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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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  1.  
    The proposal is to insulate a concrete ceiling (beam and block with 8cm concrete over) with glass wool. Is there any advantage or need to put a VCL (aka plastic sheet) between the ceiling and the glass wool or would any water vapour coming through the concrete go through the glass wool and be dealt with by the loft ventilation without issues.
    • CommentAuthortony
    • CommentTimeFeb 20th 2019
     
    I am worried about draughts in and through the beam and blocks bypassing your insulation. If it were mine I would do combined vapour and air tightness barrier under the beam and blocks and insulate any voids in it after having made doubly sure no draughts can get in round the edges.
    • CommentAuthorgyrogear
    • CommentTimeFeb 20th 2019 edited
     
    Certainly not to contradict Tony, but having two such concrete floors (crawlspace with bedrooms above, and garage with lounge above), in my experience, draughts are no consideration at all as the structure is very well jacked, with EWI to boot.

    However, I readily admit that with a loft there may well be draughts, which ought to be busted first ??

    I'd be inclined to think no VCL required becos upward vapour drive from the room will naturally trend towards the cooler loft. Also, concrete being water-permeable (unless it is waterproof concrete...) then the material ought to absorb any vapour.

    (Presumably the room is heated, whence proposed insulation works...).

    Temperature-wise, the concrete ought to be pretty stable for most of the year (because aborbing heat from the heated volume in winter, and aborbing solar gain from roof in the summer); meaning to say that there is no real cold interface on which vapour would condense; and in event of condensing, wet ought to wick up into the (dry) concrete by capillary action.

    On the contrary, adding a VCL might be asking for trouble at a particular time of the year, viz. deep mid-winter when heating on at the max and thermal mass struggling a bit to keep up...

    Now if you are thinking of installing a pool or a spa, sauna, I might have to think again !

    gg
  2.  
    Posted By: tonyI am worried about draughts in and through the beam and blocks bypassing your insulation. If it were mine I would do combined vapour and air tightness barrier under the beam and blocks and insulate any voids in it after having made doubly sure no draughts can get in round the edges.

    No issue with draughts through the beam and block because there is 8cm of concrete over the beam and block and the underside is rendered. No chance to retro-fit an air tightness/ VCL layer below the beam and block but I reckon the render under and 8cm concrete over is fairly air tight anyway.

    It is moisture vapour coming up through the concrete ceiling I am concerned about. Would putting a VCL on the warm side of the insulation which is the cold(er) side of the concrete, given the insulation won't be 100% insulating, be of any benefit, or could it be detrimental in preventing any moisture wicking up through the insulation to be removed by the loft ventilation
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