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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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  1.  
    1950's property with 600mm space below joists

    Go....
    • CommentAuthortony
    • CommentTimeDec 20th 2021
     
    Why is fast important?

    I would get under there with insulation quilt presume 100mm between joists, then strawberry netting stapled and another 150mm across the other way again strawberry net fixed with long screws and washers
  2.  
    With the floor boards be lifted or left in situ ?
    • CommentAuthorGareth J
    • CommentTimeDec 20th 2021
     
    A rug?
  3.  
    We are renewing the subfloor as part of insurance works. Seems sacrilege not to insulate while sub floor off but the insurance aren't paying for it... So we're trying to do it as cheaply as possible for the customer but if we're doing that we're not doing our insurance work so trying to work something out for them...
  4.  
    What do you mean by renewing the sub floor?
    replacing the floor boards
    renewing the joists
    What of the existing floor remains undisturbed
    Something else ?
  5.  
    When you renovate a large area, don't the building regs require you to insulate while you are at it? AIUI that is a legal requirement, but don't know what national regs are where you are (Wales?). If so the insurance should pay.

    Like Tony said, drape net between and below the joists and fill with mineral wool. Dead easy with the floor boards off. Use garden net or breather membrane or scaffold net. Don't block any ventilation paths. Trap edges with battens.
    • CommentAuthortony
    • CommentTimeDec 21st 2021
     
    Sometimes I advise fixing 50 or 75mm eps under the joists and infill above with quilt
    •  
      CommentAuthordjh
    • CommentTimeDec 21st 2021
     
    Posted By: WillInAberdeenWhen you renovate a large area, don't the building regs require you to insulate while you are at it? AIUI that is a legal requirement, but don't know what national regs are where you are (Wales?). If so the insurance should pay.
    I believe that is broadly correct and that it does apply in Wales as well as England et al. What I don't know is whether this particular subfloor is part of a thermal element. As Peter asks 'What do you mean by renewing the sub floor?'. It might be simplest/best to discuss what you're doing with building control, since it might be notifiable and they will know. If so, then as Will says I think the insurance should pay. There's the usual weasel words in the regs about reasonable costs to complicate matters further.
  6.  
    Renewing t&g floorboards

    If you were to use strawberry netting how do you insulate under the joist to make it continuous?

    Any breather membranes etc?
  7.  
    Dangle the net in quite loose loops that hang well below the joists. When you fill with wool, they bulge out either side and meet below the joists. Alternative is to have the net dangling from battens that you fix underneath or alongside the joists so the loops of net all run underneath the joists. Or like Tony said.

    Pack the wool quite tight so it springs up against the floorboards and doesn't slump down in future.

    Dead easy job with the floorboards off, and only cost is the net and wool, so definitely not 'unreasonable' to do it (per building standards).

    Others have put VCL down before laying new floorboards, I preferred to leave mine breathable. Could lay breather membrane for airtightness and join it to walls behind skirting to fix drafts. If replacing boards I would lay t&g chipboard, joints glued, more stable and airtight than pine. For my own house I would be looking at those boards with pre-routed channels for UFH but I guess not for your insurance.

    Having a warm undrafty floor made huge difference to comfort and usability of room, highly recommended!
    •  
      CommentAuthordjh
    • CommentTimeDec 21st 2021
     
    Posted By: VictorianecoRenewing t&g floorboards
    Sorry but if that's an answer to the 'What do you mean by renewing the sub floor?' question then the sub-floor is definitely a thermal element so building control have to be involved. And hopefully means the insurance will have to pay.
  8.  
    Posted By: WillInAberdeenDangle the net in quite loose loops that hang well below the joists. When you fill with wool, they bulge out either side and meet below the joists. Alternative is to have the net dangling from battens that you fix underneath or alongside the joists so the loops of net all run underneath the joists. Or like Tony said.

    Pack the wool quite tight so it springs up against the floorboards and doesn't slump down in future.

    Dead easy job with the floorboards off, and only cost is the net and wool, so definitely not 'unreasonable' to do it (per building standards).

    +1


    Posted By: WillInAberdeenOthers have put VCL down before laying new floorboards, I preferred to leave mine breathable. Could lay breather membrane for airtightness and join it to walls behind skirting to fix drafts.

