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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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    • CommentAuthorLoftDIY87
    • CommentTimeMay 15th 2020
     
    Have resumed my DIY loft conversion during the lockdown (no excuses now!) and am almost ready to start plasterboarding. I had a builder do the structural work ie. hip to gable loft conversion with rear box dormer, and I'm doing all internals myself.

    However I'm not sure if I need a vapour barrier on the stud walls? The exterior walls are tiled with OSB (except for gable wall, which is rendered OSB). There is 90mm Celotex insulation in between the studs and I will be putting 12.5mm plasterboard over this, which will be skimmed and painted.

    Should I staple a vapour barrier to the studs before I plasterboard, or is it overkill?

    Or should I at least foil tape the insulation joins and create a vapour barrier that way?
  1.  
    ''Should I staple a vapour barrier to the studs before I plasterboard, or is it overkill?''

    Yes, I think you should, and no, it isn't.

    I don't rely on foil tape. In some cases the adhesive is not that good, and it generally is not that strong.

    If you paint the plaster with vinyl silk you will, in effect, be applying a VCL, but I'd keep it behind the plasterboard.

    If you want to be really obsessive you tape over the staples.
    • CommentAuthortony
    • CommentTimeMay 15th 2020
     
    Definitely is a vb, all coined up to the one in the ceiling AND the walls of the house below
    • CommentAuthorLoftDIY87
    • CommentTimeMay 16th 2020
     
    OK I'm thinking to go for this for the walls and ceilings, should just about cover it all:

    https://www.insulationshop.co/visqueen_vapour_barrier.html

    Would that be adequate?

    Also I got 120mm celotex in between rear dormer flat ceiling joists and rafters, and 25mm under them.

    However only 90mm in between exterior wall studs. Building control is fine with that but someone advised I should really have 15mm or so over the stud walls too. However bit late as it would involve me redoing the bathroom plumbing, which I really don't want to do!

    Would 90mm celotex in between studs + Visqueen vapour barrier suffice and not cause me any condensation problems?
    • CommentAuthortony
    • CommentTimeMay 16th 2020
     
    Probably OK but anywhere there are double studs or cold bridges round windows there are risks
    • CommentAuthorLoftDIY87
    • CommentTime2 days ago
     
    My house itself is only 3 years old so the rest of the house is already packed with insulation in the walls than 99% of homes in the country.

    I know my loft is already going to get extremely hot once it's complete (it was reaching 38-40 degrees last summer BEFORE I put any insulation in!) - so for me, the less insulation in the loft, the better.

    As I've already put 120mm in between dormer roof joists and pitched roof, plus 25mm under joists/rafters, there's plenty of insulation in the roof.

    Can I just leave it as 90mm Celotex, without the vapour barrier and without insulation over the studwork? I'm concerned the vapour barrier would also help keep heat in the house, which is strangely the opposite of what I want up there.

    What is the actual purpose of insulation over the studwork? I understand it's because the studs are not insulated, however if I do NOT use any insulation over the studs, would I then definitely need a vapour barrier over the walls to stop cold bridging? Does cold bridging cause condensation?
    • CommentAuthorJonti
    • CommentTime2 days ago
     
    I would have thought that more insulation would mean your loft would be cooler in the summer. Insulation reduces the transfer of heat through it and works on both directions.

    Or am I wrong? :confused:
    • CommentAuthortony
    • CommentTime2 days ago
     
    +1 I have 300mm of fibreglass in walls, 500mm in loft, lovely and cool in the summer, keeps heat out too
    •  
      CommentAuthordjh
    • CommentTime2 days ago
     
    Posted By: LoftDIY87someone advised I should really have 15mm or so over the stud walls too. However bit late as it would involve me redoing the bathroom plumbing, which I really don't want to do!

    Sorry, I realise my comment is very late to the party, but how does putting insulation over the stud walls affect your plumbing? Unless, of course, the proposal is to put more insulation internally (generally a bad idea when compared with external insulation). But even so, I'm not clear how it affects the plumbing.
    •  
      CommentAuthordjh
    • CommentTime2 days ago
     
    Posted By: JontiI would have thought that more insulation would mean your loft would be cooler in the summer. Insulation reduces the transfer of heat through it and works on both directions.

    Or am I wrong?

    +1
    •  
      CommentAuthordjh
    • CommentTime2 days ago
     
    Posted By: LoftDIY87What is the actual purpose of insulation over the studwork? I understand it's because the studs are not insulated, however if I do NOT use any insulation over the studs, would I then definitely need a vapour barrier over the walls to stop cold bridging? Does cold bridging cause condensation?

    A vapour barrier has nothing to do with preventing cold bridging. Cold bridging is to do with insulation. Vapour barriers are to do with water vapour movement, which can cause condensation within the walls and subsequent decay of timber and anything else organic. Vapour-permeable air barriers ('breathable membranes) are to reduce the passage of air (wind) and so reduce heat loss.

    You need insulation to reduce heat loss, which reduces the fuel consumed and the global warming caused. It also makes the interior a lot more comfortable.

    You need a vapour barrier (or a condensation risk analysis that shows one isn't needed) in order to prevent condensation that will cause decay that causes your house to fall down.
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