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General: membrane for underfloor insulation

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  1.  
    • CommentAuthorrsk1
    • CommentTimeDec 4th 2023
     
    Insulating under a suspended wooden floor, access from the cellar below. Using rockwool and recycled plastic quilt (I have various rolls to use up), full depth of the joists (180mm). I want to membrane below this to draught proof but am a bit unsure of the vapour dynamics. I think i want an airtight breathable membrane. Vapour permeable so any moisture condensing near the cold side of the insulation can escape to the ventilated cellar. Is this right?

    There are plenty airtight breather membranes available, but how do they actually work? Can't visualise how moisture can get through but not air

    I've read on here that you shouldn't have fluff insulation in contact with ext walls as it can wick moisture onto timbers. Is that right? given that timber is already in contact with ext walls, isn't that already a risk?
    •  
      CommentAuthordjh
    • CommentTimeDec 4th 2023
     
    Posted By: rsk1Vapour permeable so any moisture condensing near the cold side of the insulation can escape to the ventilated cellar. Is this right?
    Yes, exactly.

    There are plenty airtight breather membranes available, but how do they actually work? Can't visualise how moisture can get through but not air
    It's possible to make materials that are airtight but 'breathable' (i.e. allow water vapour through) and it's possible to make materials that are vapour-tight but let air through, but if I am to believe https://www.insulationsuperstore.co.uk/help-and-advice/product-guides/ceilings-and-roofing/how-does-breathable-membrane-work/ breathable membranes are neither of these. It says they let both air and water vapour through. But they stop liquid water and wind and dirt. Dunno if that is true for all breathable membranes, but.

    I've read on here that you shouldn't have fluff insulation in contact with ext walls as it can wick moisture onto timbers. Is that right? given that timber is already in contact with ext walls, isn't that already a risk?
    Yes, and no. If timber is in contact with a solid wall, or with the inner leaf of a cavity wall but below the DPC then there is a risk of decay. The inner leaf of a cavity wall above DPC should be dry, though, so that should be OK. I'd use the recycled plastic against the wall as I suspect it's more likely to resist wicking.
  3.  
    If you've concerns at the walls, use rigid insulation board on the wall surface. A sheet will go a long way, if you're ripping it to say 200mm strips. Then wool against that. A fom gun will make sure the insul boards are well sealed.

    If you can go deeper than the joist (180mm), that would be better, keeping the joists warm. Fix the the membrane to the face of the joist, fill the joist void, with 2 layers of say 150mm wool. Pull the membrane (already fixed along the now filled joist face), towards you and up the face of the adjacent joist. Fix it to that joist face, and repeat.

    That gives wool between and below the joist, supports the wool, and reduces wind-washing of the wool. Looks like a big padded duvet once finished. I use the cheapest vapour permeable membrane I can get, usually about £1/m2. I use a staple gun for fixing (lots of staples). If you want to be super secure, rip strips of plywood, or 2"x1" battens to fix over the staple-secured membrame, clamping it in place.
  4.  
  5.  
    That should have said:

    ''https://www.ecologicalbuildingsystems.com/post/suspended-timber-floor-insulation-installation-underneath

    suggests a possible approach'' but I can barely get past the various error messages to post anything! ;-(
    • CommentAuthorrsk1
    • CommentTimeDec 5th 2023
     
    Posted By: Nick ParsonsThat should have said:

    ''https://www.ecologicalbuildingsystems.com/post/suspended-timber-floor-insulation-installation-underneath" rel="nofollow" >https://www.ecologicalbuildingsystems.com/post/suspended-timber-floor-insulation-installation-underneath




    This system has an airtight membrane above the insulation and a windtight membrane underneath it.
    What is the difference? Is windtight more air permeable than airtight? Why this system rather than just an airtight underneath ? There will be some penetration of air through the floorboards into the ins, but surely this hardly counts as windwash?
    • CommentAuthortony
    • CommentTimeDec 6th 2023
     
    I prefer vapour barrier in the warm side if the insulation and mo membrane underneath . You can do vb on top of the floorboards as they add a little insulation.

    Will you mitigate the thermal bridging of the joists?
    •  
      CommentAuthordjh
    • CommentTimeDec 6th 2023
     
    Posted By: rsk1This system has an airtight membrane above the insulation and a windtight membrane underneath it.
    What is the difference? Is windtight more air permeable than airtight? Why this system rather than just an airtight underneath ? There will be some penetration of air through the floorboards into the ins, but surely this hardly counts as windwash?
    Actually the system on the page uses Intello Plus, which is a variable ("intelligent") vapour control membrane. The other system shown in the PDF shows an alternative method using different products.

    Floors are an awkward situation, because there's always the possibility of water dropping on them as well as vapour. Also the conditions underneath can vary a lot, dry or damp, still or a gale etc. So you need some combination of materials that will work in your situation. There is one rules that applies everywhere I think: no organic materials - the risk of rot is just too high.
    • CommentAuthorrsk1
    • CommentTimeDec 6th 2023
     
    Posted By: tony

    Will you mitigate the thermal bridging of the joists?


    Not planning to as I need all the headroom I can get in the cellar
    • CommentAuthorrsk1
    • CommentTimeDec 6th 2023
     
    As ever, thanks for the replies

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