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Green Building Bible, Fourth Edition
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Housing - Renovation: Rigid insulation and VCL

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  1.  
    • CommentAuthormattrgee
    • CommentTimeMay 14th 2019
     
    Hi all,

    Following on from my recent thread regarding the insulation of a suspended timber floor, I have a further question regarding the use of a VCL.

    As a recap, I am fitting Celotex between the joists of my suspended timber floor, all joints are being taped resulting in the joist tops also covered.

    I had intended to lay a polythene VCL over the Celotex and joists, however, I was advised at the weekend that the foil backing of the Celotex is a VCL in itself and laying a further polythene VCL over the top, could trap moisture. I was consequently advised not to do this.

    I have already bought a polythene VCL to lay, so really the question is: will laying it over the Celotex cause an issue or is a second barrier an advantage?
  2.  
    Your VCL, on the warm side of the sandwich, is designed to stop vapour *from the house* getting into the insulation layer and below. You are right that the taped Celotex could be a VCL in itself, if taped really well at all joints and perimeters. On the other hand you are going to be climbing all over the joists and insulation in your big boots, so is the foil as tight as you thought it was? I use a heavy-gauge VCL in such circumstances. Even if you do end up with some sort of gap between 'Celotex VCL' and 'VCL VCL', it's on the warm side, so I am not clear where the moisture that you might trap is going to have come from.
    • CommentAuthortony
    • CommentTimeMay 15th 2019
     
    I do poly for air tightness what will you be doing for that

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