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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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    • CommentAuthoralm
    • CommentTimeAug 13th 2021
    Hi all,

    After many of your helpful comments a couple of months back, we are finally in our new (old) house and decided to start at the top ... There is an attic bedroom up there but we were warned by the previous owner that there is no insulation in the roof. I want to pull the ceiling down and vault it up to the inside of the ridge, so perfect opportunity to get a load of insulation in there. I would also like to put a roof light in, we are surrounded by lovely but massive beech trees which makes the room quite dark. And whilst we are at in, start with some internal wall insulation, so just after a bit of advice;

    1. I was planning on using rigid insulation in the roof (with void above) as I know this doesn't have to be breathable, there is only a fairly small area of external wall in this room, I was thinking with a small area we should be okay just sticking the PIR boards (or whatever we use) on the wall as well - any thoughts on this? (Planning on using IWI probably cork/ wood fibre combined with aerogel around bay windows for the rest of the house)

    2. Any recommendations for ethically sourced roof lights with good U-Values and thermal performance?

    3. In terms of phasing, if we insulated the walls in this room, and later on (hopefully months not years) we do the bedroom below, and ideas / experience on the best way. Do we take it down under the floor to the top of the ceiling? (I'm going to try to attach a drawing)

    I'm sure many more questions will come up once we start, but I think that should do for now!

    • CommentAuthoralm
    • CommentTimeAug 13th 2021
    Struggling to add the drawing - hopefully this works!

    You'll want a small area of dropped ceiling below the ridge in order to allow your eaves-to-eaves cross-ventilation. If you have a breathable membrane you want a 25mm gap between insulation and slate/tile batten and if an older, non-permeable membrane, or no membrane, a 50mm gap.

    Yes, you can use PIR on the wall. I would parge-coat with lime plaster first as an air-tightness layer, glue the PIR on with low-expansion adhesive foam in full-perimeter beads with frequent cross-hatchings to reduce the risk of convection behind the boards, tape all joints and perimeters as VCL (or add a VCL if your foil has dinges), then batten vertically or horizontally. You can add a further 25mm PIR in here, or use it as a service void. Plasterboard on top. If the first layer is 50mm then with a further 25 between battens you should get a compliant 0.3W/m2K.

    Take the parge coat down below the floor to the ceiling, as you say, and prime and tightly tape joist ends.

    Ceilings will need 125-150 for 0.18W/m2K.

    Sorry, no knowledge of roof-lights!
    • CommentAuthoralm
    • CommentTimeAug 14th 2021
    Thanks Nick,

    Yes I was planning a small dropped area for lights as well. Noted.

    Just wondering, if we are using PIR on the walls so not breathable, then what would be the benefit of using lime plaster as the parge coat?

    Thanks, it’s these junctions which I’m struggling with most at the moment... you’ve made it sound relatively simple ... !!

    Hi again,

    The parge coat is so that you don't get stray air from the loosely-constructed brick wall in your insulation/wall sandwich. That could be sand/cement, but the thinking is that since you are completely cutting off the breathability of the wall to the inside you should make best efforts to keep breathability open to the outside, hence the lime. If it is not open at least one way then any moisture finding its way in by accident theoretically just stays in and festers.
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