    I would go for a VCL for air tightness as well as moisture barrier (and insurance against any big spills)
    • CommentAuthorGareth J
    • CommentTimeDec 21st 2021
     
    Out of curiosity, is breathability required in these situations?

    If the house volume is separated from the underfloor by a vcl, warm house air shouldn't be able to get through to the underfloor to cause condensation problems?
  9.  
    I have an older house where the floors have been vapour permeable for the last couple of centuries, I would be very reluctant to change that now.

    The soil (2ft beneath the suspended floor) is always cooler than the dewpoint inside the house, IE it acts as a big dehumidifier by condensing water vapour from inside the house, I believe, and storing and draining it away. Moisture vapour is always drawn to the coldest surface it can find if that is able to condense it, and I would rather have that dehumidifying surface beneath the house not inside it. A VCL would prevent that dehumidification. The soil is free draining and has presumably swallowed some spills over the centuries.

    Modern thinking is to cover the subsoil with concrete and the floor with VCL, in case moisture should somehow evaporate from the cold subsoil and condense on a surface inside the warm house, but I can't follow the physics of that!

    The importance of having the joists wrapped inside the insulation and vapour-open to the house, is so they don't become part of the condenser.

    Others may have different tastes!

    (Edit: having said all that, we do have inherited 1980s vinyl flooring in one room, which seems to have done no harm!)
    • CommentAuthorGareth J
    • CommentTimeDec 22nd 2021
     
    Another thought; in a "normal" vented underfloor and leaky floorboards situation, is any warmed house air ever going to venture beneath the floor? I'd imagine the chimney effect of a house would be constantly drawing air up from the cooler subfloor into the house. Especially during the days of open fires and leaky windows. I suspect that a poorly ventilated UF and a relatively well sealed house might be able to set up a situation where unbalanced vents setup a negatively pressurised floor space when wind is strong and in a certain direction but might be rare.

    I remember seeing carpets inflating on windy days in my grandparents 60's built house, when the open fire was going and someone opened the door. An extreme example but upwards is surely the direction of air travel?
    • CommentAuthorBenM
    • CommentTimeDec 23rd 2021 edited
     
    I used cavity slabs, and just pushed them up in the spaces between the joists. However, this was a suspended floor over a basement, so this probably sped things up a lot. I was a bit nervous about damp, so left about a 300mm gap to the outer walls incase they acted like sponges and in contact with the joists.

    https://www.wickes.co.uk/Knauf-Insulation-Earthwool-Dritherm-Cavity-Slab-32-Ultimate-100-x-455mm-x-1-2m/p/143387
  10.  
    fill it up with blow in polystrene beads
    • CommentAuthorrhamdu
    • CommentTimeJan 15th 2022
     
    Posted By: WillInAberdeenDangle the net in quite loose loops that hang well below the joists. When you fill with wool, they bulge out either side and meet below the joists. Alternative is to have the net dangling from battens that you fix underneath or alongside the joists so the loops of net all run underneath the joists. Or like Tony said.


    Is there a way to do this without raising all the boards?

    Perhaps lay the net on the ground, lay wool on top of it and then pull it up into position?

    This would mean no VCL, obviously. I wouldn't mind a bit of airtightness, though.

    By the way, what kind of net would people specify?
  11.  
    Scaff protection netting is robust, but to be honest I started following others' suggestion of using breathable membrane instead of netting to reduce risk of 'windwash'.
    • CommentAuthorShevek
    • CommentTimeJan 18th 2022 edited
     
    Has anybody used Q-Bot? They use a robot to spray closed cell polyurethane foam.

    Curious that they opted for closed cell over open cell foam. Their FAQ says:

    Independent analysis conducted on our behalf showed that closed cell foams performed better than open cell ones in suspended ground floors as they reduced the risk of damp and mould in the floor. Closed cell foams can still allow moisture to escape if they get wet and they act as a hydrophobic barrier between the damp ground and floor above. This has been matched by the results from in situ data monitoring.
    Furthermore, open cell foams require a greater thickness to achieve the same U-Value and this limits the size of the void they can be used in without causing issues with the Building Regulations.
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