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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.

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    • CommentAuthormitchino
    • CommentTimeJul 5th 2020
    What's the thing to do when retro fitting PIR insulation around existing electrical cables? I'm installing 100mm PIR between ceiling joists and 100mm on top of the joists.
    • CommentAuthorCWatters
    • CommentTimeJul 5th 2020
    Ideally lift all cables so they are on top of the insulation.

    Otherwise there is this IEE Guide..


    It appears that 1mm^2 (or 1.5mm^2) lighting cable can be fixed to joists and covered with insulation provided they are protected by a 6A (or 10A) breaker. See para 5.

    Its not so easy for 2.5mm^2 ring mains, See para 6, or heavier cabmes (eg for shower or immersion heater) para 7.
    Mitchino, was your Q re 'tucking up warm' or chemical reaction between PIR and PVC? If both, as per CW re 'tucking up warm' and AFAIK, no known risk of chemical reaction between the insulant and the PVC sheathing. EPS and XPS are the ones which make the sheathing (a) brittle and (b) stick to the PS.
    • CommentAuthormitchino
    • CommentTimeJul 5th 2020
    I'm just trying to avoid making big holes in my PIR where cables come down through it for light sockets. The main cabling will lie on top in the void.
    • CommentAuthortony
    • CommentTimeJul 5th 2020
    Short lengths should be ok
    • CommentAuthorCWatters
    • CommentTimeJul 6th 2020
    I'd make holes and fill with wool insulation.
    • CommentAuthorphiledge
    • CommentTimeJul 6th 2020
    If its only lighting cable you need to deal with then you probably dont need to consider the cable heating. As your on a green forum its reasonable to assume youre running low energy lighting. A typical 5 watt LED draws 20 milliamps so you can have 50 on the circuit before you get to 1 amp load. The typical 6 amp protection for lighting circuits is a legacy from decades ago when everything was incandescent lighting.

    One thing to consider is if you have high wattage halogen security lights supplied by cabling buryied in the insulation then may want to clip that as in the IEE guide linked to above. Alternatively change the light for an LED flood and steam on with the insulation.
    Is the ceiling the airtight layer? If so, you might want to leave an access possible to the point where the cable comes through the ceiling, so you can check it is resealed after any work to the light fittings.

    I didn't :-(
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