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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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    • CommentAuthorajdunlop
    • CommentTime5 days ago
     
    Hi all, I have a friend who is going to gradually insulate his solid wall red brick house.

    He has completed the insulation in one room and got a building inspector in to sign it off. However despite running it past Building Control it has failed to be signed off.

    I will attempt to attach his wall build up.
    It is 40mm of Celotex PIR attached to the walls with foam adhesive and all taped up (very neatly) to create a continuous vapour barrier.
    Studwork (63mm) in in front of this has 50mm of sheep's wool insulation fitted.
    All to be overboarded.

    The problem the building inspector has is the danger of condensation on the metallic vapour barrier on the PIR because it is behind the sheep wool making it a cold surface. They have suggested sticking a polythene layer over the studwork but my friend is reluctant to do this. The reason for doing it the way he did was to keep the vapour layer buried and protected from punctures and for the breathability and humidity buffering properties of the sheeps wool.

    What do you all think of the inspector's verdict? Any suggestions of what can be done?
      wall_buildup.jpg
  1.  
    Has he run a condensation calc? Free online software.
  2.  
    Looks like the screenshot is from a simple condensation model - what does that predict?
    Those models are not very good with materials like wool and brick, but probably not too bad with PIR.

    Roughly half the insulation resistance value (R) is inboard of the foil, which does seem like a bit much, unless there's something to support that approach? The foil will be roughly halfway between indoor and outdoor temperature in this model (not accounting for thermal mass of bricks) but the foil will see indoor air dewpoint, which is higher (worse) than halfway between indoor and outdoor temperature. (Not accounting for water absorption of wool).

    How about making the wool a bit thinner?
    •  
      CommentAuthordjh
    • CommentTime5 days ago
     
    Why not put the poly membrane in as the BCO asks and get it signed off? Then just ignore it, since your friend believes that there's already a working vapour barrier protected behind the thermafleece. Job done.
  3.  
    Posted By: WillInAberdeenHow about making the wool a bit thinner?

    I don't like the idea of reducing insulation to solve an otherwise solvable problem


    Posted By: djhWhy not put the poly membrane in as the BCO asks and get it signed off? Then just ignore it, since your friend believes that there's already a working vapour barrier protected behind the thermafleece. Job done.

    Classically the VCL goes on the warm side of the insulation and unless you can show a bullet proof model of no condensation I would side with the BCO.
    I agree with djh - put up the poly membrane, but leave it there unless you can show a model of no condensation up to say 85% RH with -5deg.C outside. Wet sheeps wool is a very bad idea.
    • CommentAuthorjms452
    • CommentTime5 days ago
     
    IMO the BCO is correct. The interface between the wool and Celotex will end up going mouldy.

    If they are worried about small defects in the VCL they can put in a smart membrane that will let any moisture escape in the in the summer in place of poly (e.g. Intelleo plus)
    • CommentAuthorajdunlop
    • CommentTime5 days ago
     
    Thanks for the replies. Doesn't sound like there is any great alternative to putting something on the room side of the wool.

    I did think a smart / intelligent membrane might be the way to go but quite a large unexpected cost (also doesn't help with the puncture issue).
    •  
      CommentAuthordjh
    • CommentTime5 days ago
     
    Posted By: ajdunlopDoesn't sound like there is any great alternative to putting something on the room side of the wool.

    Would have been better to plan ahead and put 50 mm or more of the PIR and less wool?
  4.  
    Posted By: ajdunlopThe reason for doing it the way he did was to keep the vapour layer buried and protected from punctures and for the breathability and humidity buffering properties
    How about: swap the sheep wool for 60mm of EPS or PIR to fill the studs. Poly VCL over the studs. Add 25mm cross battens in front of the VCL and fill that space with sheep wool and services, plasterboard on the front (or woodfibre and lime plaster instead of the wool/batten/pb).

    Gets the humidity buffering, the VCL is on the warm side of most of the insulation but is safely buried behind the services, no organic insulation behind the VCL.
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