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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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Ventilation: Condensation under loftboard

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    • CommentAuthorbenawhile
    • CommentTimeJan 5th 2020
     
    I don’t so far have the problem, but a general question with regard to boarding over a loft storage space.
    I have found info relating to condensation forming on the underside of loft boards if an air space is not left between the top of the insulation and the underside of the board, but cannot find any definite statement about whether standard loft board is, or should be, water vapour permeable.

    There definitely is information on the net to imply that ”building materials “ such as ceiling plasterboard, ARE permeable, and so on pre 1970s houses there is a risk of condensation on the underside of loft board.

    Is it best to assume loft board is not permeable, or are there some board materials that have a certain amount of permeability?
    • CommentAuthortony
    • CommentTimeJan 5th 2020
     
    It is best to leave a 50mm through ventilated void between the top of the insulation and any boarding

    Even with an impermeable ceiling or vapour barrier this is still important
    • CommentAuthorbenawhile
    • CommentTimeJan 6th 2020
     
    OK, will do. Following from this, as I cannot verify whether standard loft board is vapour permeable, would I reduce risk of condensation under the boards if I drilled vent holes in them?
    • CommentAuthortony
    • CommentTimeJan 8th 2020
     
    yes, but insufficiently

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