentTimeSep 10th 2009
     
    As I see it, a continuously clamped membrane joint doesn't need tape anyway - but a real challenge to make really continuous. And then there's still the rest of it, a magnet for punctures, whether looking short-term, or over a lifetime!
    • CommentAuthortony
    • CommentTimeSep 10th 2009
     
    puncture problem is almost the same for foil back boards

    how do you join up the sheets or dont you?
  1.  
    I thought foil back board was only used for what is discribed as a vapour control layer (vcl) rather than a vapour barrier (vb)
    • CommentAuthortony
    • CommentTimeSep 10th 2009
     
    same thing
    •  
      CommentAuthorfostertom
    • CommentTimeSep 10th 2009
     
    It's called 'vapour check plasterboard' because it's nowhere near being a real vapour barrier. What's the resistance of a mere vapour check? Could be anything, from near-perfect barrier, to negligible. So it's not even any help in those situations where you actually want a mild (but quantifiable) inboard vapour resistance e.g. in the 'breatheable wall' rule-of-thumb, where inboard resistance should be five times outboard, but all on the low-resistance end of the scale.
    • CommentAuthorTimber
    • CommentTimeSep 11th 2009
     
    James

    There is no such thing as a vapour barrier. You will never get a full barrier to the passage of moisture vapour. You ONLY need to control it. hence it is called a vapour control layer. The term barrier should not be used.


    As for punchers, so what? a SMALL puncher in a vcl will not cause a problem. with a timber frame wall, moisture vapour that does get into the wall will be able to dissapate.

    If there is a large hole or rip is a different matter, but there are many many ways to mitigate against that happening.

    As for breathable constructions, well whatever floats your boat. But even breathable walls have a vapour control layer! Its just not polythene.


    Timber
    • CommentAuthorJohan
    • CommentTimeSep 11th 2009 edited
     
    The puncture issue is very much dependent on the frame construction. If you build for example (outside->in):

    1. External cladding (E.g. OSB/Panelvent/woodfibre board)
    1. Vertical 38x89 with insulation between studs
    2. Horisontal 38x89 with insulation between studs
    3. Vapour barrier
    4. Vertical 47x47 with insulation between studs
    5. Internal cladding (E.g. Plasterbord/OSB/Prticleboard/timber)

    You'll have a 47mm cavity for your wiring and any screws in the walls, making puncturing a substantially smaller issue. (You'll also have 230mm of insualtion and pretty much eliminate any cold bridges, but that's a different story...)

    How you solve this in a masonary construction I don't know. I was hoping Tony would have some suggestions! :)

    Posted By: fostertomAs I see it, a continuously clamped membrane joint doesn't need tape anyway - but a real challenge to make really continuous. And then there's still the rest of it, a magnet for punctures, whether looking short-term, or over a lifetime!
    Clamping is good. However if you're trying to achieve a very high level of air tightness you should seal the joints properly.
    • CommentAuthorTuna
    • CommentTimeSep 11th 2009
     
    If you separate out air tightness and vapour control issues, and are aiming for an air tight structure, could humidity be controlled by a MHRV system?

    That is, if your structure is inherently airtight (eg. SIPs, Hempcrete, ICF or any of the continuous material construction techniques), then with MHRV the internal moisture levels should be maintained at a level where a vapour barrier is not necessary? I'm imagining the structure constantly 'breathing out' the moisture ingress rather than physically blocking it's path.

    This is an area where I struggle to understand the different advice and practices, particularly where what works with one construction technique appears to be completely counter to standard practice in another.
    • CommentAuthorTimber
    • CommentTimeSep 11th 2009
     
    As tuna mentions above, you must separate out the deffinition of a vapour control layer and an air tightness membrane.

    Timber
    • CommentAuthortony
    • CommentTimeSep 11th 2009
     
    Tuna -- I worry about you! you say,"then with MHRV the internal moisture levels should be maintained at a level where a vapour barrier is not necessary?"

    No this is certainly not true

    Indoor humidity levels will generally be higher than out door levels especially in the colder months -- even with MHVR. Severe problems will arise with timber construction where moisture condenses within the structure or insulation if there is nothing to stop it this will cause mould. rot and disasters.
  2.  
    If one was to plasterboard a lounge wall that joins onto a garage, what would be the preferred method? It was previously rendered

    The asbestos removal company has essentially stripped it back to brick....
    • CommentAuthorrevor
    • CommentTimeJun 9th 2021
     
    What I would do (not necessarily preferred) is dry line with Gyproc thermaline board has insulation and vapour control layers in one product.
    https://www.british-gypsum.com/products/gyproc-thermaline-pir?tab0=0
  3.  
    What is the building regulation where you are (in Wales?) for insulating walls that separate heated from unheated spaces ?

    In Scotland it requires U=0.22 for 'alterations' to existing walls, such as re-lining, so you need to add about 100mm thickness of PIR or 150mm of polystyrene. Higher standards apply for 'conversions' or newbuilds.

    What are you planning to do for insulation - once you decide that, then the airtightness and vapour measures should become clear?

    Could go on either the garage or the room side of the wall. Considerations about joining to any other wall/floor insulation that might be existing (or future).
    •  
      CommentAuthordjh
    • CommentTimeJun 9th 2021
     
    Why is today's conversation posted at the end of one that is twelve years old? Why isn't it a new one?
    •  
      CommentAuthorfostertom
    • CommentTimeJun 9th 2021 edited
     
    Was going to say 'fair enough, to benefit from the context of already discussed wisdom' - but I see in 2009 I'm still quoting the now discredited
    Posted By: fostertom'breatheable wall' rule-of-thumb, where inboard resistance should be five times outboard
    • CommentAuthortony
    • CommentTimeJun 9th 2021
     
    I like separate vb, cheaper too and can be joined to adjoining vb’s
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