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Thread: Cavity wall insulation - moan

  1. #1
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    Cavity wall insulation - moan

    So at the moment I'm doing a lot of work to my hose, I've just removed the chimney breast - which was a total pain in the bottom (never realised there were so many bricks involved).

    Anyway I've just removed my outside wall plasterboard (living room) as I'm fitting 100mm celotex insulation and rewiring some stuff. In the past I had cavity wall insulation installed - the kind where you get holes drilled from the outside and they pump the insulation inside the wall. I was very surprised to find that when I removed the plasterboard that most of the insulation was only filling about half the cavity due to the weight of it over time compressing the insulation.

    Anyway what a waste of heat, If I had know about that I would have changed it years ago - so If you have that type of insulation, I recommend you change it to kingspan/celotex - It's not expensive from B&Q - £56 for a 2400x1200x100mm sheet and £6 for a sheet of plasterboard. - most walls round my way only need 3 of each, when you consider that's less than £200 to do that - and the money you'd save in gas/electric from heating - it's worth getting done the next time you decide to decorate.

    Moan over.

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  3. #2
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    Having experience of both cavity wall insulation (a 1970's house in Bristol insulated with rockwool) and solid interior & exterior wall insulation on the previous/current 1920's solid-walled house (now has both types of insulation on most walls) in London, I think you are over-simplifying the picture.

    Cavity insulation is relatively very quick, easy and cheap, and does work even if not done totally right! Interior/exterior solid insulation is far more material / time/labour/cost intensive and also works very well. Neither is "perfect" but "you pays your money and makes your choice".
    PS: we have a lot of exterior solid walls (900 sq ft floor area End-of-Terrace WITH FOUR OUTSIDE WALLS, and quite a large conservatory - GO FIGURE, because I have!) and not a "huge " boiler (Bosch Worcester 28/30 GSI combi), and we are now sitting in lightweight clothing in a temp of around 24C, with an outside temp of, probably, near 0C (frost is forming on the cars outside).

    PPS = cost/time
    In around 1987 the Bristol house (which is smaller than the London one) took 1/2 a day and cost around £250 to cavity-wall insulate.
    The London house was interior-insulated before I went to Bristol and probably cost around £1k and 10's of DAYS of work, and then, in 2015, £7.5k (but I got a 50% Green deal rebate! ) for exterior insulation over about 60% of the exterior walls (that took weeks of work from the contractor).
    Last edited by jallen01; 09-12-2017 at 11:52 PM.

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    @jallen01, I'm doing the work myself so it's only parts I am paying for - obviously removing plasterboard requires the room to be decorated afterwards, so takes more time and cost, but I can assure you that cavity wall insulation sags over time and ends up as a big lump which fills the bottom to middle of the wall with a huge air gap on the last 3rd of the wall + you end up with loads of unsightly filled holes on the outside of the building.

    Personally I think that doing the job properly and making sure your walls are properly insulated is preferable if you can do that, or have the money to pay someone else to do it, obviously not everyone is a skilled tradesman - but removing plaster board and fitting insulation and screwing new plasterboard is a relatively easy task for most people. Then get a decorator/joiner in to refit skirtings and plaster the joins if you can't do that yourself.
    Last edited by mrdude; 10-12-2017 at 11:46 PM.

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    mrdude
    You are probably correct about cavity wall insulation sagging (probably mainly the loose sort - like rockwool!).

    OTOH, interior insulation as you are, and I did, fitting does also have its drawbacks, particularly in rooms with small rooms where the additional wall thickness can noticeable reduce the useable size of those rooms. That said, I advised my current next-door neighbour to have their front bay-windowed bedroom internally insulated when it was renovated last year, and he later told me that it has made a vast improvement to the temperatures in that room (probably, especially tonight!). I also specified internal insulation of the walls of the quite large "cinderblock-walled" conservatory we had installed around 20yrs ago - and it really has made a difference to the winter temps in that room.

    So, overall, I am very in favour of internal wall insulation - but the work involved has to be taken into account as that is substantial.

    PS: it's your decision, but is it worth getting the existing sagged cavity wall insulation "topped up" to reinstate it to the full height of the walls ?

    PPS: When I replaced (myself!) the floor boards of the through lounge, I also hung plastic netting between the joists and laid 50mm of rockwool on top of that netting - made a big difference to the temp in that room (obviously would not work with a solid floor, but also had 50mm of Jabolite ("polystyrene") insulation laid below the top screed of the floor in that conservatory = nice warm floor to walk on, even in this weather! )
    Last edited by jallen01; 11-12-2017 at 10:01 PM.

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    @jallen01, it's made no difference to the wall thickness, the cavity has an air gap and then the joists/noggins are 100mm depth, so I put 100mm insulation in + then I added that bubble wrap insulation with the stuff with the foil, before putting the new plasterboard on, the wall is the same thickness as before - so the room is the exact same size (minus the chimney breast - which has made it slightly larger).

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    @mrdude
    I assume your house must be quite modern with a brick outer shell, "insulation-filled" (which it now isn't!) air-gap and a plasterboard inner surface - that's not the same as older (not much!) buildings where both the outer and inner shells are brick. Therefore, you can obviously make your new total wall thickness the same as it was before you started the work. Those of us with the older styles of buildings can't do that - so let us both compare "like with like" from now on.
    OTOH, with 100mm of Cellotex or similar insulating material (which, effectively, is roughly what I now have), the building should be pretty warm in weather like we are currently having .
    BTW: congrats for doing this all work yourself as I do know what hard graft this sort of thing can be.

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