cavity wall insulation, wall joiners and BCO

Discussion in 'Builders' Talk' started by martin68, Aug 11, 2018.

  1. martin68

    martin68 New Member

    extention 1.PNG insulation1.PNG Hi guys, again! I recently posted a thread about my BCO wanting me to install cavity wall trays over my lean to roof extension, I did this to keep him happy, but now he's picked up on another issue in the fact that I have placed insulation boards (celotex) between my cavity which is only 50mm without an air gap (i had to build above existing 50mm cavity walls which already had insulation in the cavity. My BCO said that i will get damp through the wall (it will be a pebble dash rendered wall) so I will have to clad the exterior to prevent this.
    I have been looking at many "pro" builds on youtube since, and have found that many are placing insulation without a gap into the cavity, see attached screen shots. So why is my BCO telling me I can't do this?
    Also now i'm worried for his next visit, that I built a cavity extension wall " T " from my excisting house wall but have only used wall joining bars and not removed the blocks in between to create a vertical damp break.
    But again, looking at the pro videos on youtube all they use is wall joiners without creating a damp break, again see pic attached.
    So before he pics up on that, is it allowed to simply but a cavity wall against an external existing wall without a vertical damp break?
  2. Jord86

    Jord86 Well-Known Member

    Furfixes/starter bars are fine. You butt a cavity wall up to it but run a 300mm disc cutter through the existing wall down the centre of where your new cavity will go right the way through the wall, then insert a vertical DPC all the way up to wall-plate level.
    blarblarblarblar likes this.
  3. Jord86

    Jord86 Well-Known Member

    What your brickies have placed is not celotex, its rock wool slabs. I agree with the BCO that you need a gap inside the cavity.
    blarblarblarblar and stuart44 like this.
  4. stuart44

    stuart44 Active Member

    As above those are cavity batts. Normal insulation boards require a cavity, although there are some types that can be used as full fill.
  5. stevie22

    stevie22 Active Member

    Those batts are normally full fill.
  6. sospan

    sospan Well-Known Member

    As the previous posters have said, there are quite a few varieties of insulation and each type has its own fixing and gap requirement. The ones in the pictures are designed to fill the cavity and don't let moisture cross the cavity. Celotex needs a gap and special fixing on the wall ties to hold it in place.

    If there is an existing build that has been up for over a year building control are rarely interested unless it is dangerous
  7. diymostthings

    diymostthings Well-Known Member

    You could ask if 50mm cellotex with foil faces would be acceptable...
  8. stuart44

    stuart44 Active Member

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