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Insulating Exterior Wall Internally

Discussion in 'Building' started by Jitno123, 31 May 2017.

  1. Jitno123

    Jitno123

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    Hi,

    Im considering insulating a wall internally as this room gets very cold as soon as the heating is turned off. Its a downstairs living room, and my plan is to insulate the exterior wall. We had insulation blown into the cavity walls within the house, and insulated the suspended floor to reduce draught, however it is still cold in this one room. When i asked my builder about this, he recommended to 'dot and dab' 50mm EPS 'jablite' insulation to the wall, then plasterboard and skim. When i asked can we do the same with celotex, he said its difficult to stick that on the wall.

    My questions are:

    Is EPS insulation safe to use internally?
    Is it possible to dot and dab celotex onto a wall?
    What method would you guys recommend for insulating this wall?

    Many thanks
     
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  3. John D v2.0

    John D v2.0

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    EPs works OK, about half as good as celotex but a lot cheaper. The only issue is it can't have cables touching it as it destroys the cable insulation.
     
  4. Mr Chibs

    Mr Chibs

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    I might have it wrong, but I don't think you should be using that for walls, internally, fire risk... also it's not as good, thermally, as foam.


    Here's what I've done.

    25x50mm battens, then 50mm eco-therm [foil both sides] on top of battens, aluminium tape all joints, then plasterboard and skim.
    Consensus seemed to be, to have an air gap behind boards.

    It's time consuming in a old house, where walls are not flat.

    Oh, and you need long fixings 70mm for insulation and 80mm for plasterboards.

    PS I've been paying around £15 for 50mm 2.4x1.2 board.



    Read more: https://www.diynot.com/diy/threads/insulating-external-solid-wall.482237/#ixzz4iefSmFaI
     
  5. chappers

    chappers

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    you can dot and dab celotex backed plasterboard no problem.
     
  6. Jitno123

    Jitno123

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    Thanks for the info. £15 a sheet board?? Very good price, care to share?
     
  7. Jitno123

    Jitno123

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    Is EPS bad to use indoors?
     
  8. evdama

    evdama

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    eps is nasty weak 70's stuff- similar effort to celotex with half the payback
     
  9. Mr Chibs

    Mr Chibs

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    Local merchants... yes, yes, plus VAT
     
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  11. endecotp

    endecotp

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    You should go for quite a lot more than 50mm.
    I'd use Celotex or similar. There are various ways to fix it and the plasterboard. One option is to buy combined celotex + plasterboard boards and dot+dab them; that's relatively quick and easy, but maybe not the cheapest.
    It sounds like your builder lacks appropriate experience.

    The plasterboard provides fire resistance.

    Windows, radiators, sockets etc. will all make it more time consuming, as will re-decorating afterwards.
     
  12. Mr Chibs

    Mr Chibs

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    Out of interest, why should we do more than 50mm, is it just to get the U-value down.
     
  13. JohnD

    JohnD

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    50mm of Celotex has about the same insulating power as 100mm of fibreglass, so for a wall, it will make a huge difference. Doubling the thickness of Celotex will not double the energy saving, the cumulative effect of additional thickness diminishes quickly.

    You will have to look up the figures and calculate heat loss compared to your bare wall, but I think you will be very pleased with the saving in money and the improvement in comfort.
     
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  14. endecotp

    endecotp

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    You will get the U-value down more by adding more or better insulation.

    50mm of jablite has a thermal conductivity of about 0.75 W/m^2K.
    A cavity wall with no insulation will have a thermal conductivity of about 1.5 W/m^2K.
    Combining those will give a total conductivity of about 0.5 W/m^2K.
    So adding 50mm of jablite will reduce heat loss through the wall to one third.
    But building regs require 0.18 W/m^2K (IIRC).
    So you're still losing 2.5 times as much heat through the wall as a new-build home would.

    50mm of celotex has a thermal conductivity of about 0.45 W/m^2K.
    Combining that with a cavity wall (as above) will have a thermal conductivity of about 0.34 W/m^2K.
    That reduces heat loss to less than one quarter but is still almost twice as bad as a new-build.

    100mm of celotex would get you to new-build standard.

    The additional cost of using celotex rather than jablite is small, in the context of paying someone to do it, dealing with sockets, radiators, re-decorating etc. The additional cost of using more than 50mm is also small but it does lose more room area, which is difficult to quantify.


    But you say you've had your cavities insulated.
    One possibility is that this has failed to work in this room, explaining why it is still cold. In that case the numbers above apply.
    But if the wall does have insulated cavities then the numbers change, making more insulation more appealing.


    The rule of diminishing returns does apply, but I'd think of that in terms of e.g. draught proofing and windows.
     
  15. Mr Chibs

    Mr Chibs

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    So 200mm and job done:D

    Cheers for the update, should have said mine is solid wall, haven't the space to have 100mm in all rooms.
     
  16. chappers

    chappers

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    50mm on all outside walls will make a noticeable difference, do yourself a favour and use board bonded to celotex
     
  17. motorbiking

    motorbiking

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    50mm on top of a filled cavity is plenty. i'd second celotex PL plasterboard bonded to celotex. you don't have to dot n dab. You can mechanically fix, but it can be easier to adjust levels with dabs. Avoid battens as they just reduce the performance.
     
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