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Worth insulating the interior of an exterior cavity wall?

Discussion in 'Building' started by sc0ps, 7 Jan 2010.

  1. sc0ps

    sc0ps

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    Hi All,

    Looking for some opinions/advice please. I have a standard semi-detached 1930 house which has cavity walls. At the moment these cavities aren't filled, but I am going to be getting this done soon.

    Bearing this in mind, is it still worth taking down the old plaster on the internal side of the walls and replacing with gyproc thermal boards 27mm. I can't go with any thicker boards than this because then it will be sticking out over the inner stringer of the stairs. At the moment I am just looking at doing the hall/stairs/landing and small box/study room, but will eventually do all the external walls when those rooms are re-done.

    I should mention that there is some blown plaster on the walls already but not enough in its own right to warrant taking off completely and starting again.

    Other people I have canvassed are of the opinion that all insulation is good/worth it. It's not really about the time/cost, it's whether or not it's worth doing it seeing as the cavities will be filled.

    Thanks in advance.

    Steve
     
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  3. mointainwalker

    mointainwalker

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    Since you say you are going to get the cavity insulation done soon, I take that to mean you've investigated it.

    What insulation values do suppliers give for their cavity fill ?
     
  4. sc0ps

    sc0ps

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    Hi,

    All the website says about the cavity fill is:-

    "...There are several different types of insulation:

    · Bonded bead (white polystyrene beads)
    · Glass wool (Yellow or white in colour)
    · Rock wool (Grey/brown in colour)
    · Urea formaldehyde foam (white foam)
    Note: both glass wool and rock wool are known as ‘mineral wool’.

    All systems of CWI have been tested, assessed and approved by the British Board of Agrément or the British Standards Institution. All are suitable for their purpose.

    Except for Urea Formaldehyde foam, the systems can be used in all parts of the UK.

    All systems have a similar insulation value. ..."

    Looking up the value for blockwork with cavity the Uninsulated U-Value is 0.957 W/m²K, with 50mm inusulation this comes to 0.340 W/m²K. The 27mm gyproc thermaline plus boards have a value of 0.63 W/m²K.

    Regards
    Steve
     
  5. PrenticeBoyofDerry

    PrenticeBoyofDerry

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    The cavity is worth filling, as for the boarding out could be over the top and will also lose you internal space.
    Cavity fill to recommended values, basecoat and skim walls.
     
  6. mointainwalker

    mointainwalker

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    Re: Your thermal values

    I cannot find a site giving cavity wall values however the Gyproc you mention has a Thermal Resistance (that's an R value ) of 0.63 therefore a U value of 1.6

    Can you give a link to the site for the cavity wall values ?
     
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  8. sc0ps

    sc0ps

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  9. mointainwalker

    mointainwalker

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    it's an interesting web-site but you won't get the values they give.

    It doesn't explicitly say so, but I don't think this is retro-fit insulation therefore the figures they give are for boards or wool installed during build and the U 0.34 is for PIR which is a high-performing insulant which is apparently not available for retro-fit ( not on your list ).

    They have also included the plasterboard, which is fair to give you the value of the whole wall, but you need to subtract that to be able to evaluate - or calculate - the value of c-w insulation as a stand-alone process.

    Subracting the pb and the higher value of PIR and replacing that with polystyrene gave a value of U 0.42 but that is still for poly board. I don't know if bead-filling has a lower value because of potential gaps etc.

    Financially i don't think the Thermaline is worth doing ( just my opinion )

    If you get U 0.40 ( adding back in the pb value) with the c-w, then this board improves the total to U 0.32.

    That means you are saving 0.08W/ m2/K , K being the difference in temp in degrees between inside and outside.

    If it is 20 C inside and 0 C outside then you are saving 1.6 W /hr/m2.

    I usually use a formula of K x 24 ( hrs) x 100 (days) to work out cost vs saving and that gives 3840 W or 3.84 kW

    Again keeping it simple , I always cost 1 kW as costing 10 p so you have saved 38.4p for 1 m2 over what I class as a heating season ( K exaggerated compensated by fewer days to keep it manageable )

    A quick check gave me a m2 price of pnds 7 for this board, excluding transport, fixing. Spending over 7 pnds to save 38 p/ yr doesn't look a good deal to me.

    If anyone finds a hole in my workings, I apologise in advance but it's easy to forget the odd calculation when there are so many add-ons etc.

    If you go ahead with the c-w , please let us know the outcome.
    .
     
  10. alittlerespect

    alittlerespect

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    See my earlier response to AIQ which should answer your question.

    One further comment - I would leave the cavity alone - as some insulation material can cause more problems than they solve - check out some of the discussion threads on this subject. it would be a far safer bet in your particular circumstances to insulate the external of the wall/s.

    Regards
     
  11. sc0ps

    sc0ps

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    Hi,

    Thanks for all the replies - I have taken all the plaster off the interior walls now (most of it came off far to easily for my liking hence justifying doing).

    I am going to put thermal board up on the inside (it's not really about cost - more about getting a warm house finally for next winter!) and wait and see on the cavity wall insulation - the jury is still out on it, I appreciate that there are potential problems with doing it, but you only ever hear about the problem fills, not the countless hundreds which have been done fine?!? Still, need to do a bit more research about it.

    Cheers
    Steve
     
  12. Nige F

    Nige F

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    problems can be snots on wall ties - original builders ...Damp patches and smell :mad: from foam fill...stopped using years ago...Balls flowing out through holes in internal walls ...not enough glue used . Don`t have poly balls done :idea: Airbricks to underfloor voids not sleeved through properly...original builders... check carefully , though installers " should"check/ correct. must be some more to justify having freezing walls ;)
     
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