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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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      CommentAuthordjh
    • CommentTimeFeb 15th 2020
     
    Posted By: bardoHello all, I've decided to go with Knauf omni fit glass wool. Though still looking at how to best support the insulation. Just using a membrane or mesh doesn't do it for me and I also see the potential issues with boarding up with OSB as was my original plan. There is a polypropylene product on the market which has holes punched through for ventilation and liquid release. This looks good however the price is way too high. And besides if I used this approach I would also want to cover with a glued or taped breather membrane on top to prevent wind. Has anyone on this forum come up with or heard of an alternative solution? I can see that a strong synthetic mateiral in this situation has merit.

    Sorry but it's late and although I've reread the thread I seem to have lost the plot. What was the problem with OSB? And why is plastic now the right answer, especially when its main benefit of water/airtightness is compromised by holes?

    We're talking about supporting insulation in a void under the ground floor, yes?
  1.  
    Scaffold debris netting?
    • CommentAuthorbardo
    • CommentTimeFeb 15th 2020 edited
     
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    • CommentAuthorbardo
    • CommentTimeFeb 15th 2020
     
    Posted By: djhSorry but it's late and although I've reread the thread I seem to have lost the plot. What was the problem with OSB? And why is plastic now the right answer, especially when its main benefit of water/airtightness is compromised by holes?

    We're talking about supporting insulation in a void under the ground floor, yes?


    So I was all set to take the 9mm OSB route however there was concern expressed on this forum about the lack of breathability and permeability. I moved on to looking at Omni-fit glass wool and their tech person does not recommend OSB. I was exploring the use of coroplast and making holes in it and then glueing a layer of breather membrane on the inside of the tray covering the holes. More work yes, though this would create both a sturdier structural support which would also prevent wind coming in and allowing water a way out.
    • CommentAuthorbardo
    • CommentTimeFeb 15th 2020
     
    Posted By: GreenPaddyHi Bardo,

    cut from an earlier message above...
    -------------------------------------
    What about running a foil faced breathable membrane under the JJI's, running perp to the JJI's?

    Start by fixing a timber 25x50 batten to the external block underbuild, at JJI underside height. Staple (and glue) the VPM to that batten, to give a seal to the wall. Then, VPM to first JJI, passing under it, bring the VPM up the vert face of the lower flange and onto the top side of that flange, fold back on itself, staple through the double thickness of VPM onto the top side of the flange, then go back down side of flange, and across to next JJI, passing under it, before repeating.

    The foil should face the solum! That will give a well supported continuous layer, at full JJI depth, not losing the depth of the lower flange. You could actually let the foil VPM droop below the JJI's, by say 100mm, and even put 400mm of the cheap loft roll 44 (Uvalue for that arrangement = 0.12W/mK).

    That's deff how I'd do it, and not just on paper, have done that on a previous project. Remember to fluff up the wool insulation as you install it, so it's got lots of air en-trained in it, and not squashed, as it comes out of the roll.
    --------------------------------------------

    The foil faced vapour permeable membrane should be around £2/m2, but see what your local merchant has. You don't NEED to use foil faced, but it's a bit tougher, and enhances the Uvalue a little. Also, there is the possibility of getting some condensation on the outer surface, when the strange weather occurrence of very cold for several days, then much milder damp weather, so the foil face gives a slightly more water repellent surface.

    You could just get the cheapest house wrap VPM, at 50p/m2. I don't use netting, as I feel a continuous layer is better, not least in reducing wind washing, but it's not wrong.

    I would lay a vapour check on the top of the JJI's, before fixing the flooring, to improve air tightness, but don't ask the question on here, as you'll get lots of different equally well argued reasons. Make the floor air tight in what ever way you see fit (PU glue on every board edge etc) is a must. You've a breathable house, so continue that through the floor, into the void, and show that on your drawings so the BSO can confirm he's happy with that approach.


    Hi, looking again at the option you mentioned. Not so easy to overlap membrane in some areas of house by stud walls though I guess I could over come that with fresh battens to start and stop. I've looked at some foilbacked breather membrane however I wonder how they would deal with liquid spillage. If I go down the glass wool route then I need to ensure the material can get adequate air and dry out if needed.
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      CommentAuthordjh
    • CommentTimeFeb 15th 2020
     
    Note that breathable membranes are VAPOUR permeable, but are WATERPROOF. Keeping liquid water out of a building is their purpose in life.
    • CommentAuthorbardo
    • CommentTimeFeb 15th 2020
     
    Posted By: djhNote that breathable membranes are VAPOUR permeable, but are WATERPROOF. Keeping liquid water out of a building is their purpose in life.


    Thanks for the reminder - I get confused. Earlier in the thread you suggested a permeable board fixed to the underside such as panelvent. I wonder if such a board would provide sufficient ventilation? And what would happen if there was a spillage? Would it ruin the board?
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      CommentAuthordjh
    • CommentTimeFeb 15th 2020
     
    Sorry I don't understand what you mean by 'sufficient ventilation'? Why do you require ventilation? Panelvent is airtight like most boards. I don't believe a spillage would ruin a board, but if it was continually wet that might cause problems.
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