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Suspended floor insulation

2

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  • ic
    ic Posts: 3,338 Forumite
    Part of the Furniture 1,000 Posts Photogenic Name Dropper
    I met a guy that was a big advocate of using insulated tile backer boards for insulating everything.  He was adamant they'd be strong enough to replace floorboards, resulting in an inherently insulated floor.  I'm not convinced.  Something like this: https://www.diy.com/departments/q-board-backerboard-h-2400mm-w-600mm-t-30mm-pack-of-2/1919363_BQ.prd 
  • ic said:
    I met a guy that was a big advocate of using insulated tile backer boards for insulating everything.  He was adamant they'd be strong enough to replace floorboards, resulting in an inherently insulated floor.  I'm not convinced.  Something like this: https://www.diy.com/departments/q-board-backerboard-h-2400mm-w-600mm-t-30mm-pack-of-2/1919363_BQ.prd 
    Insulation seems to be an afterthought with this product? Mainly for providing a flat surface.
  • Chickereeeee
    Chickereeeee Posts: 1,274 Forumite
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    FreeBear said:
    FreeBear said:
    Chickereeeee said: I do wonder how much heat is lost through floors.
    10-15% is the generally quoted figure.

    The % figures often used are misleading at best. (100% of heat is always lost from house however good the insulation). Generally the figures are for a house with no insulation at all, so carpets/underlay would greatly reduce this %. But loft and cavity wall insulation would increase it!
    True.
    It would be better to state xxKWh of the energy is lost through one particular element of the building. But without a complete & detailed assessment, only generalisations can be made. Reduce the heat loss by insulating and draught proofing as best as possible, and the cost of heating will be reduced.

    Well, yes, but 'as best as possible' is a bit vague, and could be hugely expensive! The question was, rather, what is optimal.
  • T&G boards are, themselves, pretty darned good insulators.
    I'm pretty sure that, by far, the biggest cause of heat loss due to suspended T&G floorboards is the draught.

    Interesting.  I've just had new T&G engineered flooring glued down over existing (non T&G) floorboards and have been wondering if there is any point now in also putting in insulation between the joists from below (it's a basement cellar below and the joists are accessible).  It's already a big improvement.
  • Thanks for all replies. I was previously checking boards as thick as 100/150mm, and they were too expensive. 25mm definitely sounds more reasonable 😁

    So to get similar performance to 25mm board, how much mineral wool would be needed?

    We'll be most likely laying the insulation from above. 
  • Bendy_House
    Bendy_House Posts: 4,756 Forumite
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    edited 27 October 2021 at 11:38PM
    I suspect you'll need to fill the full depth of your joists with mineral wool, to match a ~1" layer of sealed, high-quality rigid insulation - which shouldn't be a problem.
    I understand the usual method is to drape some form of lightweight plastic netting over the joists, and push it down the sides of each joist to form a hammock that'll then take the wool.
    If you go this method - and I guess it's a perfectly fine alternative - then I'd personally also be looking to add a layer of paper or similar (the kind of stuff that's sometimes placed under carpet underlay) before boarding back down, so that the floor is also draught-proofed. And don't forget to seal around the perimeter :smile:
  • ic
    ic Posts: 3,338 Forumite
    Part of the Furniture 1,000 Posts Photogenic Name Dropper
    This is a good read - https://www.ecologicalbuildingsystems.com/post/best-practice-approach-insulating-suspended-timber-floors

    Advocates using a membrane to stop the draughts and effect of wind reducing the effectiveness of insulation, and stop things living in the insulation...
  • I'll be putting a laminate flooring with underlay. Will this not draft proof the floor? 
  • FreeBear
    FreeBear Posts: 17,214 Forumite
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    Polin_ka said: I'll be putting a laminate flooring with underlay. Will this not draft proof the floor? 
    Yes, it will stop draughts coming up through any gaps in the floor boards.
    If you are taking the skirting boards off, get yourself a foam gun. Put a thin bead of expanding foam around the perimeter of the room to seal the gap between floor & wall. Let the foam harden, trim back, and refit the skirting. The foam gun will allow you to put down a thin bead of foam in a controlled manner. Well worth using one over cans with a built-in nozzle/tube.
    Taking the skirting off gives you the opportunity to run the laminate up to the wall and avoid the need to use scotia trims.

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  • Hi all, thanks for all replies. We've decided to first tackle the floor under our extension. The crawling space is enough to move comfortably (well, as comfy as it can get) . We are not sure about the rest of the house yet, but the extension can do with insulating for sure.

    We want to use PIP boards. What would be an acceptable thickness? I think someone mentioned 25mm before. Does it make sense to upgrade to 100mm? I obviously want to spend as little 😉 the building Reg for renovation in Scotland are pretty strict 🙄 I think I should be achieving value of 0.18 or 0.15. 

    Also, can we use any brand or is Kingspan and celotex considerable better than those lesser known names? 
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