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Loft insulation guidance

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Hi all, 

I have a 1970 brick built house. I found woodworm in the loft, removed all insulation and treated the woodworm myself (quite a task). I now need to replace the insulation but I don't want to cause damp or other issues.

The problems:

1. Joists are only 75mm high, old insulation was only that thick as I have loft boards throughout that I need for storage. I did make the floor a little higher in places by running CLS on top of the joists so I could add a little more insulation but I don't want to lose height in the central part
2. There was no gaps between the plasterboard below or loft boards above
3. The spacing between joists is not consistent so standard materials that are precut won't work
4. Importantly I do not want to use fibreglass as I want an itch free, clean solution

Ideally I want a cheapish, itch free, non problematic solution that ideally gives me better jnsulative properties than I had.

Would appreciate any advice, especially about whether I need a vapour barrier etc. I don't understand when you do actually need one!

I did look at celotex but was worried that the reflective bit could cause mould

Help!!!!!

Comments

  • FreeBear
    FreeBear Posts: 18,163 Forumite
    Tenth Anniversary 10,000 Posts Name Dropper Photogenic
    edited 15 June 2024 at 6:33PM
    Don't worry about the pre-cut rolls not fitting between the joists. Break the rolls apart on the cuts, and push it down between the timbers - My joists vary between 280mm and 330mm centres, so I just took the 380mm widths and stuffed it in (also have 50x75mm joists). On top of that, I have 150mm laid at 90° and a further layer of 200mm to top it all off.
    Extra long (300mm) loft legs will lift the boards up and give plenty of room for the insulation. You don't need a vapour control membrane as any moisture getting in to the insulation will evaporate out through the top as long as the loft space is well ventilated.
    If you don't like fibreglass, rockwool would be the main alternative. Sheep's wool gets darned expensive, and isn't really worth the extra expense in my opinion.
    Her courage will change the world.

    Treasure the moments that you have. Savour them for as long as you can for they will never come back again.
  • flashg67
    flashg67 Posts: 4,125 Forumite
    Part of the Furniture 1,000 Posts Name Dropper Photogenic
    The recycled stuff is relatively cheap, itch free and easy to tear for odd spaces. Example - https://highloft.co.uk/products/highloft-eco-friendly-thermal-insulation-roll?
     I've used it in 2 houses with no damp issues . If your loft is vented at the eaves, don't stuff it too far into the eaves and block the ventilation
  • ThisIsWeird
    ThisIsWeird Posts: 7,935 Forumite
    1,000 Posts Second Anniversary Name Dropper
    edited 16 June 2024 at 8:59AM
    I read Zeb's concerns as being; he cannot use precut rigid insulation in between the joists (and that's been answered - use normal fibre 'loft' type), and he has concerns about condensation forming within the closed  sandwich of ceiling, to densely-packed loft insulation, to loft floor boards.
    Ie, the presence of the loft flooring would greatly reduce the ventilation of this insulation layer, so could that cause issues? Would cond percolate up through the ceiling p'board, and not be able to escape?
    Have to say, that is something I have wondered about myself.
  • FreeBear
    FreeBear Posts: 18,163 Forumite
    Tenth Anniversary 10,000 Posts Name Dropper Photogenic
    ThisIsWeird said: Ie, the presence of the loft flooring would greatly reduce the ventilation of this insulation layer, so could that cause issues? Would cond percolate up through the ceiling p'board, and not be able to escape?
    Have to say, that is something I have wondered about myself.
    The sensible thing to do is to get the boards up on legs so that there is an air gap above the insulation - This would allow for any moisture to escape before it becomes a problem.
    If a vapour control membrane is fitted, it needs to go above the plasterboard and below the joists. So ceiling down, membrane up, reboarded with PB, and then skimmed. A lot of work for little gain. Although, it does give opportunity to get the insulation right up to the edges of the walls - Having taken a few ceilings down here, I've not bothered with a membrane as I don't consider it necessary on a ceiling.

    Her courage will change the world.

    Treasure the moments that you have. Savour them for as long as you can for they will never come back again.
  • Chickereeeee
    Chickereeeee Posts: 1,286 Forumite
    Part of the Furniture 1,000 Posts Name Dropper Combo Breaker
    1) to get the minimum recommended amount of loft insulation, you  need at least 270mm of mineral wool, or about 150mm of rigid (PIR) insulation. So, you have to trade head height against insulation level and installation difficulty.

    2) 'By the book' you should have a moisture impermeable membrane on the warm side (between the ceiling and the insulation) and a permeable membrane on the cold side (between insulation and loft space) but almost nobody does that, in a well-ventilated loft. However, there should be a 50mm or so gap between the insulation and any loft boards, to let moisture escape.

    3) Mineral wool is MUCH easier to install than rigid PIR, but less  insulation value.

    4) The glass-fiber mineral wool made by e.g. Knauf is not very itchy (I did not find it so at all). The alternative is rolls of Rockwool, but this is about 4 x as expensive. Rockwool is also denser, lays better, and is less affected by any moisture that does get through.
  • FreeBear
    FreeBear Posts: 18,163 Forumite
    Tenth Anniversary 10,000 Posts Name Dropper Photogenic
    Chickereeeee said: 3) Mineral wool is MUCH easier to install than rigid PIR, but less  insulation value.

    4) The glass-fiber mineral wool made by e.g. Knauf is not very itchy (I did not find it so at all).
    Mineral wool & fibreglass have about the same thermal conductivity at around 0.04 W/mK. EPS/XPS (polystyrene) is ~0.035 W/mK. PIR/PUR comes in at 0.022 W/mK. And just for comparison, spray foam insulation is 0.039 W/mK - Pretty darned poor for a polyurethane foam and needs at least 250mm to meet current requirements. Not that anyone in their right mind would use the darned stuff to insulate a loft....
    Fibreglass & mineral wool are both non-combustible. All the others listed above will burn quite nicely and release toxic fumes as it does so.

    As for itchiness of the Knauf EcoRoll, I agree, it is no where as itchy as fibreglass of old. Still worth wearing PPE when installing the stuff though.


    Her courage will change the world.

    Treasure the moments that you have. Savour them for as long as you can for they will never come back again.
  • Chickereeeee
    Chickereeeee Posts: 1,286 Forumite
    Part of the Furniture 1,000 Posts Name Dropper Combo Breaker
    FreeBear said:
    Chickereeeee said: 3) Mineral wool is MUCH easier to install than rigid PIR, but less  insulation value.

    4) The glass-fiber mineral wool made by e.g. Knauf is not very itchy (I did not find it so at all).

    Fibreglass & mineral wool are both non-combustible. All the others listed above will burn quite nicely and release toxic fumes as it does so.

    As for itchiness of the Knauf EcoRoll, I agree, it is no where as itchy as fibreglass of old. Still worth wearing PPE when installing the stuff though.


    Just for clarity, both Rockwool and Knauf loft roll are called 'mineral wool' by their manufacturers. Rockwool is made from rock (duh!) and most of Knauf's products are made from glass - sometimes they call it 'glass mineral wool' sometimes just 'mineral wool'.

    I beleive Knauf ALSO do a mineral wool made from rock, which they call Rocksilk.

    Confused me when I was looking recently.
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