Timber bay window insulation

Discussion in 'General DIY' started by KarlT1, 12 Feb 2021.

  1. KarlT1

    KarlT1

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    Hello all,
    I have to replace the crumbling lathe and plaster on our square bay window in our bedroom before the plasterers come to skim all the walls and ceiling.
    I would like to insulate the cavity while I have access, but what do I use without causing any damp issues?

    I was thinking sound insulation slabs from rockwool?

    Please advise.

    Many thanks
    Karl
     
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  3. Mike13

    Mike13

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    what depth of gap is there that you could fill with insulation? that might limit your choices. Also, what is the outer wall made of?
     
  4. KarlT1

    KarlT1

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    Outer wall i assume is also lathes and render, I base that assumption on the fact i had to remove the lathe and render apex above our bedroom bay window (same bay window as original question) back in spring last year as it was damaged...

    Depth of the void for insulation, well I don't know that yet as I haven't removed any of the L&P. I am starting tomorrow morning, I think around 50mm though, maybe a tad more..
    But with the current frosts I wanted to be ready with knowing what to buy so I can get it done in a day.
    Looking at some YT videos this PM, celotex seems to be the material of choice.

    Hope you guys can help though.
    Thanks
     
  5. KenGMac

    KenGMac

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    two possibilities to consider?

    1/. Cut a hole into the L/P and use a tape measure?

    2/. Punch several small holes, poke a thin rod. or even a pencil to give you an idea of the width of the void??

    Ken.
     
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  7. KarlT1

    KarlT1

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    Thanks, in about 12hrs I'm ripping it all off anyway.
    I appreciate everyone's input so far, including other posts I have made, but I really need to understand what my options are to insulate.

    If it's 50mm void, what options do I have?
    if it's 75mm void, what are my options?
    and so on.

    i see Celotex comes in various thickness, if there is an option to suit my void, shall I go for that or something else?

    Thanks again

     
  8. Mike13

    Mike13

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    If the frames are timber then you should use a breathable insulation. Celotex will work, but does rely on a vapour barrier at the outside of the structure to prevent condensation in the middle of the wall. Google for wood fibre insulation, as it is a bay window you will not need to buy a lot and any "internal to the wall" condensation gets a chance to diffuse away.
     
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  9. KarlT1

    KarlT1

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    So all the lathe and plaster is off, void is 100mm deep.
    Looks like timber cladding on the outside then I guess batons with L&P render over that.

    I was thinking about the vapour barrier, pointless doing it from the inside.

    So surely drytherm cavity wall insulation would be best?
     

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  10. Mike13

    Mike13

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    Drytherm or Wood fibre insulation will both work, work out which gives the better insulation value and/or which is cheaper, they are both quite easy to cut, but the dritherm will tend to blunt your knife/saw more than the wood fibre will do.
     
  11. DIYnot Local

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