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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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    • CommentAuthorMarkyP
    • CommentTimeJan 6th 2021
     
    Hi all,

    I'm renovating and extending my house and adding loft rooms. At the design stage I missed a pretty rotten cold bridge at the gable rafter line where the blockwork inner skin is a heat loss path to the outside. I have some marmox xps backer board which I am considering running a 300mm strip of along the roof line over the face of the inner skin blockwork fixed by tile adhesive and a few screws. The rest of the gable wall will be plastered with hardwall flush with the face of the marmox board, the joint taped and the whole lot then plaster skimmed. The marmox board is only 10mm of XPS but i have some spare and fitting will be easy enough, and it will at least reduce the cold bridge and extend the direct heat loss path to around 550mm - 300mm marmox plus rafter level insul depth so i think worth doing.

    Question is will the joint on the face of blocks between marmox and hardwall crack? I'll use wide scrim and I think it will be OK but plasterer is less sure. It will be all fixed to the same underlying blocks so ought not to be under much strain. I have done similar scrimmed joints between backing plaster and plaster board over same underlying blocks before without issue.

    Anyone ever tried anything like this? Am looking also at thicker marmox but too thick becomes a pain to match with the backing plaster so realistically max would be 20mm board or maybe 10mm aerogel/6mm mgo board but have just seen the prices!
    • CommentAuthortony
    • CommentTimeJan 6th 2021
     
    When I built my gable I omitted the inner skin of blocks all together tying the outer skin to the roof cavity insulation joins ceiling insulation

    Not quite sure what a gable rafter line is in your case
    • CommentAuthorMarkyP
    • CommentTimeJan 6th 2021
     
    A picture may help, the red line shows the position of the insulated board. Plaster joint with board on face of blocks at lower red line.
      wall2.jpg
    • CommentAuthorMarkyP
    • CommentTimeJan 6th 2021
     
    Sorry to be clearer the redline shows the proposed position of the marmox xps backer board at the area where the cold bridge is worst, fixed to face of inner skin following the rafter line on the gable. Below redline would be float and set wet plastered up to insulation board. Board and hardwall skimmed to flush finish, joint scrimmed
    • CommentAuthortony
    • CommentTimeJan 6th 2021
     
    Lintel looks bad too. Watch out for draughts entering anywhere they can.

    I see it cracking, I would do whole wall
    •  
      CommentAuthordjh
    • CommentTimeJan 6th 2021
     
    Where's the gable wall insulation?
    • CommentAuthorMarkyP
    • CommentTimeJan 6th 2021
     
    It's a cavity wall fully filled with mineral wool, subsequently EWI'd. The ewi carries over the face of door frame and lintel lip. The worst issue is the top face of the gable block inner skin which is uninsulated under the tiles. The wall cavity plus ewi amounts to 250mm of insulation, dritherm in cavity, graphite eps ewi.
    •  
      CommentAuthordjh
    • CommentTimeJan 6th 2021
     
    OK, thanks. I think your plasterer is right to be concerned but a wide scrim may well solve it. You're likely to get cracks at the junction with the roof anyway and the line along the edge of the Marmox should only be a hairline if it does occur. Decorator's caulk should fix if it does happen.
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